This "sine wave" UPS is a 1.5kVa unit that was cast off from by someone for the same reason most UPSs are cast off: Its internal battery went bad. Rather than simply replacing the battery, its previous owner simply got another UPS and asked the question "Do you want this?"
Instead of replacing the internal battery, the DC connections for the batteries were brought out and a bank of six 12 volt lead-acid batteries was wired up (two sets of three parallel batteries connected in series amounting to a nominal 200-ish amp-hours at 24 volts) to provide the needed 24 volts and a DC circuit breaker was added for safety, this battery capacity allowing the unit to run for far longer than it could have on the original battery.
While this UPS was more efficient than the previous unit and produces a fairly nice sine wave rather than the typical, ugly "modified sine wave" there was a price to pay: RF Interference (e.g. RFI) that was present whether the unit was active or on standby.
But first, a few weasel words:
- This project involves high voltages and/or currents: Do not attempt to construct a similar device unless you are thoroughly familiar with electrical safety and the wiring of such devices.
- If you use an external battery bank with a UPS it is imperative that you include some sort of current liming, such as a fuse or circuit breaker rated for both the expected current and battery voltage. Such devices are available from auto-parts stores.
- If you use an external battery bank with a UPS you must determine if this battery bank is "mains referenced" internally by the UPS or not. If it is mains referenced (e.g. connected directly or indirectly to the mains AC voltage) then the low-voltage DC terminals will pose a line voltage safety hazard and care must be taken - the least of which is enclosing the battery and any exposed DC terminals to prevent accidental contact. This UPS's battery terminals were isolated from the mains and given that the room in which the UPS has restricted access, the low-voltage battery connections themselves were deemed to be "adequately safe" left exposed.
- YOU are responsible for the safety and any liability if you choose to do something similar to what is described on this page. You have been warned!
The completed AC/DC filter. This
box contains a "brute" force L/C filter
for the battery (DC) leads as well as
separate filters for the AC mains in/out
Click on the image for a larger version.
The radio frequency "grunge" from the UPS manifested itself on the HF ("High Frequency" or shortwave) bands in several ways. Most obvious was a loud "buzz" every 40-50 kHz on the lower (80 and 40 meter) bands, but there was also more subtle interference that pervaded these and higher bands: A background "hiss" that might initially escape the notice of the casual listener until one realizes that this noise masked signals that should have still been perfectly audible. If one switched the receiver to AM it would be observed that this "hiss" was subtly modulated at twice the mains frequency, 120 Hz.
To determine the extent of this sort of interference the typical procedure is to start turning things in the house off one-at-a-time (or, more reliably, the reverse: Turn everything off at the breaker panel and then turn on one thing at a time) until the culprit is found. This was done with the UPS and the magnitude of its "radio interfering" nature was determined. Clearly, this "grunge" was being conducted from the power leads going in and out of the UPS.
Further experimentation revealed the true extent of the noise: Even with everything disconnected from the UPS - that's to say, with it running on its battery, unplugged from the mains and the load disconnected - there was still detectable noise getting to the antenna and a quick check with a portable shortwave receiver proved that the remainder of this noise seemed to be being radiated by the physically-large battery bank and the wires that connected it.
While the proper application of a "brute force" AC line filter would likely quash the noise conducted in and out of the UPS on the mains power leads, something else would be required to minimize/eliminate the noise emanating via the DC lines.
"Brute force" line filters:
One of the best ways to eliminate or minimize the amount of RF energy that might be conducted out of a potentially interference-generating device is to apply a combination of inductance and capacitance to that line as depicted schematically in Figure 2.
With the noise coming from the power supply (the UPS in our case) capacitor "C4" effectively "shorts" this RF noise to both sides of the power line, leaving the AC signal (pretty much) unchanged. Inductor L1 consists of two equal windings on a ferrite core and it is practically invisible to signals that are equal and opposite, but it acts as a series choke for signals that are "common mode" - that is to say, equal on both sides of the power supply's mains leads - such as the RF noise energy.
On the "mains" side of the filter capacitor C3 reinforces the common mode again while capacitors C1 and C2 shunt any remaining RF energy from the power supply - its impedance now made much higher by inductor L1 - to ground - which would be the metal enclosure in which everything was mounted.
As it happens, these filters are available on the surplus market, and we would need three of them:
- One for the AC mains connection to the UPS.
- Another for the UPS's AC output
- A third one for the battery connection to the UPS.
Putting the filters in a box:
Ideally, one would have put the UPS and the batteries in a large metal box and passed the power leads in and out of this box only via the filters, but this simply wasn't practical, so the next best thing had to be done: Put the filters in a single, metal box that would be electrically bonded to the UPS chassis and make the connections in and out of the UPS using short leads. By keeping the leads short and bonding our new filter box to the UPS, we'd do our best to limit the number and length of conductors that carried the RF interference and, most importantly, preventing RF circulating currents from finding their way on external connections.
The end of the box with the mounted outlets. As noted,
one pair of these outlets is connected to a relay to allow the
connected devices to be remotely controlled.
Click on the image for a larger version.
With the short-as-possible conductors between the UPS and the filter, they will have minimal ability to directly radiate RF while the RF conducted on these lines will be filtered by the circuitry in the box itself with the bonding of the two boxes minimizing differential RF currents. The power "to" the UPS was made with a short length of "SO" cord with a female connector on it while the power "from" the UPS is via a short cable with a male connector, plugged into one of the UPS's outlets - and being the ONLY thing plugged into the "dirty" AC output of the UPS.
When installed, the filter box was placed underneath the UPS as it had a slightly larger footprint - and to minimize the length of the "noisy" DC and AC power leads from the UPS, along with the lead used to bond the two cases together.
To connect the DC, the cable coming from the batteries was effectively cut so that there was just enough of it emerging from the UPS to connected to the "output" side of the filter: On both sides of this cable heavy "ring" lugs were attached and these were connected to the studs of the DC filter. To eliminate the probability of accidental shorting, the ground stud on the "input" side of the filter was removed and near both connectors a plastic wire clamp (one may be seen in Figure 6, below) to keep the positive wire in a fixed position and rotating into and shorting to the other stud.
To the remaining ground stud of the DC filter was attached a short piece of heavy (8 AWG) green wire with a ring lug on each end, this wire being visible in Figure 6. The other end of this wire was attached to a marked grounding screw on the UPS chassis to bond the two boxes together and minimize RF circulating currents and to prevent the UPS chassis itself from being a source of radio frequency interference by direct radiation.
Taking the 20 meter (14 MHz) amateur band as an example, the UPS caused an extra 2 "S" units or so of noise above that of the typical ionospheric noise floor when it was powered up, before the filter was installed - this, being detected on a Carolina Windom antenna lofted between two trees high above the cabin's roof. After this filter was installed the noise from the UPS was completely undetectable on any HF band, revealing other weaker low-level noises from other devices - the quieting of some of these will be discussed in later installments.
After the filter was installed and the unit was powered up again with the loads connected, the noise was barely audible in the FT-817 and from across the room, it went away completely when the green wire was connected, bonding the UPS and filter chassis together. If the radio was moved to within a foot or two of the UPS I could start hearing the "hash", but it seemed to be emanating only from the coils of AC cables "zip"-tied to the back end of the unit. Because of the short length of the wires - and their being close to the metal case and not near any other wires into which this noise could be coupled it is unlikely that these short conductors will radiate any detectable noise at a distance greater than a few feet.
Links to other articles about power supply noise reduction found at this site:
- 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.
This page stolen from ka7oei.blogspot.com