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Help Identifying unknown Transformer Answered


I got this transformer from an old ups, there are four wires on the primary side black, blue, green , yellow and two on the secondary red and blue. This transformer used to charge 12v 7.5ah battery also converts it back to 220V in backup mode.
Tried testing its output and got this

(Mains 220V)
Black to ground
Blue to mains-> 7.9v on secondary

Connecting Blue and Green to mains and it was a shot circuit and blown my fuse. I want to use this for making a decent power supply 12v+ output, is this possible?

I also saw this video https://www.youtube.com/watch?v=mc4GZM-kFUY&hd=1
The guy is using his ups transformer for generating hv to power cfl using tip122, i am not sure how to do this using my transformer.

I need to make use of this transformer somehow, otherwise its just eating dust at my home.


Well, i just plugged it with bridge rectifier and 1000u cap and its working ok now.

When i connect Black to ground and blue to phase i get 12v

yellow to phase i get 8v

green to phase i get 7v

Even tested with voltage doubler, 24 volts stable.

Yeah... I have plenty of power supplies now for my projects. At least this transformer is being used now instead of eating dust.

Can i use that transformer as a center tape transformer 12-0-12

I have the same problem with the same type of transformer

It is taken from MERCURY UPS but the transformer has an other part no.


So please help me to find out how to use it

Thanks in advance

Well, there is another problem, even though i connected it to the bridge rectifier and capacitor, this big and bulky transformer is only good for small things which works on low current. When i hooked it up to my 12v cfl driver, it didn't even blink. Even though the output from the transformer was 12v, there was a huge voltage drop upon connecting it to the cfl circuit, almost 5v decreased, 12 to 8v. It all suggests that this transformer is of very low amps... How do u think i should deal with this. Is there any way i can increase its current capabilities?

You can't change the power output except you want to rewire the transformer.

First it would make sense to measure the resistance between the different wires.

Since it will be low you need a proper Ohm meter or use a fixed DC current and a voltmeter.

It it really was used to get the 220V back I would be surprised, so please check if the outlets of the UPS are connected to the transformer more or less directly.

Reason I doubt it is because it would be a pretty weird ciruit to do the job.

Once you got the resistance sorted you will know what wires can be used for a proper test.

IMHO the safest way is to feed 12 AC into the secondary and measure what comes out the other end - might be better to use something that can handle the load or to put a resistor between source and transformer to limit the current.

I doubt that would be safer--there would still be upwards of 220V on the primary. Safer yet would be to connect a 48V transformer to the primary, then multiply the secondary voltages by 4.5 or 5 to equate to 220 or 240V on the primary coil.

Frankly, if done correctly, it's OK to test with full line voltage. That means connecting with wire-nuts or terminal blocks and likewise covering any exposed wire ends. No wires hanging loose. No fiddling with wires or meter unless the primary is unplugged. Respecting the high voltage-- I.E., not touching anything when powered.

Even when using a low-voltage transformer to replace the line voltage it's still connected somewhere to the mains...and that can be just as much a danger, depending on how it's done.

His problem is that needs to identify the correct connections for the primary first, so no real use of feeding this side for a test IMHO.

And safety should not be an issue:

1. The current will be minimal.

2. It is FOR TESTING, and I see no bigger risk in testing the output of a transformer than to do the same on your power outlet.

3. If someone messes with mains voltage I have to aussume he/she knows that rules and dangers and is smart enough not to touch the bare wires/contact with the hands.

Oh, your way will work. I only take issue with "the safest way" statement.

1) No, not really. Not into a short, human or otherwise. Maybe if we had a tiny 12V transformer that saturates at 5mA. Or if we had the body mass of an elephant, then the inherent current-limiting character of the "chained" transformers might be demonstrably safer...

But this transformer has beefy secondary wires, and reversed-wired with a decent sized 12V transformer (2 amps is not uncommon), the primary could supply plenty of current. Probably in excess of 100mA @ 220V, and that's way deadly.

2) Totally with you on that.

3) I don't think you can stress safety enough. I won't assume safe practices--I get lots of PMs about the HV in tube gear, and always include a safety "lecture." Makes me feel better, anyways.

If you feed 12V at 100mA into the transformer and assume a near perfect transformer for the ease of calculating you end up with around 5mA on the primary @ 230V.

Since the transformer won't be perfect it will be less.

Wire gauges don't matter here, only what goes in and the ratio of the windings.

And if you use a 12V, 500mA power adaptor like we all have around for our toys to plug into the next socket you would be surprised how little comes out of the other transformer as the feeding one will break down in voltage due to the extrem low resistance of th beefy secondary.

But I agree, safety third!

Rule 1. Check rule number 3!
Rule 2. In case you missed first, skip directly to the next rule!

Rule 3. Never ever assume it is fine and nothing will happen! Safety is a must and all rules and regulations in your country/state of residence must be followed!

You don't "feed" amperes--you "feed" volts. Amperes are drawn by the load, up to as many amps as the inductors can provide.

I stand by that comment, where I referenced a 2A 12V transformer. Connecting a 2A 12V transformer (very common) to the secondary of the other transformer could easily supply 100mA of current at the primary, at approx 220V.

My 5mA 12V transformer example was a little joke; a theoretical way to limit current. I don't think such a tiny power transformer exists, off the shelf. If we had one, maybe it would work. Or, it might fail in our circumstance, which might lead to an internal short, and that could have other consequences.

"And if you use a 12V, 500mA power adaptor like we all have around for our toys to plug into the next socket you would be surprised how little comes out of the other transformer as the feeding one will break down in voltage due to the extrem low resistance of th beefy secondary"

Don't confuse DC resistance with impedance @ 60Hz.

And are your saying your test approach can't work? No, I think it will. Check out the Real McTube II by Fred Nachbour. As Fred says: "Larger units (transformers) will work just fine..." Your voltage losses won't be significant. Not enough to consider this "safer."


3 years ago

Don't assume black is common for transformer wiring colors, or AC in general (in the US, white is neutral and black is "hot"). Still other variations on the primary to try (carefully).