Possible use as an Isolatin Xformer?

[GrayLox]

This is a question that could turn into a suggestion, I hope. I have a transformer from a bad computer UPS. The unit rating was 1100 VA as a backup PS. The transformer has three windings. The best I can determine, without a schematic, is that there is a main primary where line voltage of 117/120 VAC was supplied, a secondary low voltage that was rectified and filtered to charge the sealed gel cells, and a third winding...use unsure. I am thinking of two possibilities....A stepup winding to get the voltage back up to line level when the mains drop out (which I realize would require the first, low voltage secondary to be switched to a 60 HZ battery driven 12 volt supply), or maybe as a isolation winding used as part of a line conditioner circuit.
I'm not seeing any usable markings on this transformer, so I guess I will need to power it up slowly and see what the other windings yield. The low voltage secondary is a massive wiresize winding. I estimate it's #8 solid (I can see part of it). Is there a prctical way to determine what the output current would be? Loading it on a rather large cooled resistor with an ammeter maybe? I'm reaching here! I'm hoping I can use it as an isolation transformer and a very solid low voltage high current supply.

Thoughts?

Tony
_________________
Tony

No trees were harmed creating this message, but several electrons were terribly inconvenienced.

My docter told me I had A.D.D. He said "It is a complex disorder, blah, blah, blah." I didn't pay attention to the rest.

[Mikeinkcmo]

Hi Tony,

Its possible the two windings could both be 115AC and the secondary used to charge the batteries. Depending on design I guess it could have been also used as the output tranny as well.

Do you have an isolation transformer now? A variac? An sine wave audio generator?

Just trying to see what you have available to test it.

Mike
_________________
Mike Harrison, KE0ZU.
Out in the country, not far from Jessy James' farm.

[GrayLox]

Mike,
I have a Tenma 72-1095 which serves as a variable/isolation power supply, but have no signal gen of any type, not a variac per se. Can I just power it up slowly and watch the output on a scope? can a ratio be calculated that way, or is is simpler to just meter the output and compare to the input I have meter or two for that.
I guess I'm trying to get a feel for the current I can expect from the low voltage winding as well....I'm pretty sure the "other" primary is 1:1 to the main input....want to be absolutely sure though.

Thanks,

Tony
_________________
Tony

No trees were harmed creating this message, but several electrons were terribly inconvenienced.

My docter told me I had A.D.D. He said "It is a complex disorder, blah, blah, blah." I didn't pay attention to the rest.

[Tom Herman]

[Norm Leal]

Hi Tony

Transformer may have been wound for 120/240 volt use? If so you will have two primary windings. This will make a nice isolation transformer. Use 1 primary winding for input, the other isolated output.
_________________
Norm

[Tom Herman]

[GrayLox]

I had thought about the possibility of dual voltage use, but I actually have this pulled transformer, and a complete (but dead) ups just like the one I pulled from. It is labeled only for 120 VAC 12 amps supply, but I suppose the transformer could have been used in other units. I'll attempt to track the circuit back in the one complete unit to see if the other windings just dead end somewhere, as though they may be an alternate 220 VAC input. I just thought that they may be using it to step up the battery voltage to 120 again when the UPS actually kicks in. In which case this would be a nice isolation unit plus a heavy low volt supply. There are no other transformers in the unit that I see doing that.
If I trace the suspected 120 VAC primary back through the circuit breaker and power switch and get to the power cable (and whatever sensing circuitry there is) I'll power the thing up with that winding and see what there is on both the secondary and the other "primary". The one I suspect of the real primary is colored black and white, so I'm fairly certain it is the correct primary for 120 VAC.

Got some errands to run this PM, so this evening I hope to spend some time with it. I'll post results. I have a few of these dead units, and they may be a good source for isolation transformers.

Thanks alot, guys!

Tony
_________________
Tony

No trees were harmed creating this message, but several electrons were terribly inconvenienced.

My docter told me I had A.D.D. He said "It is a complex disorder, blah, blah, blah." I didn't pay attention to the rest.

[GrayLox]

Results so far:

Applied 120VAC to primary, current with no load on either secondary was ~250ma.

First secondary winding (heavy winding) with no load measured 15 VAC.
Second secondary (tertiary?) with no load measured 15 VAC.

Now I am beginning to think the tertiary winding is merely a supply voltage for the board, and not as I thought (and hoped), a equal but separate 1:1 winding with the primary. So no isolation transformer here. Good heavy low voltage supply could be built based on this transformer I believe.

Now I need a way to progressively load the heavy secondary while monitoring the primary to see what current the secondary is capable of. The entire unit was labeled 12 amps, so I assume (dangerous word there) that as I load the secondary, apply 120 VAC to the primary while watching the primary draw, stop safely under 12 amps, and measure current through load of some type on the secondary. Now what to load the secondary with....any suggestions? Or am I going about this all wrong? Impedance test of secondary maybe?

I suppose this is still considered test equipment of some sort, as it will end up on my bench as a power supply of some type.

Thanks for all suggestions!

Tony
_________________
Tony

No trees were harmed creating this message, but several electrons were terribly inconvenienced.

My docter told me I had A.D.D. He said "It is a complex disorder, blah, blah, blah." I didn't pay attention to the rest.

[Tom Herman]

Hi Tony!

I suspect you're right! I think what happens is the unit converts 110 VAC to a bit over the amount necessary to charge the batteries. Then the batteries supply a separate inverter and give you 110 VAC...
If this is one of those inverters that feeds thorugh and runs all the time (i.e., the device is run from the transformer output and inverted, then the windings would take the full current.
This begs the question, if you had two identical transformers, you could wire one up normally to go 110 to low voltage, then hook the other one up backwards (low voltage to low voltage) and pull out 110 VAC and get isolation that way....

-Tom

[Tom Bavis]

If the secondary is self-leaded, the gauge should give you an idea. Usually about 500-750 circular mils per amp (c.m. = wire diameter in thousandths of an inch squared) Also, since it's the main secondary, you can assume it's most of the VA capacity of the transformer, which you can estimate from its size.

[Dean Huster]

[Chris108]

If you've got another dead UPS with an identical transformer that can be salvaged, you are all set.

As has already been suggested, simply connect the 15-volt secondaries back-to-back, and take your isolated 120-volt output from the primary of the second transformer.

The phase of the secondary windings is important; if one is flipped, it will oppose, and you'll get smoke instead of volts for an output. Therefore, the first time you bring this up, put a 60-watt to 100-watt incandescent lightbulb in series with the AC input. If it lights up noticably, the connections on one of the secondaries probably need to be flipped. If the bulb stays out, and you've got a measureable voltage on the primary of the "output" transformer, you are good to go after disconnecting the bulb.

This arrangement will be a tiny bit more lossy than a single transformer. The efficiency of modern transformers, even cheap ones, is quite good, like around 98%, but the first one still has to carry the losses in the second one. So it would be prudent to de-rate the output a few percent, e.g. if you've got 800 VA transformers, the load should not exceed 750 VA. This only holds if the two transformers are identical; if you use two dissimilar transformers, the one with the smaller VA capacity should be used as the "output," and it determines the largest load that can be handled.
_________________
Chris

"Engineering without numbers is just opinion."

[GrayLox]

[Dean Huster]

Phasing is the issue if you have more than one secondary and you're tying all like secondaries together to get the maximum possible magnetism in the core. So, if you have (for instance), an 8v secondary and a 12v secondary on each, if you hook one of them up with the wrong polarity, the transformers will eat each other up. Yum, yum.

So, with the light bulb in series and the 8v secondaries connected together, if you then connect the 12v secondaries together and the light bulb lights up, it means that you have to reverse the two connections of the 12v secondary. Reverse them and the light bulb should remain off. If there were three secondaries and you finally got the first two connected correctly (in phase), then you'd connect the third one of each together. Lit bulb, reverse the connection on the third secondary. "Dead" bulb, it's connected OK and you're good to go for an isolation transformer.

There are other ways to insure the correct connections, but this is probably the easiest and most-foolproof.
_________________
Dean, Electronics Curmudgeon
Contributing editor emeritus of the "Q&A" column in Poptronics magazine, R.I.P.

[GrayLox]

Ah, Dean I see where Chris was going. My plan was to use only the high current secondary windings to step the voltage back up to line voltage. I did properly describe the transformer as having 2 separate secondary windings, both producing 15 VAC, but one I is a small gauge wire and I was proposing that it was used to power the circuit board for other functions. Of course I can see that if both sets of secondary windings were connected, there would be a 50/50 chance of an out of phase magnetic field collapse, leading to problems. I was not planning on using the low current secondary windings as part of the step up circuit. I had actually considered using the tertiary 15 VAC winding from what I shall call the "feed" transformer to run a pancake fan in the enclosure I am planning on building to house this rig. I realize it will need to be rectified to run the common fan from a PC enclosure, but I actually have an 12 VAC fan in the junk somewhere......been a while, but I'll find it or build a rectifier circuit for the DC fan, no problem. A pilot light (15 V from the "fed" transformer?) and a meter....might make a cool package. If so, I'll post a pic.

Thanks all for the help!
_________________
Tony

No trees were harmed creating this message, but several electrons were terribly inconvenienced.

My docter told me I had A.D.D. He said "It is a complex disorder, blah, blah, blah." I didn't pay attention to the rest.

[Chris108]

Sorry if there was any confusion. My remarks about phasing the secondaries only apply if you connect two or three of them back-to-back. If only one is connected, it won't make any significant difference if the wires are flipped or not.

With two identical transformers, connecting all the respective secondaries together will give you the most load capacity. Leaving one or more disconnected reduces the overall capacity because there's that much less magnetization in the core of the second transformer. You can still draw low voltage AC power for fans, pilot lights, etc., from across the secondaries whether they are connected together or not.
_________________
Chris

"Engineering without numbers is just opinion."