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Transformers Answered

Hi There!

I have here a transformer from an old UPS (Belkin 550VA), I just wondering if I can use it as a battery charger, my problem is I cannot locate the PCB connected to it. Available information I have here are in one side there are 3 wires Blue, Brown & Red on the other side Black, Red, Brown, yellow, Blue. It is also labeled on the top

GP 0751
Class B Viking B-2

Can someone help me with this little project?

Thanks in advance!



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1 year ago

i have this transformer in my ups, as it is not in use any more now, i want to use this transformer for any other project. i noticed that it has 5 wires, 1 blue, 2 x white which are connected inside transformer's insulation and 2 x black which are also connected inside transformer's insulation. then i measured the resistance between these terminals, which are as below

both white have 0 resistance,
both black have 0 resistance,
both white and both black wires have 0 resistance between them,
blue to both white and blue to both black wires have resistance of 9 ohm equal.

what further i notices without damaging transformers protective insulation, that blue wire is connected to a copper wire which is thinner then the wire connected with both black and white wires.

the thing i concluded but not yet tested at my test bench is this transformer is a multi tap type. it's blue/thinner copper wire which is most probably primary wire is also connected to the white/thicker copper wire which is further end at black. Black and white wires are connected to secondary winding but white wire is also connected to the end of primary wire which is not theoretically/ logically understandable for me.

any body to answer me, please reply. thanx


6 years ago

Please ask me Vin and Vout of this transformor


Reply 5 years ago

Ok, i'll bite. Whats the Vin and Vout of that transformer? ;)


9 years ago

If you can't find any information on this transformer by doing google searches, then try this procedure.

WARNING - You will be using potentially leathal voltages and can possibly cause fire. If you don't understand what you are doing , don't know how to use a multimeter, don't know how to wire basic circuits, don't know how to wire mains voltages safely, DO NOT go any further, or find someone who does. Read all the instructions before you do anything. If anything doesn't make sense, either instructions or measurements, stop, rethink and ask for advice.

DISCLAIMER - Sorry about this but I gotta say it. I will not be held responsible if you injure or kill yourself or others, or cause damage of any sort to your or others property by following the instructions below. You proceed at your own risk.

I'd guess that the side with the 3 wires would of been connected to the mains. The primary (mains) is probably 1 winding with a center tap. To check this I would use a multimeter to check the resistance between the wires. The pair with the highest resistance will be the ends of the winding. The resistance between the 3rd wire and each of the ends should be half, this is the centre tap.

The other side is probably the secondary (output) side. Since there are 5 wires, I'd guess that you have either one winding with multiple taps OR 1 center tap winding + 1 standard winding. You can check this using a multimeter.

If it's a multi-tap then all the wires will show some low resistance between them. If it is the other arangement you'll get 3 wires showing low resistance between them and the other 2 showing low resistance between them but not to the other 3.

Note: the resistances on the primary side will probably be in the 100s to 1000s ohms. The secondary resistances will be very low, possibly even under 1 ohm, so they may look like shorts depending on your multi-meter.

WARNING - If the secondary windings have a higher resistance than the primary winding, we've got the windings the wrong way round or you have a step-up or high voltage transformer. If so STOP as these instructions won't work with this type of transformer.

As for checking the output voltages you can do a low voltage check first and then try full mains or do the full mains only.

BASIC SAFETY - When doing any tests with any live voltages, especially mains, please make sure that all wires are terminated and/or insulated. You can use an insulated terminal block or some other safe method to hold the wires for measurement. There should not be any loose wires that you can aciddently come into contact with. Do not touch any wires with bare hands when the power is connected, always disconnect power at the plug when you need move wires. Please be careful.

For the low voltage check you need an other transformer or AC plug pack of around 10 V AC (9-16V) at 1-2 Amps. Connect this transformer output to the primary wires. For 220-250V countries the end wires only, don't use the center tap wire just insulate it. For 110-120V countries join the two end wires and connect the transformer to the center tap and the joined end wires. Use a fuse between the low voltage transformer and your transformer, of the same rating or lower as the transformer ie 2 Amp transformer use 1-2 Amp fuse..

Then you can measure the voltages from the secondary windings and then calculate the true output voltages. True Voltage = measured voltage x(Mains/test voltage).

eg using a 10VAC test transformer, the mains is 110 VAC, output voltage is 1.09 VAC, Vt=1.09 x (110/10) = 11.99V (12V).

Note: measure the test voltage on the primary side as well as most transformers will not produce exactly their marked voltage. A 10VAC transformer can drop to 9 VAC or even 8 VAC. All measurements must be done on the VAC mode on the multimeter.

The full mains test is the same as the low voltage test but without the low voltage transformer. You connect the mains to the primary windings as for the low voltage test with a fuse (1Amp or less). For 220-250V countries the end wires only, don't use the center tap wire just insulate it. For 110-120V countries join the two end wires and connect to the center tap and the joined end wires. You can then measure the output voltages to get the real life voltages.

None of this will tell you how much current you can draw from the secondary. This is dependent on the size and construction of the transformer, and the thickness of the wires on the output side. Generally bigger transformer = more power, thicker wire = more current. I guess you need to try it in your circuit and see if it gets too hot.  The transformer should be warm to the touch, not hot.  Again use a fuse in case something goes wrong.

I hope this helps.


9 years ago

The wires, as you may have already ascertained are on one side connected with the center one being a center tap. 

A quick and dirty transformer tester  (testing mostly for shorts) can be put together by building a circuit that takes low amperage dc (from a battery for instance) and converts it to low voltage/amperage AC. 

Running this though one side of the transformer, with a volt/ohm meter on the other side, should give you an approximate voltage ratio from one side the other  (step up,  step down, or simply an isolation transformer). 

CAUTION:   if there is a tremendous step up,  (or a large step down, wired the wrong way) you could end up with lethal output voltage.  Take all precaution NOT to get electrocuted.