[RFI] UPS Radiated Interference

wa4tuk wa4tuk-rf at comcast.net
Sat Jan 19 21:28:29 EST 2008

The units I salvaged used the transformer to trickle charge the battery 
and then the transformer was driven by an inverter from the 12 volt 
battery to made pseudo sine waves when needed but not all the time. The 
trickle charge function was RF quiet. The UPS inverter was switched on 
quickly by a relay when the AC line fails so there is a short time with 
no AC present that a good PC power supply could withstand. So these were 
more accurately "standby" power supplies because they allow a brief 

I guess there is a savings in copper and shipping costs to use a 
switcher for the battery charge.

You might shop by favoring the heavier units and at stores that allow no 
hassle returns. I'm wondering now if almost all UPS manufacturers have 
gone to switching types due to energy efficiency requirements.

If you do locate a quiet model please let me know and I'm sure the list 
would be interested.


Bob K0NR (email list) wrote:
> Pat,
> Yes, I unplugged the equipment that the UPS is powering, so it really is
> the UPS.
> I said "switching regulator" but it might be more accurate to say
> "switching inverter" (DC battery to 120 VAC).
> These are el-cheapo off brand UPSs.
> Bob
> wa4tuk wrote:
>> They certainly could be switching supplies; however, the ones I've
>> disassembled after failing have had iron core laminated transformers.
>> These were APC and Tripp-Lite brands. In fact, I've contemplated using
>> them in reverse as filament transformers. It could be that newer
>> models have all moved to switching supplies.
>> When these UPS's are asked to provide emergency power they do make an
>> strong RF buzzing howl.
>> I would guess that you pulled the PC's and monitors AC cord to be sure
>> it wasn't the PC/monitor that was making the noise? Many newer PC's
>> have a small switching supply that is always on to support
>> "wake-on-lan" and other features. And the monitors just go to a
>> standby low power mode. That's part of the reason my PC's are fed from
>> a switched outlet strip.
>> Pat
>> wa4tuk
>> Bob K0NR (email list) wrote:
>>> I tracked down some RFI problems on the 2M band to several small
>>> Interruptible Power Supplies in my house. These are the half-a-shoe-box
>>> size units intended for use with PCs. I assume the switching regulator
>>> inside is the source of the noise.
>>> I am thinking my best strategy is to replace them with higher quality
>>> units (rather than try to suppress the noise).
>>> Any suggestions on models of UPSs that don't radiate?
>>> Any other ideas?
>>> Bob K0NR
>>> Bob Witte K0NR
>>> bob at k0nr.com
>>> Technical Coordinator
>>> ARRL Colorado Section
>>> _______________________________________________
>>> RFI mailing list
>>> RFI at contesting.com
>>> http://lists.contesting.com/mailman/listinfo/rfi
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