A short-term capacitor-based "UPS" for mini (NUC-type) PCs

The problem

Figure 1:
A "Beelink" small form-factor PC.  This unit sports a Ryzen
processor and runs from an external 19 volt supply.
Click on the image for a larger version.

Very small computers (so-called "NUCs" - a term that we'll use generically throughout) of recent manufacture are energy efficient and are increasingly used in lieu of full-size desk top PCs.  Many of these use external power supplies - often referred to as "bricks" - of the sort also used to power and charge laptops.

Even if one has a UPS (Uninterruptable Power Supply) attached to their computer - or especially in the case of a "whole house UPS" (e.g. Tesla Power Wall or equivalent) there are instances during which the transition between power grid going down and the UPS picking up the load may not be fast enough to prevent the computer from rebooting or just crashing and hanging.

For this article, we are looking at the case when the power supply for a "NUC" (small form-factor power supply) is incapable of "riding" through the aforementioned UPS transition.  In this instance at least part of the problem has to do with that unlike the power supply in a desktop computer - which are physically bigger and have a comparatively large reservoir of energy storage in the form of big filter capacitors - the small power supplies used for these small computers have a comparatively small energy reserve - and unlike a laptop, there is no onboard battery to serve as a backup.

Returning to the "whole house UPS" and, to a lesser extent its much smaller "desktop" counterpart used to back up only critical gear, it may take more than 100 milliseconds for the power to resume after the grid is lost - depending on the nature of the outage:  Owning a Tesla Powerwall - and talking to others with this and similar (non-Tesla) systems - they all seem to share a common trait:  Sometimes they switch quickly enough that nothing reboots, but other times they take much longer to switch (sometimes more than 500 milliseconds) and many computers - even desktop PCs with more capacitive energy storage - fail to carry through the transition.

Details about the replacement power supply The MeanWell power supply that works with this Beelink NUC  is P/N:  GST90A19-P1M  which may be found at Jameco Electronics and it is Jameco Part Number 2223486 (link) and from Digi-Key as Part Number 1866-2156-ND (link).  This unit is rated for 19 volts at 4.74 amps - much greater than the supply that is likely to have been supplied with the PC and it has the needed 5.5mm O.D./2.5mm I.D. coaxial power connector with center positive.  Other NUCs will have different power requirements and connector types and polarities so it is up to YOU to determine what might work for your computer. As noted, it has good power factor correction (PF of 0.9 or better) and produces very little to no radio frequency interference - unlike some power supplies of "unknown" brands.  As a bonus, it so-happens that this supply works perfectly with my older Asus ROG laptop as well!

For this reason it might be reasonable to have a smaller (and, presumably a "fast") UPS to carry the computer through this transition although it seems a bit silly to have a UPS when one already has one for the entire house - but all it needs to do is to run for a few seconds, so even UPS batteries in poor condition will likely suffice.

In the case of a very small form-factor computer such as a NUC, we could contrive a means of providing power for just long enough for the UPS - whether desk-top or whole-house - to do its job.  It, too, needs only last long enough - perhaps a second or so.  If it's powered via a "brick" power supply this task is a bit easier and it is those devices with external power supplies that this article addresses.

Carrying through the interruption

In the specific case of the "NUC", these are often (but not always) powered via an external DC power supply.  In my case, I have a Beelink NUC using a Ryzen 5700 that is powered from a 19 volt supply.  In communicating with others who own this same mini PC it's clear that it's shipped with a wide variety of different power supplies from different manufacturers - some of them with ratings that seem a bit low for the expected power consumption of the computer - so I replaced it with a good-quality MeanWell unit (see sidebar) which not only has more robust ratings, but its input is power factor corrected - a very important consideration when powering it from a UPS! ^{See comment #1 at the end of this article}

In testing, neither the original or MeanWell power supply had enough reserve capacity to consistently carry it through a UPS transition - particularly if the computer was "busy" and consuming maximum power

The major reason why there is this concern is that this Beelink computer is located at the remote site of the Northern Utah WebSDR where power bumps and outages causing the load to switch to the UPS are very frequent - and occasionally, this causes the computer to "hang" (and not reboot!) requiring that the power outlet be remotely switched off - and back on.  As this is not a "public-facing" computer (it does WSPR monitoring) its outage may not be immediately noticed.  It's worth reiterating that the desktop-type computers usually have no issues with these transitions.

What to do?

Figure 2:
The Tecate SCAP PBLS-3.5/21.6 capacitor module.
This unit contains the necessary voltage equalization circuitry.
Click on the image for a larger version.

I did not want to put a separate mains-powered UPS on this computer and while I could have figured out a battery-based solution, this seemed overkill as I literally needed it to power the computer for less than one second - plus I didn't want to have batteries that would eventually "age out" and need to be replaced.  The obvious solution seemed to be the "supercapacitor" - devices with Farads of capacitance, capable of storing enough energy to power the computer for a very short period of time.

In perusing the DigiKey catalog I found at least two useful candidates:  One capacitor of 1.25 Farads with 540mΩ of internal resistance (Tecate P/N: SCAP,PBLS-1.25/21.6) and another of 3.5 Farads with 260mΩ of internal resistance (Tecate P/N: SCAP,PBLS-3.5/21.6), each rated for 21.6 volts - both suitable for use with a 19 volt supply.  These are actually capacitor modules, consisting of eight 2.7 volt capacitors of 10 and 20 Farads each, respectively, and containing simple circuitry to assure that the voltage across each of the internal capacitors was balanced.  It's worth noting that the voltage equalization circuitry itself will consume a small amount of current (perhaps as high as a few 10s of milliamps) - particularly as one approaches the maximum voltage rating and this must be considered in the design of the support circuitry.

It's important to note that these won't actually function as a UPS in the traditional sense:  These capacitors can store enough energy to power the computer for a short time - only for a few seconds at most - but this is more than enough to carry it through for the few hundred milliseconds of drop-out that might occur during a UPS transition. 

Using the supercapacitors

The problem with using a supercapacitor is that when they are discharged, they look like a dead short, meaning that you probably cannot simply tack them in parallel with a power supply:  To do so would stress the power supply - putting it into current limiting at best, possibly causing it to "trip out" and go offline, or in the worst case, damaging it - so provisions must be made to regulate the charging of the capacitor.  The diagram in Figure 3 shows the circuit surrounding the capacitor.

Figure 3:
Schematic of the supercapacitor NUC UPS.
A standard outboard power supply is used - typically the one supplied with the computer, but it could
be another unit - probably of better quality - as noted in the article.
Click on the image for a larger version.

How it works

For charging, we are using old and "newer old" tech here - R1 is a simple series resistor of 100 ohms with a power rating of 3-5 watts which will limit the current to around 200mA, tapering off gradually as the capacitor charges up.

In parallel with R1 is F2, a 100 milliamp self-resetting thermal fuse (e.g. "Polyfuse").  This device is really a thermistor and when "excess" current flows through it, it heats up and the resistance skyrockets, greatly reducing the current flow.  The way that it is used here means that when the power supply is first connected (and the capacitor is fully-discharged) there's a brief inrush of current until F2 "blows" (gets hot) at which point it takes only 15-20 milliamps to keep it in this state at which point R1 is handling most of the current.  As the capacitor charges and the voltage differential across R1 decreases, the current through the 100 ohm resistor will also drop - but F2 will also gradually cool down as the voltage across it decreases - but the current will also increase - but never more than approximately the 100 mA rating.

Figure 4:
Internals of the UPS.  The support circuit was constructed
on a small piece of prototype board (left) while the LEDs to
indicate the status are on the right.  The rear panel (far left)
has the power cable and coaxial power connector.
Click on the image for a larger version.
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The use of F2, the 100mA fuse, results in much faster charging of the capacitor than with the 100 ohm resistor alone.  In testing with a 3.5 Farad capacitor, it took about an hour for the capacitor's terminal voltage to be within a hundred or so millivolts of the power supply voltage with just R1, the 100 ohm resistor - but it took about 9 minutes with the addition of F2.  As an added bonus, when the capacitor is nearly fully charged (within a volt) the thermal fuse would allow much more current than the 100 ohm resistor would be only about 10mA or so and the charging rate would slow to a crawl - plus the equalizing circuit within the capacitor module draws a few milliamps meaning that it will never get closer than 200-500 millivolts of the power supply voltage.

The reason for this is that with the addition of F2 - and the fact that at this low voltage drop it will have cooled off and have a resistance of between 3 and 10 ohms - the capacitor's resting voltage will be within a few 10s of millivolts of the power supply rather quickly.  This is important as even a few hundred millivolts of extra charge on the capacitor will measurably extend the "run" time.  Attaining this sort of "full charge" could be done with a solid state circuit using FETs and op amps,  but it would be fairly complex:  This approach - with a single, inexpensive component - is nice and simple.  The use of F2 also overcomes the small current consumption of the capacitor module's equalization circuitry:  A few milliamps of current from this circuitry would drop the full-charge voltage by as much as a few hundred millivolts without F2.

A maximum charge current of 200-300mA seems reasonable as that would not put a significant amount of burden on the power supply - which must be able to power the computer and charge the capacitor at the same time.  I also considered the use of a simple transistor-type current limiter which would maintain a constant current until the capacitor got to within a volt or so of the supply voltage, but decided that it probably wasn't worth the added complexity - and I would still have required something like F2 to bring the capacitor right up to the supply voltage.

The "Charge" LED works by detecting the voltage crop across R1:  If it exceeds approximately 0.6 volts, Q1, a PNP transistor, is turned on, pulling its collector high, turning on LED2.  When this LED goes out, the capacitor will be within 0.5-0.6 volts of full charge.  The "Ready" LED (LED2) is in series with D2, a 15 volt Zener diode and it will start illuminating when the voltage across the capacitor exceeds about 17 volts for an old-tech AlGaInP LED (with a 2.1 volt threshold) or about 18 volts for a more modern GaN LED.  In a "standby" state, the "Charge" LED will have extinguished and the "Ready" LED will be on indicating the unit's readiness.  Neither of these circuits are perfect, but they give a "good enough" indication of the state of the device and let the user know that things are working.  A little bit of extra circuitry could dispense with the Zener-LED circuit and simply turn on the "Ready" LED when the red "Charge" LED goes off - effectively indicating the same thing.

Figure 5:
The completed UPS with the two LED indicators on
on the front panel.
Click on the image for a larger version.

An "ideal" diode - in real life

Parallel with R1 is a diode (D1) that is reverse-biased when the capacitor's voltage is lower than the supply voltage, preventing current flow other than through the resistor.  While I originally considered using an "ordinary" diode - which would have a voltage drop of about 0.6 volts for a standard silicon or around 0.4 volts for a high-current Shottky type - I decided to do something different:  Use an "ideal diode".

A voltage drop of 0.3-0.6 volts from a typical diode would represent an immediate voltage drop from the capacitor - and since the voltage on the capacitor will drop as it's discharged, the "diode drop" would represent less time that the computer could be powered by it, alone.  A hypothetical "ideal" diode would have zero voltage drop in the forward direction and block current in the reverse - and fortunately, something pretty close to that actually exists these days!

As it turns out, such a thing actually exists - and it is pretty inexpensive.  This implementation of an "ideal" diode is really a module with several components:  The specific modules that I used (which I got from Amazon - five for US$10) use the Diodes Incorporated DZDH0401DW chip along with an AGM30P05A P-channel FET along with a 100k and 1 Megohm resistor.  These "diodes" are rated for a maximum stand-off voltage of 26 volts and a steady-state current of 10 amps, but could probably handle 15 or even 20 amps for brief periods.

The way that these work is that the DCDH0401DW has a comparator that is used to detect the minute voltage drop between the "input" of the "diode" (on the "+" side of the FET, actually) and the "output" (the "-" side):  If the voltage on the input is higher than the output, the P-channel FET is turned on, allowing it to conduct from the input to the output.  If the voltage on the input is NOT higher than the output, the FET is turned off, preventing current from flowing from the output to the input.  The use of a P-channel FET allows the switch to be placed in the positive lead which permits the negative side of the power sources - the power supply and the super capacitor - to be connected together.  Incidentally, the FET is wired such that even if it weren't "on" at the moment that it might need to conduct, it's intrinsic diode would conduct, anyway, albeit with a 0.6 volt drop, but since the DZDH0401DW chip responds within a few microseconds at most, the FET would be very quickly turned on.

Figure 6:
The back panel of the supercap UPS.
The original power supply plugs into the jack while the
short cable needs to be just long enough to get to the
back panel of the PC.
Click on the image for a larger version.

When the FET is on, its resistance is on the order of 5.5 milliOhms which means that if there's three amps flowing through it, less than 20 millivolts will be lost - about 1/30th of that of a standard silicon diode - and since there is so little voltage lost, there will be a similar fraction of heat being produced as well.

As you may have noticed in the schematic diagram of Figure 3, there are actually three connections to this "diode":  The anode, the cathode and ground - the ground being required because not only does the comparator/control chip need power, but the gate of the P-channel FET needs to be pulled negative with respect to its source.  The "overhead" current of the FET and comparator/control chip is only on the order of 175 microamps according to the data sheets so it's power consumption is practically negligible in our application.

The other components in the circuit include D2 - a 15 volt Zener diode along with LED1 and R2 for current limiting:  This LED will illuminate if the applied voltage exceeds about 17 volts and functions as a "Power" indicator.  Transistor Q1, a PNP, is connected across R1 via current-limiting resistor R4 and when the voltage drop across R1 exceeds about 0.6 volts, its collector will be pulled toward V+, causing LED2 to illuminate, indicating that the capacitor is charging.  When this LED goes out, this indicates that the capacitor is - at the very least - "mostly" charged.

The final component is F1 - a self-resetting thermal fuse (e.g. "polyfuse") which could have a rating of anything between 5 and 9 amps.  As the capacitor can deliver a large amount of current when shorted, this is provided as protection.  A "normal" fuse of 6-10 amps would suffice here, but I happened to have the polyfuse on hand.

Variations on a theme:  Backing up a 12 volt PC.

As noted, this unit was built using the 3.5 Farad capacitor - but it should be capable of doing its job with the lower-cost (and physically smaller) 1.25 Farad unit.

The described unit is also designed to be used with a NUC/PC that operates at 19 volts - a common voltage used by laptop computers.  Many of these small computers use 12 volts - and while one could possibly tack a small battery across the power supply, the use of a capacitor-based backup would mean that there would be no battery that would have to be checked/replaced on a routine basis.

The circuit depicted in Figure 3 - designed for 19 volts - would have to be modified slightly, as follows:

• D2, a 15 volt Zener, would be changed to a 9 volt device for a 12 volt bus.  This would better-represent the charge state of the capacitor for a 12 volt supply, causing it to illuminate once it had charged to better than about 11 volts.  The "Ready" LED would illuminate at voltages above that of the 9 volt Zener plus the LED's forward voltage.
  
• R1, a 100 ohm resistor for the 19 volt device would be changed to somewhere between 47 and 62 ohms but still a 5 watt device.
  
• The capacitor described is rated for 21.5 volts - which is probably overkill for a 12 volt power supply.  A 16 volt capacitor would be a better choice.  Additionally, a lower-voltage capacitor module will have commensurately lower internal resistance which improves efficiency - and for a 12 volt power supply where voltage droop due to Ohmic losses is arguably more important, it would be best to keep it below 400mΩ.   Possible capacitors for 12 volt use include:
• Tecate 2.0F, 13.5 volt, 360mΩ:  SCAP,PBLS-2.0/13.5
• Cornell-Dublier 2.5F, 18V, 334mΩ:  DSM255Q018W075PB 
• Tecate 5.6F, 13.5 volt, 185mΩ: SCAP,PBLS-5.6/13.5 
• It's worth mentioning that while a "12 volt" computer may operate from a supply voltage that is nominally 12 volts, it's worth checking to make sure that it's within the safe operating range of the capacitors that you choose.  For example, the Tecate capacitors listed above (for the "12 volt" system) can operate safely only up to 13.5 volts, ruling out the use of a power supply that operates in that range - but the Cornell-Dublier capacitor with its 18 volt rating would work nicely over a slightly wider range.

Conclusion

Figure 7:
The supercap UPS, on the shelf next to the PC -
now in service at the Northern Utah WebSDR!
Click on the image for a larger version.

As can be seen from the photos, the capacitor and support circuitry was placed into a plastic enclosure:  The two LEDs were placed on the front panel and labeled while the back panel has a female coaxial power connector that matches that of the computer and power supply along with a short cord terminated with the same type of male power connector used by the PC - which happens to be the common "5.5mm x 2.5mm" type with the outside shell being negative.

To install the UPS, the PC was powered down and the device inserted into the power lead - the power supply plugging into the UPS and the short cable plugging into the PC.  After a bit less than 10 minutes, the "Ready" LED illuminated - followed soon after by the "Charge" light extinguishing - but since the charger is current-limited, the PC could be powered up immediately after installation - not needing to wait for it to fully charge.  Of course, any testing of the device to determine its ability to "ride through" an interruption should wait until the capacitor has fully-charged.

As can seen in Figure 7, the UPS was placed on the shelf next to the PC that it supports.  With the PC under a "moderate" load (about half of the maximum power consumption) the power supply was unplugged briefly to see if it would hold.  Interruptions of up to 1.5 seconds were tried with no disruptions of the PC with the capacitor being fully "recharged" to just a few 10's of millivolts of the maximum voltage in under two minutes due to the "shallow" discharge.  We chose not to try to see how long it really would hold the PC up, but with the UPS installed, we cycled the UPS several times and the PC happily rode through it - something that it would not do without.

In other words, success!

 * * * * * * *

Comment #1:

In this article, I mention that having a power factor corrected power supply is particularly important when running from a UPS.  If your UPS is running power supplies without power factor correction, it may well be that it will trip out due to overload at around half of its wattage rating:  The real clue is to closely look at your UPS's specifications and note that it has a "volt-amp" rating (which is more of a true indication of its capability) that is much lower than its "wattage".  What's worse is the "spiky" nature of the input current of a non power-factor corrected power supply which may put even more stress on a UPS - or even the circuit breaker/wiring - than the numbers would indicate. 

For example, using a "Kill-A-Watt" - a relatively inexpensive power analysis device - we measured the power factor of the originally-supplied power supply and found it to be 0.45.  What this means is that if computer were pulling 65 watts and the power supply were 85% efficient - which implies that it's needing about ( 65 / 0.85 = ) 76 "watts" - it would actually need to pull (76 / 0.45 = ) 170 volt-amps from the mains.  As the mains supply - which may be a UPS - must actually supply volt-amps, it must be capable of supplying that "170" value - which is more than 2.6 times the power that the computer is actually consuming.  The Mean Well power supply that we chose has a measured power factor of 0.94, so at an efficiency of 85%, it would be pulling only 80 volt-amps from the UPS - less than half of the load!

For more information about this, see the Wikipedia article about Power Factor (link) - and pay special attention to the section about "Non Linear Loads" which are what a typical, non power-factor corrected switching supply presents to the mains.  In these cases, the peak amperage can be several times higher than the average - and all power circuits must be able to supply these high peaks regardless of the average power, which is why a UPS, generator or even mains supply circuit must to be de-rated to accommodate devices with a poor power factor.

In other words:  If you don't use power-factor corrected power supplies on your UPS or generator, you won't be able to safely and reliably supply anywhere near its "wattage" rating - but if you do use only devices with good power factor, you will be able get much closer to its ratings without overloading it.

* * * * * * *

This page stolen from ka7oei.blogspot.com

[END]

 

A high-current DC (and AC) noise filter for UPS or RV use

A friend of mine (Glen, WA7X) acquired a 16 kVA UPS (for free!) a year or so ago - a commercial system consisting of four hot-swappable 4 kVA modules:  With his current load, he only uses one of the four modules, the rest being available as spares or providing room to grow.  Using this as a battery back-up system for important devices in his house (computers, etc.) it's active all of the time as it is an "online" UPS - that is, the inverter pulls power from the battery bank, but the battery bank is always being charged.

Figure 1:
Whiteboard diagram of the dual AC mains filter for the
UPS - See text for details
Click on the image for a larger version.

AC-side filtering:

When he first installed the UPS, he discovered that being a commercial device, it was only a "Class A - commercial" device under FCC part 15 - and it trashed the 20 meter amateur band and caused interference on a few others.  This, however, was easy to remedy as he'd asked me for advice and built a larger version of UPS noise filters that we'd implemented together in the past:  See the article "Containing RF Noise from a Sine Wave UPS" - link.  

Being capable of many kVA, the filtering for this UPS had to be built from scratch rather than using (expensive!) commercially-available filter modules, but this was easily done using readily-available ferrite toroids and bypass capacitors.

Figure 1 shows the general diagram, crudely sketched on a white board in his shop after our consultation.  The inductors are 12-14 turns of 6 AWG on FT240-31 cores, each half (phase) being an equal number of turns for best common-mode suppression as depicted in Figure 2.  Because the UPS outputs 240 volts, the 50+ amp capability of unbundled 6 AWG wire is sufficient for the envisioned load on this UPS.

Figure 2:
The inside of the dual mains filter, built
into a standard NEMA box.  The capacitors
- mostly obscured - are connected to the
blocks with the ground side bonded to the
case.
Click on the image for a larger version.

The filter uses suitably-rated parallel 0.01uF and 4700pF capacitors:  Those across the AC leads (which could have been as large as 0.1uF or so) help force the RF energy to be common-mode across the bifilar choke while the capacitors to ground on the "outside" (non-UPS) side of the filter shunt the remaining RF - which is already at higher impedance due to the choke - to the common-point ground.  Shown in red on the drawing in Figure 1 are large 43 Mix slip-on beads on the "UPS" side of the filtering to better-suppress the high frequency (VHF) components:  Ideally, one would run both conductors through each bead for net zero flux on the core, but larger diameter devices were not available at the time of construction.

The filter pictured in figures 1 and 2 completely solved the RFI problem:  One has to get within a few inches of the UPS cabinet to hear magnetically-coupled RF energy with a portable shortwave radio.

DC-side filtering:

It wasn't a huge surprise, then, when he added more battery capacity external to the UPS - 120 volts DC - and the racket on 20 meters and other bands reappeared.  Because RF is RF, the filtering method for the DC leads is exactly the same as required for the AC leads:  Common-mode choking, bypass capacitance and single-point grounding techniques. 

Considering that the UPS is capable of up to 16 kVA, the DC filter needed to be able to handle more than 100 amps at the 120 volt (nominal - about 138 volts, actual) input.  Looking about, he found a pre-made set of 6 foot long, 2 AWG, very flexible "inverter cables" at Harbor Freight (cost:  $35) that were conveniently available - easily capable of handling about 100 amps - more than enough because he was not ever expecting to load the UPS to its capacity.

Because of the size of the wire, standard FT-240 (2.4 inch/61mm O.D.) cores aren't appropriate, so Glen obtained some "Monster" size toroids (Mix 31) from KF7P.com:  These cores are about 4" (102mm) in diameter and it was possible to wind 7 bifilar turns of the 2 AWG wire onto them, yielding about 170 uH - more than enough inductance to provide adequate choking on the HF bands.

Because they were on-hand, the same capacitors were used:  0.01uF and 4700pF capacitors in parallel:   With a DC system, much larger-value capacitors (e.g. 0.1-10uF) of appropriate voltage could have also been used if lower-frequency attenuation were required.  Like the AC choke, large slip-on ferrite beads (31 mix in this case) were slipped over each of the 2 AWG wires on the "UPS" side to help suppress the higher-frequency energy.  Because of the current involved, 200 amp screw terminal strips were procured - both to terminate the connections to the wire comprising the inductances, but also provide connections to the "outside" world.

Figure 3:
The completed DC noise filter.  The bifilar-wound choke on the "monster" 31-mix core is wound with 2 AWG welding/inverter cable:  The slip-on ferrites on the "UPS" side of the DC are clearly visible.
Mostly obscured are the bypass capacitors, connected to the screw-type terminals.
Click on the image for a larger version.

There are a few caveats to making a filter like this work:

• The "ground" lead must be as near zero length as possible.  This box was bolted directly to the box containing the AC input/output filter described above  (which, in turn, is bolted to the UPS cabinet) to establish a single point ground where the RFI on the AC in/out leads and the DC leads come together:  Connecting the two boxes with just a few inches of wire caused noticeable degradation in its performance!
  

• The cables connected to the UPS must be considered to be "dirty", carrying a lot of RFI, and must be kept as short as possible.  Additionally, one must keep other wires away from these "noisy" leads to prevent interference from being re-coupled into them!
  

• The external battery bank itself has its own fuse, at the battery bank:  Do not even think of connecting a high-current power source like this without some sort of short-circuit protection!

As with the AC filter, this one appears to be completely effective with no conducted noise being detected on the leads of the external battery connection.

Where might these techniques be applied?

The filters shown above are simply "scaled up" versions of those described previously on this blog (links below) to handle higher voltage and current.  A few instances where these techniques might be useful include:

• Adding higher battery capacity to an existing UPS.  You may own a UPS that will power your gear, but simply has too little battery capacity for the desired run time - and adding external battery capacity safely (e.g. fused, insulated) is one way to do this.  As in this case, adding more capacity caused radiation of RFI which had to be suppressed.
  

• Suppress noise from an existing UPS.  Many modern UPSs are likely to create RFI - and these pages show how that might be mitigated.
  

• Suppress noise from an RV power system.  Many RV (recreational vehicles) have power converters (AC to DC for charging batteries) and inverters (DC to AC for running mains-voltage devices) that are likely to generate RFI.  The techniques described on these pages show how it is practical to prevent the conduction/radiation of RFI on both AC and DC leads.

Thanks to Glen, WA7X, for supplying the pictures:  I just scribbled down diagrams and notes and gave him a few capacitors - he's the one that actually built the thing!

Related links:

Links to other articles about power supply noise reduction found at ka7oei.blogspot.com:

• Containing RF noise from a "pure" sine wave UPS.   Even when it is not operating your sine wave UPS may be producing a lot of HF radio interference!
  

• Completely containing switching power supply RFI - link.  Sometimes it can be difficult to quiet a switching power supply, so it may be necessary to put it in a box with strong filtering on all of the conductors that enter/leave.
  

• Minimizing VHF (and HF) RFI from electronic ballasts and fluorescent tubes - link.  Electronic light ballasts, like many switching power supplies, operate in the LF frequency range so "cleaning them up" at VLF/LF/MF frequencies can be a challenge.
  

• Quieting high current switching power supplies used in the shack - link.  This page describes techniques that can be used to reduce the amount of RF energy produced by switching power supplies that you may be using to power your radios.  Again, higher-inductance chokes may be required at VLF/LF/MF frequencies.
  

• Reducing switching supply racket - link.  This describes techniques that can be used to beef up the filtering for switching supplies in general.
  

• Quieting a 150 Watt Samlex Sine Wave inverter - link.  This talks about using familiar methods - if you've been reading the other links - to suppress RF noise on a Sine Wave inverter.
  

The large, ferrite toroids and beads used on this project were obtained from KF7P.com - link.

* * *

This page stolen from ka7oei.blogspot.com

[End]

An automatic transfer relay for UPS/Critical loads, for the ham shack, generator backup, and home

It is quite common to use a UPS (Uninterruptible Power Supply) to keep critical loads - typically computers or NAS (Network Attached Storage) devices - online when there is a power failure as even a brief power failure can be inconvenient.  Like any device, a UPS occasionally needs to be maintained - especially the occasional replacement of batteries - and doing so often necessitates that everything be shut down.

A simple transfer relay can make such work easier, allowing one to switch from the UPS to another load - typically unprotected mains, or even another UPS - without "dumping" the load or needing to shut down.

This type of device is also useful when one is using a generator to provide power:  Rather than dumping the load when refueling the generator, another generator could be connected to the "other" port, the load transferred to it, and the original generator be shut down and safely refueled - such as during amateur radio Field Day operations.

Figure 1:
Exterior view of the  "simple" transfer relay depicted in Figure 2, below.
The "Main" power source is shown as "A" on the diagram.
Click on the image for a larger version.

But first, a few weasel words:

• The project(s) described below involve dangerous mains voltages which can be hazardous/fatal if handled improperly:  Please treat them with respect and caution.

• Do NOT attempt a project like this unless you have the knowledge and experience to do so.

• While this information is provided in good faith, please do your own research to make sure that it suited to your needs in terms of applicability and safety.

• Do not presume that this circuit or its implementation is compliant with your local electrical codes/regulations - that is something that  you should do. 

• There are no warranties expressed or implied regarding these designs:  It is up to YOU to determine the safety and suitability of the information below for your applications:  I cannot/will not take any responsibility for your actions or their results. 
  

• With a standard DPDT relay, it's possible that one set of contacts can "weld" (get stuck) causing half of it to switch to one source.  The hazard of this is that this could "back-feed" power to the grid if the power is off not to mention possibly damaging equipment - the load equipment due to improper voltages and/or the UPS itself.  There are relays with mechanical interlocking contacts which make it virtually impossible for this to happen that might be considered.
  

• It is recommended that this type of device not be plugged in permanently if connected to the mains grid:  If used on the grid, the "B" side would be best connected only if you are transferring from "A" to "B", minimizing risk.
  

• You have been warned!

The simplest transfer relay:

The simplest version of this is a DPDT relay, the relay's coil being powered from the primary power source - which we will call "A" - as depicted in the drawing below:

Figure 2:
The simplest version(s) of load transfer relays - the load transferred to "A" ("Main") upon its presence, switching to "B" (Aux) in its absence.
The version on the left uses a relay with a mains-voltage coil while that on the right uses a low-voltage transformer and relay coil - otherwise they are functionally identical.
Click on the image for a larger version.

How it works:

Operation is very simple:  When the primary power source "A" is energized, the relay will pull in, connecting the load to source "A".  Conversely, when power source "A is lost, the relay will de-energize and the load will be transferred to the back-up power source, "B".  In every case that was tried, the relay armature moved fast enough to keep the load "happy" despite the very brief "blink" as the load was transferred from one source to another.

Two versions of this circuit are depicted:  The one on the left uses a relay with a mains-voltage coil while the one on the right uses a low-voltage coil - typically 24 VAC.  These circuits are functionally identical, but because low-voltage coil relays are common - as are 24 volt signal transformers - it may be easier to source the components for the latter.

Figure 3:
The interior of the "simple" transfer relay.  Tucked behind the outlet is the
DPDT relay with the 120 volt coil, the connections made to the relay.
using spade female spade lugs. The frame of a discarded light switch
is used as a mounting point for a standard "outlet + switch" cover plat
with neon panel lights being mounted in the slot for a light switch.
The entire unit is housed in a plastic dual-gang "old work" box.
Click on the image for a larger version.

The actual transfer takes only a few 10s of milliseconds:  I have not found a power supply that wasn't able to "ride through" such a brief outage but if a UPS is the load, it will probably see the transfer as a "bump" and briefly operate from battery.

Why a DPDT relay?

One may ask why use a DPDT (Double-Pole, Double-Throw)  relay if there is a common neutral:  Could you not simply switch the "hot" side from one voltage source to another?

The reasons for completely isolating the two sources with a double-pole relay is multi-fold:

• This unit is typically constructed with two power cords - one for each power source.  While it is unlikely, it is possible that one or more outlets may be wired incorrectly, putting the "hot" side on the neutral prong.  Having a common "neutral" by skimping on the relay would connect a hot directly to a neutral or, worse, two "hot" sides of different phases together.

• It may be that you are using different electrical circuits for the "A" and "B" power in which case bonding the neutrals together may result in circulating currents - particularly if these circuits are from disparate locations (e.g. long cord.

• For readers outside North America:  While typical outlets are 120 volts, almost every location with power has 240 volts available which is used to run larger appliances.  This is made available via a split phase arrangement from a center tap on the distribution transformer which yields 120 volts with respect to the neutral.  It is because of this that different circuits will be on different phases meaning that the voltage between two "hot" terminals on outlets in different locations may be 240 (or possibly 208) volts.

• There is no guarantee that a UPS will "play nice" if its neutral output is connected somewhere else.  In some UPSs or inverters the "neutral" side may not actually be near ground potential - as a neutral is supposed to be - so it's best to let it "do its thing."

How it might be used:

With such a device in place, one simply needs to make sure that source "B" is connected, and when  load "A" - typically the UPS, but it could be a generator -  is disconnected, everything will get switched over, allowing you to performs the needed maintenance.

UPS maintenance:

When used with a UPS, I have typically plugged "A" (Main) into the UPS and "B" (Aux) into a non-UPS outlet.  If you need to service the UPS, simply unplug "A" and the load will be transferred instantly to "B".  Having "B" as a non-UPS source is usually acceptable as it is unlikely that a power failure will occur while on that input - but if you choose not to take that risk, another UPS (or a generator) could be connected to the "B" port.

I have typically kept input "B" (Aux) plugged into non-protected (non-UPS) power as a failure of a UPS would not likely interrupt the power to the backed-up device(s) - but if you do this you must keep an eye on everything as unless it is monitored, the failure of a UPS may go unnoticed until there is a power failure! 

This same device has also been used in a remote site with two UPSs for redundancy, not to mention ease of maintenance.  One must, of course, weigh the risk of adding yet another device (another possible point of failure, perhaps?) if one does this.

Generator change-over:

During in-the-field events like Amateur Radio Field Day such a switch is handy when a generator is used.  It is generally not advisable to refuel a generator while it is running even though I have seen others do it.  If, while gear is running on a generator, it is necessary to refuel it - another generator can be connected to input "B" and once it is up to speed (and switched out of "Eco" mode if using an inverter generator) input "A" is un-plugged  for refueling, checking the oil, etc.

If you are of the "OCD" type, two generators can be used:  The generator on "A" would be running the gear most of the time, but if it drops out, a generator on "B" - which will have been under no load up to that point - will take over.

Disadvantages of this "simple" version of the transfer relay:

For typical applications, the above arrangement works pretty well - particularly if power outages and maintenance needs are pretty infrequent - and it works very well in the "generator scenario" where one might wish to seamlessly transfer loads from one generator to another.

It does have a major weak point in its design - and that's related to how the relay pulls in or releases.

For example, many UPSs or generators - especially the "inverter" types - do not turn instantly "on", but rather they may ramp up the voltage comparatively slowly, but by its nature the relay coil may pull in at a much lower voltage than nominal - say, 80 volts.  When a load is transferred at this lower voltage, it may momentarily cause the power source to buckle, causing the load to be dropped and/or the relay to chatter briefly or, possibly simply cause the load to drop owing to too-low battery voltage.  The typical "work around" for this is to allow the "A" source to come up fully before plugging back into it - which is fine in many applications.

A "slow" pull-in on a relay can also be hard on relay contacts - particularly a "slow" rise the voltage from power source "A" - in which the contacts may not close quickly enough to prevent extensive wear.  In severe conditions, this can even result in one or more of the contacts welding (sticking together) which is not at all a desirable condition.  For this reason it is a good idea to use a relay with a significantly higher current rating than you are planning to pull.

A slightly more complicated version:

What can help this situation would be the addition of a short delay, after power source "A" is applied  but before the load is transferred to it - and better yet, we would like this load to be transferred only if its voltage is above a minimum value:  The circuit in the diagram below does this.

Figure 4:
This version of the transfer relay offers a short delay in transferring to load "A" as well as providing a low-voltage lock-out/detect.
The relay is a Dayton 5X847N - a 40 amp (resistive load) DPDT contactor with a 120 VAC coil.  This relay is likely overkill, but it should handle about anything one can throw at it - including capacitor-input power power supplies that tend to be very hard on relay contacts due to inrush current.  Not shown on the diagram above:  It's recommended that a "snubber" circuit consisting of a 100 ohm resistor and 0.1 uF capacitor - of appropriate voltage rating - be connected across the contacts of RLY2 to suppress back-EMF that might damage its contacts when this relay opens.
Click on the image for a larger version.

How it works:

This circuit is based on the venerable TL431 - a "programmable" (via resistors) Zener diode/voltage reference - U1 in the above diagram.  A sample of the mains voltage is input via T1 which, in this case, provides 9-12 volts AC which is then half-wave rectified by D1 and then smoothed with capacitor C1.  LED D2 was included on the board mostly for testing and initial adjustment - but it also establishes a 8-12 milliamp static load to help discharge C1 when the mains voltage goes low - although the current consumption of the relay does this quite well.

Figure 5:
An exterior view of the version of the transfer relay depicted in Figure 4,
above.  The unit is mounted in a 6x6x4" electrical "J" box.
The 10 amp rating is a bit arbitrary and conservative considering that
the contactor itself is rated for 40 amps and the fact that capacitor-input
supplies are likely to be connected to it.
Click on the image for a larger version.

The DC voltage is divided down via R2 and R3 and this is further filtered with capacitor C2, with R3 being adjustable to provide a variable threshold voltage to U1.  The combination of R2 and C2 causes the voltage at their junction to rise comparatively slowly, taking a couple seconds to stabilize.

When power is first applied, C2 is at zero volts, and will take a couple seconds to charge.  When the wiper of R3 exceeds 2.5 volts, U1 will suddenly turn on (conduct), pulling the "low" side of the coil of relay RLY2 to ground, turning it on which, in turn, will apply current to the coil of RLY1.  When it does, the base of transistor Q1 is pulled toward ground via R6, turning it on and when current passes through R4 into the junction of R2 and R3, the voltage will rise slightly, resulting in some hysteresis.  For example, if R3 is adjusted so that RLY2 will be activated at 105 volts, once activated the voltage threshold for U1 will be effectively lowered to about 90 volts.

If power source "A" disappears abruptly, RLY1 will, of course, lose power to its coil and open immediately - and a similar thing will happen if the voltage goes below approximately 90 volts when RLY2 will open, disconnecting power to RLY1 - and at this point Q1 will be turned off and it will require at least 105 volts (as in our example) for RLY1 to be activated again.  Diode D4 may be considered optional as it will more-quickly discharge C2 in the even the power on "A" goes away and suddenly comes back, but it is unlikely that its presence will usefully speed response.

As noted in the caption of Figure 4, the relay used is a Dayton 5X847N which has a 120 volt coil and 40 amp (resistive load), self-wiping contacts.  While 40 amps may seem overkill for a device with an ostensible 10 amp rating as depicted in Figure 5, it is good to over-size the relay a bit, particularly since many loads these days (computer equipment, in particular) can have very high inrush currents due to capacitor-input rectifier, so a large relay is justified.

Note:  The 5X848 is the same device, but with a 240 volt AC coil while the 5X846 has a 24 volt AC coil:  All of three of these devices are suitable for both 50 and 60 Hz operation.

Circuit comments:

Figure 5:
Inside the transfer relay unit.  The large, open-frame DPDT relay is in the
foreground while the 12 volt AC transformer is tucked behind it.  Mounted
to the wall behind it (upper-left in the box) is the piece of prototype
board with the smaller relay and delay/voltage sense circuitry.
Click on the image for a larger version.

U1, the TL431, is rated to switch up to 200 milliamps, but it's probably a good idea to select a relay that will draw 125 milliamps or less.  Because the contacts of the relay are simply switching power to the main relay (RLY1), RLY2 need only be a light-duty relay.

When I built this circuit I used a 5 amp relay with a 9 volt coil because I had a bunch of them in my junk box and in checking it out, I found the coil resistance to be 150 ohms meaning that at its rated voltage, it would draw 60 milliamps.  The voltage across C1 when RLY1 was not active was measured at about 16 volts so it was presumed that with the load of the relay that this would drop by a volt or two meaning that a series resistor that would pass 60 milliamps across 6 volts (the difference between the 15 volt supply and 9 volt coil voltage) should be used - and Ohms law tells us that a 100 ohm, 0.5-1 watt resistor would do the job.

Adjustment:

A variable AC supply (e.g. a "Variac") is essential for proper adjustment.  To start, the wiper of R3 is adjusted all of the way to the "ground" and then the applied AC voltage is set to 105 volts - a nice, minimum value for U.S. power mains.  Then, R3 is adjusted, bringing the voltage on its wiper upwards until RLY2 and RLY1 just close.  At this point one can lower the input voltage down to 80-90 volts and after capacitor C2 discharges, the relays will again open and one can then move the voltage back up, slowly, and verify the pull-in voltage.

Figure 6:
The back side of the front panel of the J box:  A large, square hole was cut
in the front and an plastic dual gang "old work" box with its back
cut away was used to facilitate mounting of the two outlets  to the front panel.
Adhesive was used around the periphery to prevent the box from sliding
around on the front panel.
Click on the image for a larger version.

If less hysteresis is desired, the value of R4 can be increased to, say, 22k.  Note that despite the operation of Q1, some of the hysteresis is cancelled out by the voltage across C1 decreasing under load when the circuit is triggered, by the current through RLY1, so a bit of hysteresis is absolutely necessary or else the relays will chatter!

Construction:

As can be seen in figures 5 and 6, a 6x6x4 inch gray plastic electrical "J" box was used to house the entire unit - a common item found in U.S. home improvement stores.  A pair of "duplex" outlets were mounted in the front cover by cutting a large square hole in it and using a modified "old work" box with its back removed, giving a proper means of mounting the outlets.

A pair of front panel neon indicators indicate the current state:  The "B" indicator simply indicates the presence of mains voltage on that input while the "A" indicator is wired across the relay's mains-voltage coil and is thus indicative of the delay in the relay's closure.

The circuitry with the TL431 and RLY2 is constructed on a small piece of prototype board, mounted to the side of the box using stand-offs.  The 9-12 volt AC transformer - the smallest that I could find in my junk box (it's probably rated for 200 milliamps) is also bolted to the side of the box.  Liberal use of "zip" ties are used to tame the internal wiring with special care being taken to absolutely avoid any wire from touching the armature of the relay itself to prevent any interference with its mechanical operation!

Final comments:

Both versions work well and the "simple" version depicted in figures 1 and 2 is suitable for most applications.  For more demanding applications - particularly those where a transfer may occur frequently and/or the mains voltage may rise "slowly", the more complicated version is recommended.

Again, if you choose to construct any of these devices, please take care in doing so, being aware of the hazards of mains voltages.  As mentioned in the "Weasel Words" section, please make sure that this sort of device is appropriate to your situation.

This page stolen from ka7oei.blogspot.com

[End]
 

Shunt regulation of series-connected lead-acid batteries to equalize the voltage

There are many instances where series lead-acid batteries are connected - including:

• 12 volt systems using two 6 volt "golf cart" batteries in series
• 24 or 48 volts in a UPS (Uninterruptible Power Supply) consisting of several 6 or 12 volt batteries
• A UPS system that requires much more than 48 volts - more on this, later.

One issue that arises with any system in which cells/batteries are series-connected is that the voltages across them will ultimately be unequal.  For example, in a hypothetical 24 volt system where the nominal float voltage would be about 27.1 volts it is likely that the two "12 volt" batteries will be slightly different.  If the voltages are "pretty close" (within 0.1 volt or so) there is probably on reason to be worried - but as batteries age, they will inevitably drift apart:  Before you know it you might end up with one battery at, say, 14.2 volts where it will be evolving gas (or if it is a "sealed" battery like an AGM, will start to lose electrolyte) and the other battery will then be around 12.9 volts where it will be chronically under-charged, leading to loss of system capacity.

In either case - if the battery is exposed to a consistently high or low voltage - the life of the battery will be reduced - possibly dramatically!

Figure 1:
A small pile consisting of four shunt regulators.
For convenience, these are outfitted with spring-loaded
alligator clips which allows easy installation and removal -
and eliminates the need to sandwich a ring lug on the
battery terminal and potentially increase the total series
resistance of the bank by doing so.
Click on the image for a larger version.

While there are certainly other considerations, keeping the series-connected batterys' voltages in check will certainly go a ways toward maximizing system longevity.

First, a couple words of warning:

• Batteries are high-current devices:  Shorting can cause injury/fire, so be very careful!  Remove metal jewelry - particularly any rings, or at least cover them with tape.
• Some types of batteries - such as "flooded-cell" lead acid - can weep a bit of sulfuric acid which can cause burns on skin and damage to clothing.
• Some UPS systems are not isolated from the mains power and pose an electrical shock hazard:  Always assume this to be the case and take precautions (e.g. power down/disconnect.)
• Even moderate/low voltages can cause electrical shock.
• While this information is presented here in good faith, it is up to you to do research about its validity before implement it, taking responsibility for doing so.
• It is up to you to research and implement safe procedures when working on this - or any - electrical gear and you are solely responsible for any damage/injury that may result.

"How do you know this?"

A quick check of manufacturers' specifications and recommendations will reveal that exposing batteries to either too-high or too-low voltage will compromise longevity, so it would seem to be "pound foolish" to ignore it happening.

As a "case study" I maintained, for several decades, several multi-battery UPS systems - the largest of which was a 300kVa system considering of two banks of forty 12 volt batteries in series that were wired in parallel (e.g. 542 volts nominal) - and with a total of eighty batteries, things are going to drift around!

Having an available BMS (Battery Monitoring System) I could easily track the voltages on the batteries - and inevitably, they started to drift apart as the batteries aged, were exposed to slightly different ambient temperatures (e.g. the ones higher up will be a degree or so warmer the the lower ones) and manufacturing variances.  Knowing full-well the implications of batteries that were drifting apart, I soon devised a simple shunt regulator, described on this page:

• Not all cells are created equal!  (Shunt regulation of batteries...) - Link

These "shunt" regulators are very simple:  Just several diodes (including a Zener) plus an LED that allows a selectable amount of "leakage" current so that "better" batteries (e.g. those with lower self-discharge/leakage current) aren't exposed to elevated voltages by the current of the "other" batteries.  By "tweaking" things one can balance the system so that all batteries will obtain very close to equal voltages.

Despite the fact that only the current through a few lowly LEDs (20-30 milliamps, maximum) is microscopic compared to the currents seen when the UPS was "on battery" and fully-loaded (several hundred amps) this system worked well for several years - but it required frequent adjustment as the batteries aged and their own internal leakage currents started to change:  The battery monitoring system's "live" voltage readout was an invaluable aid to allow tweaking - but this task eventually got "old".

A better shunt regulator:

The "LED+Zener+diode" arrangement has a sharper voltage-current "knee" than a simple resistor, but it was not quite "sharp enough" so I decided to upgrade to a circuit that would respond much more forcefully with increased voltage - and I chose to use the venerable TL431 "programmable Zener".  This chip is ubiquitous, appearing in almost every PC power supply ever made:  It has a temperature-stable onboard voltage reference, and it can handle up to 100 milliamps - several times the current of the original circuit.

This circuit is represented in Figure 2, below:

Figure 2:
Schematic and mechanical layout of the shunt regulators depicted in Figures 1 and 3.
Click on the image for a larger version.

This circuit is very simple in its operation:

• U1, the TL431 will turn "on" if its reference voltage exceeds 2.5 volts, drawing current.
• R4, the potentiometer, is adjusted so that the "reference" terminal is 2.5 volts at the desired shunt voltage.  A 10 turn potentiometer is strongly recommended as the setting of the precise voltage will be both "fiddly" and easily disturbed if a single-turn pot is used.
• When the battery voltage is below the adjusted shunt voltage, U1 is "off", the circuit drawing a few hundred microamps.  This is likely to be less than the self-discharge current of battery itself.
• When the battery voltage is above the adjusted shunt voltage, U1 will turn on:  The LED will illuminate - the brightness roughly proportional to the shunt current - and the bulk of the current will flow through R2.
• Because the voltage will be high enough to activate the shunt only when the battery bank is being charged, these shunts will have negligible load when the bank is actually being used (e.g. power being drawn due to a power failure.)
• R2 (and R1/LED1) limit the maximum current that is likely to be drawn by the circuit in the event that voltage cannot be drawn down below the threshold voltage.  This can occur during bulk charging of the battery banks and it can also occur if the sum of the threshold voltages of the individual shunt regulators is lower than the float voltage - something that could happen on a system that adjusts the float voltage with temperature (discussed below).

In practice, if we were to set the shunt voltage is set to 13.55 volts, the lead resistance connecting the circuit to the battery will result in a very sharp "knee", the circuit going from "off", drawing a few hundred microamps, to "on" and drawing nearly 100 milliamps - in just a few millivolts or 10s of millivolts  - depending on the gauge of the wire used to connect to the battery.

Figure 3:
Internal and external views of the shunt regulator.
These were built "dead bug" rather than on a circuit
board, using sleeving to insulate conductors that might
otherwise touch.  Braided silicone-fiberglass tubing
was used to insulate the large resistor as it is designed
to handle a bit of heat with the remainder of the circuit -
and the top of the braided tubing - being secured with
heat-shrinkable tubing:  The end of the LED and
potentiometer protrude from the end of the tubing.
Click on the above for a larger version.

In operation the it is suggested that the voltage threshold be adjusted to just light the LED at the ideal float voltage in order to force enough current through the charging system to assure that a small amount of current is flowing through each battery - or through the shunt regulator.  In systems that keep the batteries maintained at a constant temperature the float voltage will remain constant, but in some cases - where the batteries are in an uncontrolled climate environment - the temperature and the float voltage may vary - typically being slightly increased, by many "smart chargers", at low temperatures and decreased at high temperatures.

The information in Figure 2 suggests a voltage that works out to be about 2.26 volts/cell, which is a reasonable value for a temperature range between 10C and 35C (50F-95F).  If the charge voltage rises above this value, the shunt regulators will start to conduct - but the voltages across the battery will be equalized, provided that they were adjusted to the same voltage:  The heat produced - even though it may be just a few watts - will not be of detriment in extremely cold conditions to the performance of the battery.  At high temperatures the lower voltage being produced by the charger may not "trigger" the shunt regulator at the average voltage, but it will still keep any errant cells from straying too far in voltage from the ideal:  Because self-discharge and leakage currents of batteries increases at higher temperatures, it is arguably more important that measures to be taken to keep everything equal!

The diagram in Figure 2 shows two options -  Values for a circuit to be used with a "12 volt" battery and values for a "6 volt" battery - but the operation and set-up is identical in each case as described on the drawing itself:

• Set the wiper of R4, the 10 turn potentiometer, the wiper is at the "ground" end.
• Set an adjustable power supply for precisely the desired shunt voltage.  For normal "room-like" temperatures with Lead Acid batteries, 13.55 and 6.775 volts is recommended for 12 and 6 volt types, respectively.
• Connect the unit to the power supply and adjust the potentiometer so that the LED just illuminates.   Note that even a few millivolts will make a significant difference in LED brightness which means that one adjusts several in on session and if they are "approximately" the same brightness, they will be really close to each other in threshold voltage.
• It is strongly recommended that all units in a particular battery bank be adjusted to the same voltage.

Construction:

It was convenient, at the time, to construct these circuits in a "dead bug" manner (see Figure 3) with no circuit board:  Once the layout was determined - and thoroughly documented - it took only a few minutes to assemble each unit, trimming/bending/insulating/soldering the leads in assembly-line fashion.

The entire circuit was covered with insulating tubing - but to cover the main heat-generating component, R2, I obtained some high-temperature, silicone-fiberglass tubing.  This tubing extends beyond R2 and a small piece of "normal" heat-shrink tubing is used to cover these components and hold everything in place with the tip of the LED and the adjustment screw of R4 protruding.

Because of the heat being produced - which could be well over 1 watt - R2 should be placed as far away from other components - particularly U1, which also generates heat.  If you choose to replicate this circuit on a small board it is strongly suggested that R2 and U1 be separated - and that R4, the potentiometer, not be placed too-near R2, either.

The leads connecting the unit are color-coded for polarity and in this case, they were fitted with alligator clips which provide a convenient connection to the battery terminals.

Long-term observation:

During the time that the UPS was active, batteries were replaced only when they degraded as indicated by the resistance measurements of the battery monitoring system:  By the time the UPS was finally shut down after 22 years of operation some of the batteries were "new" and some were as old as 13 years and still within their specifications of internal resistance (and when they did finally go out of spec in terms of internal resistance, many were load-tested and their remaining amp-hour capacity was still pretty good) - and much of this is attributable to the fact that these shunt devices did a very good job of confining the "float" voltages of all 80 batteries to within +/- 50 millivolts - most of that variation being due to not all shunt regulator units being more-precisely adjusted than that.

To be sure, the battery monitoring system did do in-situ impedance testing and a battery was pulled and replaced when its resistance exceeded a threshold determined by observation and correlation of the "failed" battery with its actual amp-hour rating measured after the fact:  By the time the internal resistance of the battery exceeded its mark (0.005 ohms for the particular 100 amp-hour, high-current UPS batteries that were used) it had dropped below about 80 amp-hours as measured at the 20 amp rate.

This long-term observation also showed that the LEDs were a useful visual indicator:  If an LED wasn't illuminated at least dimly it meant that the particular battery's leakage current had exceeded the average of idle current (shunt regulator current plus the battery leakage current) and that its terminal voltage was dropping - something that was usually a sign that that particular battery should be watched very closely.

What about an "Equalization charge"?

It is recommended by many battery manufacturers that an "equalization charge" be applied to the batteries periodically  to raise their terminal voltage, presumably "stirring up" the internal electrolyte of flooded-cell batteries.  In such situations, the shunt regulator will try to clamp the voltage, but since it is current-limited, the batteries will still see elevated voltage:  The shunt regulator should help divide the voltage of series-connected batteries to assure that this purposely-high potential will be the same across all batteries.  (Note:  One should not equalize AGM batteries as this can lead to internal gas pressure that can be vented and cause loss of electrolyte.)

Comment:

You might ask:  "Don't I need to equalize the voltage of the individual 2 volt cells within a 6 or 12 volt battery for this to work?"  The answer is:  It would be nice if this was possible, but connections to individual cells is usually no possible.  Fortunately, in a single battery, individual cells are usually pretty-well matched as they were made at the same time and typically experience (pretty much) the same temperature throughout their lives.

If you have a system that uses individual 2-volt cells, the above circuit will not work at that low voltage - but there is a version of the TL431 (e.g. the TLVH431) that will work at the "2 volts" of individual cells.  Its maximum current rating is lower than the TL431, but it should be adequate for many applications.  The circuit in Figure 2 would have to be modified slightly to accommodate these changes (e.g. adjust values of R2 and R3 - R1 and the LED would be eliminated as the voltage would probably be marginal/too low for it to work) - but that might be the subject of another article.

* * *

Another related page at ka7oei.blogspot.com:

• Lithium-Iron Phosphate (LiFEPO4) batteries revisited - equalization of cells - Link

This page describes a circuit of similar function, also based on the TL431, that is used for equalizing LiFEPO4 cells.  With component changes to adjust the voltage threshold, it could be modified for 6 or 12 volt batteries.  Because it uses power transistors, it can handle much more shunt current - but note that even with the large 300 kVA UPS, 50-80 milliamps, the current capability of the TL431 itself, was sufficient to keep relatively healthy batteries equalized.

One saving grace is that like most power systems, the UPS was rarely "on battery" which meant that even at just 50 milliamps or so, even an imbalance of a few amp-hours would eventually be equalized - assuming that the batteries themselves were run "completely down".  With a low-voltage disconnect built into a system that cut off the load at the 25%-30% level, this should never happen, so a bank that consists of multiple batteries - even those with slightly different amp/hour capacities (due to manufacturing differences, age) should "track" reasonably well upon recharge.


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This page stolen from ka7oei.blogspot.com