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]

 

Modifying an "O2-Cool" battery fan to (also) run from 12 volts

A blog posting about a fan?  Really?

Why not!

Figure 1:
The modified fan on my cluttered workbench, running
from 13 volts.
The external DC input plug is visible on the lower left.
Click on the image for a larger version.

This blog post is less about a fan, but is more of an example of the use of a low-cost buck-type voltage converter to efficiently power a device intended for a lower voltage than might be available - in this case, a device (the fan) that expects 3 volts.  In many cases, "12" volts (which may be anything from 10 to 15 volts) will be available from an existing power source (battery, vehicle, power supply) and it would be nice to be able to run everything from that one power bus.

Background

Several years ago I picked up a 5" battery-operated DC fan branded "O2 Cool" that has come in handy occasionally when I needed a bit of airflow on a hot day.  While self-contained, using two "D" cells - it can't run from a common external power source such as 12 volts.

Getting 3 volts

Since this fan uses 3 volts, an obvious means of powering it from 12 volts would be to simply add a dropping resistor - but I wasn't really a fan of this idea (pun intended!) as it would be very wasteful in power and since doing this would effectively defeat the speed switch - which, itself is just a 2.2 ohm resistor placed in series with the battery when set to "low".

The problem is that the fan itself pulls 300-400 mA on high speed.  If I were to drop the voltage resistively from 12 volts (e.g. a 9 volt drop) - and if we assume a 300mA current - we would need to add (9/0.3 = ) 30 ohms of series resistance to attain the same speed on "high" as with the battery.  The "low speed" switch inserts a 2.2 ohm resistor, and while this works with its original 3 volt supply, adding this amount to 30 ohms would result in a barely noticeable difference in speed, effectively turning it into a single-speed fan.  By directly supplying the fan with something close to the original voltage, we preserve the efficacy of the high/low speed switch.

Fortunately, there's an answer:  An inexpensive buck converter board.  The board that I picked - based on the MP1584 chip - is plentiful on both EvilBay and Amazon, typically for less than US$2 each.  These operate at a switching frequency of about 1 MHz and aren't terribly prone to cause radio interference, having also been used to power 5 volt radios and even single-board computers (such as the Raspberry Pi) from 12 volts without issues.

These buck converters can handle as much as 24 volts on the input and provide up to 3 amps output - more than enough for our purpose - and can also be adjusted to output about any voltage that is at least 4 volts lower than the input voltage - including the nominal 3 volts that we need for the fan.

An additional advantage is the efficiency of this voltage conversion.  These devices are typically 80% efficient or better meaning that our 300 mA at 3 volts (about 0.9 watts of power) would translate to less than 100mA at 12 volts (a bit more than a watt).  Contrast this to the hypothetical resistive dropper (an efficiency of 25-30%) discussed earlier where we would be burning up nearly 3 watts in the 30 ohm resistor by itself !

Implementation

One of my goals was to retain the ability of this fan to run at 3 volts as it would still be convenient to have this thing run stand-alone from internal power.  Perhaps overkill, but to do this I implemented a simple circuit using a small relay to switch to the buck converter when external power was present and internal power when it was not, rather than parallel the buck converter across the battery.

If I never intended to use the internal "D" cells ever again I would have dispensed with the relay entirely and not needed to make the slight modifications to the switch board mentioned below.  In this case I would have had plenty of room in the case and freedom to place the components wherever I wished.  In lieu of the ballast of the battery to hold the fan down and stable, I would have placed some weight in the case (some bolts, nuts, random hardware, rocks) to prevent it from tipping over.

The diagram of this circuitry is shown below:

Figure 2:
Diagram of the finished/modified fan.
On the left, J1 is the center-positive coaxial power connector with diode D1 and self-resetting
resetting thermal fuse F1 to protect against reverse polarity.  The relay selects the source of power.
Click on the image for a larger version.

The original parts of are the High/Low switch, the battery and the fan itself on the right side of the schematic with the added circuits being the jack (J1), the self-resetting fuse (F1), D1, R1, the buck converter and the relay (RLY).

How it works:

When no external power is applied, the relay (RLY) is de-energized and via the "NC" (Normally-Closed) contacts, the battery is connected to the High/Low switch and everything operates as it originally did.

External power is applied via "J1" which is a coaxial power jack, wiring the center pin as positive:  The connector that I used happens to have a 2.5mm diameter center pin and expects an outer shell diameter of 5.5mm.  There's nothing special about this jack except that I happen to have it on-hand.

When power is applied, the relay is energized and the high/low switch is disconnected from the battery but is now connected, via the "NO" (Normally Open) contacts, to the OUT+ terminal of the buck converter.  

Ideally, a small 12 volt relay would be used, but the smallest relay that I found in my junk box was a 5 volt unit, requiring that the coil voltage be dropped.  Measuring the relay coil's resistance as 160 ohms, I knew via Ohm's law that it required about 30 mA (5/160 = 0.03) and if we were to use 12 volts, we'd need to drop (12 - 5 =) 7 volts.  The resistance needed to drop 7 volts is therefore (7/0.03 = ) 233 ohms - but since I was more likely to operate it from closer to 13 volts much of the time I chose the next higher standard value of resistance, 270 ohms to put in series for R1.

Figure 3:
Modification of the switch board.  The button is
the positive battery terminal and traces are cut to
isolate it to allow relay switching.
Click on the image for a larger version.

The diode D1 is a standard 1 amp diode - I used a 1N4003 as it was the first thing that I found in my parts bin, but about any diode rated for 1 amp or greater could be used, instead.  Placing it in reverse-bias across the input of the buck converter means that if the voltage was reversed accidentally, it would conduct, causing the self-resetting thermal fuse F1 to "blow" and protect the converter.  I chose a thermal fuse that has several times the expected operating current so I selected a device that would handle 500-800 mA before it would open.

Modification to the switch board

The High/Low switch board also houses the positive battery contact, but since it is required that we disconnect the battery when running from external power, a slight modification is required, so a few traces were cut and a jumper wire added to isolate the tab that connects to the positive end of the battery as seen in Figure 3.

Figure 4:
The top of the board battery board. The
connection to the Batt+ is made by soldering to
the tab.
Click on the image for a larger version.

Near the top of the photo in Figure 3 we see that the trace connecting end of the 2.2 ohm resistor has been separated from the battery "+" connector (the round portion) and also along the bottom edge where it connects to the switch.  Our added jumper wire then connects the resistor to the far end of the switch where the trace used to go and we see the yellow wire go off to the "common" contact of the relay.

In Figure 4 we can see the top of the board with the 2.2 ohm resistor - but we also see the wire (white and green) that connects to one of the tabs for the Battery + button on the bottom of the board:  The wire was connected on this side of the circuit board to keep it out of the way round battery tab and the "battery +" connection.

The mechanical parts

For a modification like this, there's no need to make a circuit board - or even use prototyping boards.  Because we are cramming extra components in an existing box, we have to be a bit clever as to where we put things in that we have only limited choices.

Figure 5:
Getting ready to install the connector after
a session of drilling and filing.
Click on the image for a larger version.

In the case of the coaxial power connector, there was only one real choice for its location:  On the side opposite the power switch, near the front, because if it were placed anywhere else it would interfere with the battery or with the fan itself as the case was opened.

Figure 5 shows the location of this connector.  Inside the box. this is located between two bosses and there is just enough room to mount it.  To do this, small holes were drilled into the case at the corners of the connector and a sharp pair of flush-cut diagonal nippers were used to open a hole.  From here it was a matter of filing and checking until the dimensions of the hole afforded a snug fit of the connector.

Figure 6:
A close-up of the buck converter board with the
attached wires and BATT- spring terminal.
The tiny voltage adjustment potentiometer is
visible near the upper-left corner of the board.
Click on the image for a larger version.

Wires were soldered to the connector before it was pressed into the hole and to hold it in place I used "Shoe Goo" - a rubber adhesive - as I have had good luck with this in terms of adhesion:  I could have used cyanoacrylate ("Super" glue) or epoxy, but I have found that the adhesive bonds of these tend to be a bit more brittle with rapid changes of temperature, mechanical shock or - most applicable here - flexing - something that the Shoe Goo is meant to do.

Because this jack is next to the battery minus (-) connector, a short wire was connected directly to it, and another wire was run to the location - in the adjacent portion of the case - where the buck converter board would be placed.

Figure 6 shows the buck converter board itself in front of the cavity in which it will be placed, next to the negative battery "spring" connector.  Diode D1 is soldered on the back side of this board and along the right edge, the yellow self-resetting fuse is visible.  Like everything else the relay was wired with flying leads as well, with resistor R1 being placed at the relay for convenience.

Figure 7:
The relay, wired up with the flying leads.
Click on the image for a larger version.

Figure 7 shows the wiring of the relay.  Again, this was chosen for its size - but any SPDT relay that will fit in the gap and not interfere mechanically with the battery should do the job.

The red wire - connected to the resistor - comes from the positive connector on the jack and the "IN+" of the buck converter board - the orange wire is the common connection of the High/Low switch, the white/violet comes from the "OUT+" of the buck converter and goes to the N.O. (Normally Open) contact on the relay, the white/green goes to the N.C. (Normally Closed) relay contact and the black is the negative lead attached to the coil.

Everything in its place

Figure 8 shows the internals of the fan with the added circuitry.  Shoe Goo was again employed to hold the buck converter board and the relay in place while the wires were carefully tucked into rails that look as though they were intended for this!

Now it was time to test it out:  I connected a bench power supply to the coaxial connector and set the voltage of my external test power supply at 10 volts - enough to reliably pull in the relay - and set the fan to low speed.  At this point I adjusted the (tiny!) potentiometer on the buck converter board for an output of 3.2 volts - about that which could be expected from a very fresh pair of "D" cells.

Figure 8:
Everything wired and in its final locations.  On the far left is
the switch board.  To the left of the hinge is the relay with the
buck converter on the right side of the hinge.  The jack and
negative battery terminal is on the far right of the case.
Click on the image for a larger version.

The result was a constant fan speed as I varied the bench supply from 9 to 18 volts indicating that the buck converter was doing its job.

The only thing left to do was to make a power cord to keep with the fan.  As is my wont, I tend to use Anderson Power Pole connectors for my 12 volt connections and I did so here.

As I also tend to do, I always attach two sets of Anderson connectors to the end of my DC power cords - the idea being that I would not "hog" DC power connections and leave somewhere to plug something else in.  While the power cord for the fan was just 22 gauge wire, I used heavier wire (#14 AWG) between the two Anderson connectors so that I could still run high-current devices.

* * *

Does it work?

Of course it does - it's a fan!

The relay switches over at about 8.5 volts making the useful voltage range via the external connector between 9 and 16 volts - perfect for use with an ostensibly "12 volt" system where the actual voltage can vary between 10 and 14 volts, depending on the battery chemistry and type.

Figure 9:
The fan, folded up with power cord.  The two
power pole connectors and short section of heavy
conductor can be just seen.
Click on the image for a larger version.

Without the weight of the two "D" batteries, the balance of the fan is slightly precarious and prone to tip forward slightly, but this could be fixed by leaving batteries in the unit - but this is not desirable for long-term storage as leakage is the likely result.

Alternatively, one may place some ballast in the battery compartment (large bolt wrapped in insulation, a rag, paper towel, etc.) or simply by placing something (perhaps a rock or two) on the top.  Alternatively, since the fan is typically placed on a desktop, it is often tilted slightly upwards and that offsets the center of gravity in our favor and this - plus the thrust from the airflow - prevents tipping.

This page stolen from ka7oei.blogspot.com

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Automatically re-connecting low-voltage cut-outs for 12 and 24 volt lead acid battery systems

In a previous post I described a simple circuit that provided a low-voltage cut-out that could be used in a battery-operated system - see the article "A latching low-voltage disconnect for 12 volt lead-acid and lithium batteries".

That circuit - intended mostly for lithium-based batteries - required manual intervention to "reset" the device, intended for those situations where you wanted to provide manual intervention in resetting the circuit to prevent causing harm to the battery - and maybe the gear connected to it.

Figure 1:
The completed 24 volt low-voltage cut-out.  R5, the cut-off voltage
adjustment, is the blue 10-turn potentiometer near the middle and
R8 is the black, single-turn potentiometer near the top, to the left
of Q1 and its heat sink.  U1 is the lower transistor-looking device
near the left side of the board.
This device was installed in an outdoor enclosure that houses a relay
point to bridge a wireless Internet connection over a hill that has
a 24 volt back-up battery system.  In the event of an extended
power outage, this device prevents damage to the batteries,
particularly in the winter where this device will
prevent the batteries from being damaged by freezing.
The small heat sink is adequate for this FET and an operating
current of 5 amps.
Click on the image for a larger version.

This circuit is different from that described in the link above, intended for lead-acid battery based systems where an automatic reconnect of the battery is required when the battery voltage rises above a threshold voltage after application of charge current.  An example of where this would be useful is a system in which the battery is charged by a main-powered DC supply that is used to keep the battery charged where the battery will power the load in the event of a power failure.

Why would we worry about this?

Related to this, let's briefly talk about maximizing longevity of lead-acid batteries:

• For best longevity, the shallower the discharge, the better.  A classic example of this is a solar-powered system where there is a daily discharge.  If the battery were sized too small such that it was discharged from "full" to 30% every day, it might last only a year or so at best, but by increasing the battery size such that it was only discharged to 75% worst-case, the cycle depth would be much shallower and the battery would last much longer.  This assumes, of course, that there is enough charging capacity to do this, which can be an issue for a solar charge system in the winter and/or periods of weather where there is little direct sun.

• Lead-acid batteries can handle being run completely down - as long as you don't keep them in that state for very long (a few days at most - as little time as possible) and don't do it very often - and the more charge you can leave on them at the deepest point of discharge, the better.  In other words, if you run an otherwise healthy lead acid battery completely dead and immediately recharge it, little immediate damage is likely to have been done other than taking a bit of life off it farther down the road as most lead-acid batteries can only handle a few hundred of such cycles before they are significantly and permanently degraded.  (The previous statement applies only to "Deep Cycle" lead-acid batteries:  Automotive "starting" batteries do not fall into this category - They can be permanently damaged by as few as dozen or two "complete" discharge cycles in some cases.)

• In cold-weather environments, while the degradation (primarily sulfation) caused by the battery being dead for extended periods is dramatically slowed, extremely deep discharge also reduces the specific gravity, raising the electrolyte's freezing point and increasing the possibility of the battery being damaged/destroyed at very low temperatures if it does get cold enough to freeze the electrolyte.  Preventing too-deep a discharge by disconnecting the load while there is still some capacity left can reduce the probability of this happening.  (This can happen with both flooded cell and AGM batteries.)

• Important:  A general rule of thumb is that above approximately 70F (approx 20C) for every 10F (approx. 5.5c) increase in temperature, the expected lifetime of a lead-acid battery is approximately halved.  Carefully consider this if the battery is to be in a location that might get warm/hot.

For several of the reasons mentioned above, a low-voltage disconnect circuit is essential in systems where there is the distinct possibility that the battery could be completely discharged - both to prevent the loss of battery capacity due to over-discharge (e.g. sulfation if the battery is left "flat" for a long time) as well as preventing freezing of the electrolyte when the battery is deeply discharged where/when that is a concern.

A simple low-voltage cut-off circuit:

While there are many ways to build such a circuit, the circuit below - designed for a 24 volt battery system (values are included for use with a 12 volt battery) - demonstrates a simple, yet flexible device.

Figure 2:
A low-voltage battery disconnect, designed for 24 volts.  It has a useful adjustment range of between 22 and 28 volts.
For 12 volts:  R2=4.7k, R4=62k, R7=47k, R11=47k, R12, R13, R14=10k.
 Components LED2 and R14 are optional, used to indicate that the circuit has disconnected the battery from the load.  (Errata:  LED2 is shown connected backwards.)

Not shown is appropriate fusing on the battery side of the circuit to protect against short circuits and/or catastrophic component failures.
Click on the image for a larger version.

This circuit, somewhat similar to that described in the other article linked above, works as follows:

The battery voltage is measured using R4, R5 and R6 which form an adjustable divider to take the battery voltage down to the 2.5 volt comparison voltage required by U1, a TL431 "programmable" zener diode.  When the voltage at the wiper of R5 goes above 2.5 volts, U1 will conduct.

When U1 conducts it pulls down the voltage at the base of Q1 via R3 causing it to turn on.  This voltage, via R12, is fed to the gate of Q1 - an N-channel power MOSFET in the negative battery lead. In this way, as long as the voltage is above the threshold set by R5, Q1 is turned on, connecting the load to the battery.  While U1 is turned on and pulling the voltage at the bottom end of R3 close to ground, the voltage at the gate of Q3 is low, keeping it turned off.

If the divided battery voltage drops below the low-voltage cut-out threshold, U1 turns off and its cathode is pulled high by R2 with resistor R1 being used to allow Q2's gate to go high and turn it off.  When Q2 is turned off, the gate voltage on Q1 goes away, being pulled to its source potential by R11.

At this same time, the rise in voltage at U1's cathode also causes Q3 to be turned on and the voltage at the junction between R4 and R5 is pulled slightly down, requiring a higher battery voltage to turn U1 back on than was required for it to be turned off, thus providing a bit of hysteresis.  It is by adjusting R5 that the amount of hysteresis (e.g. the amount by which the turn-on voltage must be higher than the cut-off voltage) may be set.

There are some "extra" components included to make the circuit more robust:

• C1, across the reference pin of U1, reduces this circuit's susceptibility to being triggered by power glitches (e.g. lightning) and stray RF energy.  If the circuit tends to trip out when a load is activated (e.g. voltage "dip" caused by battery/lead resistance)  this capacitor can be increased in value to slow U1's response - up to several microfarads.

• Capacitors C2 and C3 are across their respective FET's gates to provide RF energy protection as well as to smooth transients that might cause false triggering or damage.

• ZD1 is placed between Q1's source and gate to protect it against excess voltage that would damage the device - possibly caused by transients.  The maximum gate-source voltage rating of a typical power FET (Q1) is around 20 volts, so this is a required component for the 24 volt version.

• Finally, C4 is placed across the drain and source leads of Q1 to provide both resilience to stray RF energy as well as to suppress transients that might appear across it when the power is off.

 
Also included in the circuit is the "power on" indicator consisting of LED1 and R13 which provides an indication that voltage is present between the "Load +" and "Load -" terminals.  The optional circuit containing LED2/R14 shows when Q1 is turned off (e.g the load is disconnected) by detecting when there is a voltage drop across it.  (Note:  LED2 was accidentally drawn "backwards" in Figure 2.)

Comments:

• Important:  To simplify the design, it is the "Batt -" lead that is switched to disconnect the load.  What this means is that the negative battery lead must not be connected anywhere else or the battery disconnect will not work properly.  It is possible to design the circuit to use a P-channel FET in the positive lead, but high-current N-channel devices are more plentiful and less expensive.  Instead of a power FET, a slight modification will allow a relay to be used, instead - but at much higher quiescent current owing to the required current for the coil.

• LED1 and LED2 may indicate correctly only when there is a load connected across the "Load +" and "Load -" terminals.

• Even when the load is disconnected this circuit will draw several milliamps of current due to the quiescent load of U1, its voltage divider and LED2, the "Battery Disconnected" indicator.  Keep this in mind if you are going to use it in an application where there is a reasonable probability that battery charging may be unavailable for very long periods of time.

• The current capability of this circuit is limited by the current-handling capability of Q1, the power FET, and the amount of heat-sinking attached to it.  When choosing a power FET, not only should the current rating be considered, but also the specified "on" resistance.  For example, if a 50 amp FET is used and it has an ON resistance of 0.01 ohms, at 20 amps it will dissipate at least (20 * 20 * 0.01 = )  4 watts and will drop (20 * 0.01 = ) 0.2 volts.  The heat sink shown in Figure 1 is capable of dissipating that much power under all but the hottest ambient temperature.

Comment about Q1, the main power FET:

The power FET is not specified, as practically any appropriate N-Channel power FET will work - and here are a few selection guidelines:

• Pick a device with a current rating of at least 3 times the maximum that you expect to draw.  100-150 amp devices are fairly inexpensive, so don't be afraid to pick a far "heftier" device than you might otherwise select.  Note that while a particular FET may have a "150 amp" rating, if there is a short circuit downstream, it is possible that connections inside the FET itself may act as a quicker-acting fuse than an actual fuse intended to protect the circuit.

• Pick a device with a voltage rating of at least 4 times the battery voltage, which would imply at least a 125 volt device for 24 (really around 28) volts and 75 volt device for 12 (really around 14) volts - and there is little penalty in going higher than that given a choice of devices with the same rated current.

• With a properly functioning circuit, a heat sink is likely not required, but a small heat sink doesn't hurt - particularly if the circuit will be exposed to high temperatures.  While the "on" resistance of a saturated power MOSFET can be very low, it is not zero and at maximum expected current, the FET can produce a some heat.
• Note that a FET's "on" resistance may be a fraction of an ohm, but it is not zero so some voltage will be dropped - and heat generated - by the FET.
• Remember the formula - Power in watts = I*I*R where "I" is the current and "R" is the "on" resistance of the FET - and if power is more than 2-3 watts, consider a larger heat sink for Q1 - and after it is built, make sure Q1 isn't getting "hot" under the expected load.

The amount of current that can be handled by this cut-off-switch is essentially the limit of the chosen FET, the fusing, and the interconnection wiring.

Examples of FETs:

•  The IRF540 is rated for 25+ amps at 100 volts and this would be an excellent device for a disconnect that was intended to handle 5-8 amps at 12 volts.  This device is inexpensive and readily available - even from surplus outlets, less than $1.50 from Electronic Goldmine.

• The 45N20 is rated for 35 amps at 200 volts and would be a good choice for 10-12 amps and either a 12 or 24 volt disconnect - and is also available from Electronic Goldmine as the "SSP45N20".

(The Electronic Goldmine is used as an example of surplus electronic suppliers.  These parts are readily available from suppliers such as Digi-Key, Mouser, etc.  Always buy parts like this from reputable suppliers as counterfeit power devices are quite common.)

Adjustment:

Important:  Verify that the circuit works and is properly adjusted before placing it into service!

The procedure is as follows:

• Set R8 for maximum resistance. 

• Connect "Batt +" and "Batt -" to an adjustable DC power supply that is capable of providing between 20 and 30 volts.

• Connect a known-accurate voltmeter to the DC power supply to measure the "Battery" voltage.

• Connect a load to the output terminals ("Load +" and "Load -").  A small lamp of the correct voltage or resistor (1k at 1 watt or more) will do. 

• Increase the voltage to 30 volts:  The load will turn on. (15 volts for the 12 volt version.) 

• Decrease the voltage slowly:  The load will turn off below a certain voltage.

• Increase voltage again to turn the load back on and then adjust R5 so that the load turns off at the desired voltage  The lowest cut-off voltage that is suggested is 1.92 volts/cell for a state-of-charge of 20-25% at "room temperature", which translates to:
• For a 12 volt lead acid battery, 11.5 volts
• For a 24 volt lead acid battery, 23.0 volts
• Note:  At very low temperatures, the output voltage of a lead acid battery will drop given a constant charge level.  This means that in the case of the 12 volt battery that is cold (32F, 0C) a 11.5 volt cut-off would imply that the actual charge level of a battery would be higher than the 20-25% level indicated by this same voltage of the battery were at room temperature.

• This concludes the adjustment of the cut-off voltage.  Adjusting R8 should not affect the cut-off voltage.

• Set the power supply voltage below the cut-off voltage and slowly increase it until the load turns back on, noting the voltage at which that happens.

• Adjust R8 so that the cut-in voltage is adjusted as desired.  The cut-in voltage is much less critical, but should be well above the cut-out voltage.
• A typical cut-in voltage for a 12 volt lead-acid battery is 13 volts, or 2.17 volts/cell, but it may be set lower - to 12.5 volts (2.083 volts/cell) if quicker power restoration is required in a system that has limited charge capacity. 
• A typical cut-in voltage for a 24 volt lead-acid battery is 26 volts, or 2.17 volts/cell, but it may be set lower - to 25 volts (2.083 volts/cell) if quicker power restoration is required in a system that has limited charge capacity. 
• Note:  A higher cut-in voltage may delay restoration of the output load, but it will allow the battery to charge more before the load is restored.  An example where this may be important is in a solar-powered system where a low output from the PV array may be sufficient to slowly charge the battery, but may not be able to sustain the load:  This can happen in the morning, when the panels are covered with snow, and/or on very dark and cloudy days.

•  Re-check the cut-off and cut-in voltages:  When adjustment is needed, remember to set the cut-off voltage first and then the cut-in voltage.

Comment:

This device could be used for similar-voltage Lithium-based batteries with appropriate adjustment of the cut-in and cut-off voltages.  Note that any Lithium-based battery system should already have a "protection" circuit that protects against over and under voltage.  Particularly with LiFeO4 batteries, a circuit to equalize the voltage across the cells should be used.

* * *

This page stolen from ka7oei.blogspot.com

[End]