Using a UPS to Isolate and Test a Homebrew Offline smps

Today, a UPS with a dead battery was given to me..

A label underneath reads:
The output of this device is not sinusoidal. It has a THD of 68% and
max. harmonic of 57%.

Crazy Question: Can I make use of this UPS to create an isolated
120VAC or 170VDC source to safely test my homebrew offline smps with
an earth grounded oscilloscope?
The output power rating is ok.
I'm assuming the UPS uses a lead-acid 12V batt..
I have a car battery. Arrfff Arrfff...
To hack or not to hack?

Maybe I can dodge ordering an isolation transformer from
Digikey...(About $45.00CAD)

My local surplus electrical junk shop had a pile of transformers but
no isolation types...strange...The owner looked too....nothing.

Too poor to get an isolated Tektronics TDS digital phosphor scope
D from BC

 

The peak voltage will be somewhat lower and obviosuly there will be
masses of harmonics. As you are presumeably rectifying first, the
harmonics won't be much of an issue. The lower peak voltage might be.

Also many UPSs are only rated for *very* short run times. Say five or
ten mins max. That also may not be an issue.

 

*most* UPSs use 24 volts (two 12V or four 6 volt batteries) not 12
volts...

I'd suggest the isolation transformer is the best route to go.

 

All of the APC ones I have dissected had 12 volt Gel Cells in 'em. One
cell per unit. I have salvaged parts from 4 different models of APC.

- Tim -

 

| All of the APC ones I have dissected had 12 volt Gel Cells in 'em. One
| cell per unit. I have salvaged parts from 4 different models of APC.

At least the APC types have sinus output. Connect to 12 volts batteries in series and provide sufficient cooling.
You can connect a 24 volts power supply (lower power) for the batteries instead. Or solar panels...

Works!

 

Well, you don't need to "isolate" it from anything - just set it on
a board so it doesn't short to the concrete floor, and voila! Isolation!
Of course, unplug the input cord.

As far as using it as an input to your SMPS, it depends on what your
SMPS input wants - it could be a good test of "What kind of line crap
can my new design take?"

DO NOT USE A "CAR" BATTERY!!! They are not designed for this kind
of application - they're designed to start car engines. Use a marine,
RV, golf cart, etc. "Deep Cycle" battery.

And be careful to use the right number of them - as others have said,
a lot of UPSs use 24V.

Yeah, as long as your DUT can take the waveform.

Should be no need - but it might be interesting to get a big ol'
tranny and an oil cap and make a parallel-resonant circuit at 60 Hz
across the output. ;-)

No worries - like I said, the UPS is already isolated. (as long as it's
not plugged into the wall, which is what the batteries are for. ;-) )

Good Luck!
Rich

 

I opened up the UPS.. 12V 4.5Ahr battery.
It's a Minuteman UPS..

From posts, I'm thinking this idea is a PITA...

UPS Noise problem:
I could hook up a line filter.
(Resonator was mentioned...)
Could be ok..

UPS Undervoltage problem:
The outlet voltage is 124VAC today..
The smps label has 115VAC output. This might be ok if I'm just
tracking down transients and ringing on the homebrew smps.
Hopefully not drowned in noise from the UPS.

UPS time limited operation problem:
I could put a fan on the UPS switcher.
The UPS pcb looks loaded with control electronics... Maybe it'll auto
cut off even though the battery is still fully charged.. But, I can
get my testing done within 5min..

Replacement battery problem:
Somebody didn't like the car batt idea. So, battery shopping
required.... I might use the UPS for my computer.. However, the power
is great here .. Only 2 brownouts in one year..I don't need a UPS.

Of course, UPS's make alert sounds so...piece of tape on the piezo

Groannn
Ordering that isotransformer from Digikey is looking better
D from BC

 

Last one I did (destroy...) had a pair of 12V batteries.

The one I use for my server farm has... ten 12V batteries (yes, in
series, 120 vdc!) but it is a monster UPS--3.5 KVA, with waveform
correction and a bunch of other stuff... <g>

Personally were I to need to isolate soemthing and wanted to go that
way, I'd get a 12v to 120v inverter, better utility: done with the
project you can use it in your car or truck for general purpose AC!
<bg>

 

Maybe you can get another UPS? and put
the xformers secondary to secondary.

Ed

 

Actually got 2 UPS..same model..

When I took the UPS apart, I was surprised to find a rather large line
transformer. Almost accounts for half the weight of the unit.
Just for charging the battery?
(The battery inside is a 12V 4.5Ah batt.)
I guess the battery couldn't be charged by a smps for some reason...
The 115VAC output is done by switching.

I guess you mean pull out the 60Hz power transformers and put
secondary to secondary.. Toss the UPS smps pcb in the junk box..

I dunno....
I need 130Watt capability and less than 10% V drop from loading..
I've melted the bobbins on 2 transformers doing that with underrated
50W transformers..
D from BC

 

No, that's the main transformer. It's probably used for the inverter and
charging.

I don't think it is.

 

Yes. With the PCB and the batteries removed, there's enough
room to add the transformer from the second UPS, and you
already have the line cord and receptacles mounted to the
UPS case, so with just a little work you ought to be
good to go.

I don't _know_, either, not having done it. Based on the
weight of those transformers, my guess is they will do it.
I figure it's worth looking into, for a 0 dollar investment.

Ed

 

Just get the isolation transformer, that is a good price.