# Help with transformer. Large surge currents

##### Ash
Feb 9, 2013
424
Hi guys I'm experimenting with power supplies and am trying to rectify 240VAC to a variety of voltages. The transformer has several output pins and two input pins. I have tried connecting the transformer on the primary side to AC, while testing the outputs.

I seem to be getting large surge currents in excess of 16A, I have an MOV in parallel to the supply and a fuse holder so I can keep it safe. Because I'm connecting no load on the secondary side, is this like shorting out the supply in the windings? How would I go about testing a transformer to find which voltages I can achieve on the multiple output pins?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
1) Don't kill yourself

2) Don't work alone.

3) Is it a 240V transformer?

4) How are you measuring these currents?

5) No, having no load on a transformer is like it being an open circuit (well, not exactly -- it's a pure inductive load) but it should draw no real power -- or very little.

##### Ash
Feb 9, 2013
424
Thanks for your concern, I'm very careful and always keep a fuse on the fixed breadboard that I don't need to hold while turning on the supply. I would never get cocky with electrics, I know there's always dangers.

It's definitely a 240V transformer, but one I recycled off an old circuit about a year ago. The currents are estimated because they managed to trip the 16A over-current device at the fuse board, so I know they're in excess of 16A.

So if the transformer is ideally acting as an open-circuit why am I getting what looks like a short-circuit?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
A fuse will protect lots of things, but humans and semiconductors are 2 things they won't protect.

If it is tripping the breaker, it is possible that the transformer has a shorted turn (exactly like a dead short). This is pretty much impossible to repair. It will also cause the fuse/breaker to blow every time you plug it in.

You can also have a magnetised core. This will result in increased currents for a short while until the core gets de-magnetised.

Try placing a light bulb in series with the transformer (100W should be fine). The light bulb should be VERY dim when no load is connected to the transformer. If it is bright (and stays bright) then your transformer is an ex-transformer.

##### Ash
Feb 9, 2013
424
Okay, that is a clever idea . I'll grab a spare bulb and try that in just a moment.

Thanks Steve, I'll post my results when I'm done.

##### Ash
Feb 9, 2013
424
The transformer is an ex-transformer. I connected the 100W bulb in series with the transformer after the fuse and it lit up brightly, it showed no sign of dimming after a few seconds.

I'm going to have to find another transformer, maybe I should repeat this test on the other two recycled transformers. Again it's a great way to test a transformer considering the alternative, I'll post again after I've tested the other transformers.

Thanks Steve.

#### BobK

Jan 5, 2010
7,682
Are you sure you have identified the primary correctly?

Bob

##### Ash
Feb 9, 2013
424
Hi Bob,

I think so, I'm assuming the primary side is the one with just two input pins on this specific transformer. But how can you identify them if they have multiple pins on both sides? By the size of the windings?

#### duke37

Jan 9, 2011
5,364
This can be very difficult. A low voltage transformer out of a radio may well have a primary with several taps for different mains voltages. Check how many windings you have with an analogue meter.
The secondary, possibly low voltage, would have thick wire.

Putting the mains on a low voltage winding would generate very large voltages. I know that 1500V can blow a meter !

You could put say 6V on a winding and measure the voltages on other windings.

##### Ash
Feb 9, 2013
424
I tested the other two transformers at 240V with a 100W bulb in series, both tests lit the bulb brightly aswell. I have tried putting 5V a set of windings in order to perform atest with my multimeter, but it overheats my 2A variable voltage supply just at 2V so had to switch it off.

Is it possible that all three of these transformers have a short-circuited? Would anyone like me to upload pictures of the transformer? It would be great to learn how to identify primary/secondary windings and test for desired voltages

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Yeah, post some photos.

Upload a picture of how you're wiring up your transformers for the testing too.

It's worth measuring the resistance of the various windings too. And the inductance if your meter has that function.

#### duke37

Jan 9, 2011
5,364
If your 2A power supply is DC, no wonder it overheated. Transformers work on AC and the right frequency for the transformer must be used.

The highest voltage winding may be the one with the highest resistance but this is not always true. Putting 5V AC into one winding and measuring the voltages on other windings will give some indication of turns ratio.

You can get very high voltages with unknown transformers so be very,very careful.

##### Ash
Feb 9, 2013
424
Sorry guys was quite busy yesterday and couldn't reply.

Yeah my 2A power supply is DC. Sadly I don't have a bench power supply, just my cheap 2A DC supply. My multimeter doesn't have an inductance function , I'm going to buy either a multimeter or bench supply in a few weeks because I need better equipment.

I measured the resistance between pins on each side of the transformer and got negligible values of about 0.6-0.8Ω and 1.2-1.3Ω. Not sure what this means, but I think it should be much higher.

Still here is some pictures of the transformer.

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#### duke37

Jan 9, 2011
5,364
There are all sorts of transformers, high frequency inverter, 100V line sound output etc

If it were a mains transformer I would expect the primary to have a resistance approaching 100 ohm.
If it is a mains transformer or audio transformer it will have a laminated silicon-iron core.
If it is a high frequency transformer it will have a ferrite core.

The copper band is to keep magnetic coupling either in or out so it could be from some audio equipment.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Here is an example of where that transformer is used

The easiest way is to search for "5220"

It's in the middle of the first circuit diagram (A!) in section 7. It's a transformer used in a switchmode power supply.

Yep, connect this to the mains and watch he sparks fly.

Finding this out tool a little googling at no risk to myself I searched for

Code:
ct425v "3128 138 39731"

I'll admit to a few false starts

##### Ash
Feb 9, 2013
424
Some important pointers there Duke, I'll need remember them for identifying more transformers in the future . The transformer is used around audio and video then, which explains the copper band.

Nice googling skills Steve,

Is there still hope in re-using this transformer? I can see they use a control circuit and lots of exclamation marks haha, beyond my knowledge of things. Sure helps me identify primary/secondary though

#### davenn

Moderator
Sep 5, 2009
14,040
.................Is there still hope in re-using this transformer? I can see they use a control circuit and lots of exclamation marks haha, beyond my knowledge of things. Sure helps me identify primary/secondary though

contrary to your earlier comments about 220VAC connection.... you can see from the PSU circuit diag in that service manual, that there is NO mains power connected directly to the transformer. Now that you have done that, you may have damaged the transformer by shorting turns. you may now have a "paper weight"

cheers
Dave

##### Ash
Feb 9, 2013
424
Now that you have done that, you may have damaged the transformer by shorting turns. you may now have a "paper weight"

The important thing is I've learnt alot during this thread, perhaps this will be avoided next time. If I can't find another, ill have to buy one.

Ashley

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