A couple of transformer questions

[ShinOBIWAN]

Hey folks

I'm in the process of putting together an amplifier and though it would be a neat idea to extend the secondary lead outs on my 45-0-45v transfomer and then heatshrink them to keep things tidy. Problem is that I did the heatshrink and then realised I hadn't colour coded the ends of the lead out extensions so now have no idea which windings are what nor which lead is the start or the end of the winding so as to keep the two secondaries in phase.

I have a DMM but don't really know 100% how to diagnose which leads are what and would greatly appreciate it if someone could explain how to identify them.

These are the markings on the transformers:



One last thing, which wires should be placed in the center tap? Is it the red and yellow or black and orange? I don't think it matters does it?

[Jan Dupont]

Last thin first...
IT DOES MATTER which wires to connect to make a center tab !!

Here is what I would do;
Connect two wires as for center tap.
Power up the transformer
Measure between one free end and the center.
Do the same for the other free end.
If the above measures correct, then measure between the two free end.
If it doesn't measure correct, then change one of the wires in the center tab and start measurements again

[AndrewT]

Hi,
screw the end of each winding into separate lugs of a terminal strip.
Connect up the primaries to the mains plug.
Connect the mains plug to the mains light bulb tester you keep specially for these occasions.
Connect any two secondary terminals together with a link.
Stand back and connect to the mains wall socket and switch on.
If the bulb is lit the primaries are wrong.
If the bulb is out then no damage.
If the bulb glows, one of the secondaries are shorted, be quick because the transformer is heating up. Switch off at the mains wall socket, or safer, pull the plug out.
Move one end of the link to another empty secondary winding lug.

Once the bulb is unlit, measure the secondary voltage between the two unconnected ends.
If it reads double voltage then the two connected ends are the centre tap. Mark them 2 & 3.
disconnect from the mains.

If the voltage reads near zero, then the secondaries are connected out of phase. move one end to the other empty secondary lug. switch back on. is the bulb unlit or glowing?
sort it like before.
Disconnect from the mains.
Remove the secondary link.
Set your DMM to ohms. measure the resistance of the secondaries. The end that reads near zero ohms to 2 is labelled 1. the end that reads near zero ohms to 3, is labelled 4.

For a single bridge rectifier connect 2&3 to common on the smoothing caps. connect 1 & 4 to ~ & ~ on the rectifier

for a dual rectifier. connect 1 & 2 to ~~ on first rectifier and 3 & 4 to ~~ on the second rectifier. the + on one rectifier connects to the - on the other rectifier and this goes to power ground.

[ShinOBIWAN]

Thanks Jan and Andrew.

I was following Jans advice but then you posted Andrew and it seemed a bit more indepth(which I need because I'm near useless with this stuff). Anyway I've identified the correct wires and all is OK. I did get a bit of scare when I'd connect the wires up wrong and the transformer started to buzz, I disconnected quickly but was surprised that it had become warm in such a short space of time.

If you could just indulge one last noob question please:

Originally posted by AndrewT
[B]For a single bridge rectifier connect 2&3 to common on the smoothing caps. connect 1 & 4 to ~ & ~ on the rectifier

In the diagram on the first post, which wires would be 1, 2, 3 and 4?

Once again thanks
Ant

[AndrewT]

You're yet another that chances their arm with mains voltage.

I had never heard of this mains light bulb trick till I joined this Forum.
I never work without it now.

Starting at either bottom or from the top they are 1,2,3,4 sequentially.

If it was starting to buzz I guess you had a shorted secondary. Hundreds of amps would be flowing. Probably drawing five to ten times rated VA while you were p..issing about (equivalent to go to the loo).
Now where's the back of your hand? You need a skelp! (for our non Scots skelp = slap).

[ShinOBIWAN]

Originally posted by AndrewT I had never heard of this mains light bulb trick till I joined this Forum.
I never work without it now.

I did plug a light into the socket next to the transformer - it just stayed lit.

Seriously though, I'd have to agree with this:

You need a skelp!

I shouldn't be messing with this stuff really because I know so little but...

[AndrewT]

Originally posted by ShinOBIWAN
I did plug a light into the socket next to the transformer - it just stayed lit.

the modified equipment is fed through the light bulb circuit.
Do some reading.

[ShinOBIWAN]

Originally posted by AndrewT
the modified equipment is fed through the light bulb circuit.
Do some reading.

That was said in embarrased jest hence the smilies. I neglected the light bulb and adopted the usual ignorant stance of "Nah, won't happen to me and I'll save 5 minutes from not having to hook it up!"

[djk]

"I disconnected quickly but was surprised that it had become warm in such a short space of time."

The risk you run without a lightbulb.

The transformer is damaged, I would buy another.

Even if everything seems OK now, it may not be. I had an accident about 20 years ago with a 625VA toroid. I was using a variac and running the voltage up. I got a small buzz for a very brief instant. It was enough to damage the tape over the core and the enamel insulation of the wire on the layer next to the core. Even if it works OK now, there may be damage to the insulation that you cannot see. I still have that transformer and keep it as an object lesson. It seems OK, but has one shorted turn on the primary.

[ShinOBIWAN]

Originally posted by djk
"I disconnected quickly but was surprised that it had become warm in such a short space of time."

The risk you run without a lightbulb.

The transformer is damaged, I would buy another.

Even if everything seems OK now, it may not be. I had an accident about 20 years ago with a 625VA toroid. I was using a variac and running the voltage up. I got a small buzz for a very brief instant. It was enough to damage the tape over the core and the enamel insulation of the wire on the layer next to the core. Even if it works OK now, there may be damage to the insulation that you cannot see. I still have that transformer and keep it as an object lesson. It seems OK, but has one shorted turn on the primary.

I bought a load of these on the cheap because they're non ROHS. I don't like the sound of what your telling so rather than risk fate and potentially cause untold damage I'm going to swap it for another.