Step down PT test

[AlNewman]

So I have this old transformer that I was thinking I could find a use for.

When I tested at wall voltage through my GFCI outlet @123.5v Ac over the higher resistance leads, I got a consistent 28 ish volts.  So around 4.4:1 winding ratio.  I then switched leads to confirm around 520v Ac, and it popped my GFCI plug.

So I tried my LBL, and through one winding it lit up, and through the other, no glow at all.

Is this transformer only designed as a step down transformer, or is it hooped?  With no current, I would assume I could test both readings, but maybe I'm wrong... 

Thanks

[shooter]

did you ohm the primary windings for open?
secondary for open?


find a LOW volt AC wall wart to feed into the secondary so the primary comes out ~~~120vac


unless it's a one off transformer, step down works stepping up, on a GFI circuit, all bets are off though

[AlNewman]

Yes, I tested both windings, one read 0.4R, the other 2.9R.  There was a connector on the end, and in removing that, I did find a broken solder joint, but it didn't help or change anything much if at all. 

I plugged it into my ammeter on the multimeter, and it's drawing .144A with no load, so I'm thinking there's a problem.  I haven't dug through my stash of AC wall warts yet, but I don't have much hope.

It doesn't smell funny, and it is built with old 40's or 30's type wire, so I'm hoping a minor surgery may find the problem?  Or, it might make a good paperweight.  Kinda strange though, in all the old decrepit power transformers I've found, this is the first I've seen with a problem.

[sluckey]

I then switched leads to confirm around 520v Ac, and it popped my GFCI plug.

You can't do that!

[AlNewman]

[/quote]You can't do that!
[/quote]

Ok, I'm listening.  Why?

[sluckey]

That 28V secondary is meant for high current. That's why the DCR is only 0.4Ω. You basically put a dead short across the 120VAC house circuit!

[shooter]

plugged it into my ammeter on the multimeter, and it's drawing .144A with no load

in series with Primary, or secondary lead??

[AlNewman]

That 28V secondary is meant for high current. That's why the DCR is only 0.4Ω. You basically put a dead short across the 120VAC house circuit!

Soooo, in that case, theoretically it should be a good transformer?  I just did stupid shit?

Except for the 144 mA current... is that normal?

[AlNewman]

plugged it into my ammeter on the multimeter, and it's drawing .144A with no load

in series with Primary, or secondary lead??

Well, I guess it was in series with the primary lead.  Would be the 2.9R winding to wall voltage.

[WimWalther]

Would be the 2.9R winding (...)

2R9 .. Remember, the "multiplier" is the decimal point in this labeling system. So 1.5K is 1K5 and 0.015 Ohm is R015, etc.

Btw, the reason your xfmr pops the breaker when you try to energize the secondary, is that the sec. lacks sufficient inductance to behave as a primary.. so it pulls too much current

An xmfr primary needs sufficient inductance so that it keeps the quiescent current (the "magnetizing current") low enough to not waste power & heat (and core saturation..), but high enough to produce a field that fully energizes the secondary.

[AlNewman]

2R9 .. Remember, the "multiplier" is the decimal point in this labeling system. So 1.5K is 1K5 and 0.015 Ohm is R015, etc.

Btw, the reason your xfmr pops the breaker when you try to energize the secondary, is that the sec. lacks sufficient inductance to behave as a primary.. so it pulls too much current

An xmfr primary needs sufficient inductance so that it keeps the quiescent current (the "magnetizing current") low enough to not waste power & heat (and core saturation..), but high enough to produce a field that fully energizes the secondary.

Thanks.  So must be the primary coil that's drawing current, even without a load?
This would make a good low voltage supply.  What sort of current could I expect on the secondary at 7.5 volt amps?

[sluckey]

Thanks.  So must be the primary coil that's drawing current, even without a load?

yes.

This would make a good low voltage supply.  What sort of current could I expect on the secondary at 7.5 volt amps?

Do the math. 7.5VA/28V = 0.27A. That's very low current and not very useful. Why are you asking about 7.5VA?

[AlNewman]

Do the math. 7.5VA/28V = 0.27A. That's very low current and not very useful. Why are you asking about 7.5VA?

That's what's written on the transformer.  I wasn't sure if the VA was a direct equation across the windings or not.  So rectified, 30ish volts is probably not very useful, but as a variable power supply, 5v would be 1.5A, 6v 1.25A, 12V .625A.

It could work well to test different things.  I wasn't sure it was a step down transformer at all until you gave me a heads up about the low resistance on the winding, so thanks for that. 

I've had my eye on different power supplies lately, because there's often low voltage circuits that otherwise are very difficult to test without a regulated supply. 

Sure, I could spend $50 and get exactly what I need, but why do that when I can spend $50 and build something that may not work in all circumstances?  This is why it's fun.

[sluckey]

Post a hi-rez pic of the transformer. Include a ruler for a size reference. Show all the writing on the transformer.

[AlNewman]

Not sure how much more high rez my pictures can get here.

It is type 167060.
Volts-115
Cycles-60
V.A.-7.5

It measures approximately 4" high, 3" wide, and 3.5" deep.

[AlNewman]

Probably weighs about 2 lbs

[WimWalther]

Thanks.  So must be the primary coil that's drawing current, even without a load?

I'm not clear on what you're terming the 'primary' in this case..

But the point is that the winding you're trying to use as the primary (actually, a low-voltage secondary by design) lacks the required characteristics to work as a primary winding.

The other winding (the design primary) has nothing to do with this. It's all down to the fact that the winding you're attempting to use as the primary isn't suitable for it.

[PRR]

> But the point is that

Specifically: you can't push (much) more voltage at a winding than it was designed for. The iron "saturates", the magnetic domains crumple, inductance falls to 1/1000th of intended, the thing flows AC current like a 30" sewer on my 2-toilet house, GUSH.

Yeah, you can put 7V on a "6V" winding. But 120V on a "6V" winding is going to blow or melt something.

I am reminded of getting a 120V:30V transformer, and doing it wrong. When I hit the switch the room lights dimmed.

[AlNewman]

I'm not clear on what you're terming the 'primary' in this case..

But the point is that the winding you're trying to use as the primary (actually, a low-voltage secondary by design) lacks the required characteristics to work as a primary winding.

The other winding (the design primary) has nothing to do with this. It's all down to the fact that the winding you're attempting to use as the primary isn't suitable for it.

So in this case, I would call the primary the one that works at 120V.  So the 2r9 winding.

Specifically: you can't push (much) more voltage at a winding than it was designed for. The iron "saturates", the magnetic domains crumple, inductance falls to 1/1000th of intended, the thing flows AC current like a 30" sewer on my 2-toilet house, GUSH.

Yeah, you can put 7V on a "6V" winding. But 120V on a "6V" winding is going to blow or melt something.

I am reminded of getting a 120V:30V transformer, and doing it wrong. When I hit the switch the room lights dimmed.

Well, thanks for that.  It means I'm not the only one who's done silly things.
In my defense, I've never went out in a thunderstorm with a kite.