Power Transformer - Am I Doing Something Stupid?

[R.G.]

Here is how to best test for a shorted Pt.
On 120 volt wall power if you read over .4 volts on the meter it’s toast, I guess twice that .4 if your on 240 volt power.

I'm confused. Wouldn't that just pop that fuse??
1 ohm would only limit current to 120 amps or 240 amps if the primary was shorted, so wouldn't the fuse pop before you could read a voltage?
I can see it's a good way to measure normal primary current if your meter doesn't have an AC current setting, though.

[Stevem]

For testing secondary shorts.

[R.G.]

Ah. I understand better. You're right, a secondary short will not always pop a reasonable sized primary fuse. I've had to discuss that point with others before. Thanks!

[CraigGa]

So I got to do all the tests today,

I found absolutely no contact between any of the windings or any winding and the frame so I tried Stevem's resistor test.

I had an 8 ohm power resistor so I tried that first and with my meter on peak hold I measured 11v, the T250ma fuse blew.
I then tried with 1 Ohm (only 1/2W though) and measured 0.53v before the resistor and fuse blew.

I guess that by Stevem's definition that means it's toast.

As one last test I connected my 10v wall wart to the primary (fed through the limiter)

I've attached a picture of the results, the numbers in red are my measurements x24 (UK Mains/Wall wart voltage)

Transformer.jpg

It looks pretty conclusive that there's a problem with the lower half of the winding.

Craig

[R.G.]

Does it pass the shorted-turns test with a battery and an NE-2?

[CraigGa]

Does it pass the shorted-turns test with a battery and an NE-2?

Using this method I get a tiny flash across the primary but nothing across an of the secondaries.

xform_test.gif

[Stevem]

Let me make it very easy here, if with a 1/4 amp fast blow fuse ( or a fuse of half the amperage it should be ) on the primary side and with all the secondary's unhooked that fuse blows, then without question it's bad!

You might pull the bells off each end to check for cut thru wires and shorts in there, but other then that it's toast!

[CraigGa]

Thanks Stevem,
I'd come to the same conclusion so I've fired off an email this morning.

Craig

[R.G.]

Yep, it's toast. I agree with Stevem - If it pops a 1/4A fuse with secondaries unloaded, there is an internal problem. The results you note on the pulse-inductance test indicate that as well.

[CraigGa]

They've offered to take it back and test it so no problem,
I'll let you know how it goes.

Craig

[martin manning]

For comparison I pulled a known good power transformer off the shelf and ran the neon bulb test and an unloaded AC current test.

Does it pass the shorted-turns test with a battery and an NE-2?

Using this method I get a tiny flash across the primary but nothing across an of the secondaries.

Using a hardware store AC line voltage tester (which is a neon bulb with a resistor in series) I was able to get a good flash on the primary and on the the HV secondary, but nothing from the 5V or 6V filament windings. Power was from bench supply set at 9VDC and 1A.

Let me make it very easy here, if with a 1/4 amp fast blow fuse ( or a fuse of half the amperage it should be ) on the primary side and with all the secondary's unhooked that fuse blows, then without question it's bad!

This test does not seem reliable. With 120VAC connected to the primary I see 330mA current draw with nothing connected to any of the secondaries.

They've offered to take it back and test it so no problem,
I'll let you know how it goes.

Please do, that will be interesting.

[R.G.]

Using a hardware store AC line voltage tester (which is a neon bulb with a resistor in series) I was able to get a good flash on the primary and on the the HV secondary, but nothing from the 5V or 6V filament windings. Power was from bench supply set at 9VDC and 1A.

As always, there are a string of details on any test. Would you mind running that test again, leaving the hardware store neon across the primaries? I suspect that loading and opening the 5V and 6V windings will cause a flash then.
The asterisks and footnotes matter here. If you have a 120vac primary and a 6V secondary, there's a 20:1 ratio of voltages in the windings. The pulse inductance test loads up the core with the DC current times the number of turns, and then when you interrupt the current, the core tries to force the same current to keep flowing. All turns inside the windings experience the same volts per turn even on inductive discharge, so the voltage on the primary is still 20X the voltage on a 6V winding. If the voltage to be observed is the break over voltage of an NE2 or about 90V, then the primary is going to show 1800V. There are some windings where the self capacitance can damp that down enough to keep a neon from breaking over. Leaving the neon on a high voltage winding will give the same go-no-go test, as an internal short clamps all the turns to the same low voltage as well. So the neon would flash if left on the primary.
Actually, now that I'm thinking about it, you don't have to test all the windings. Just the primary or HT will do. An internal short ought to clamp all the turns to the same voltage, so a single test on the primary or ht will tell you whether you have a shorted turn.

I do wonder if the hardware store line tester is actually a neon these days. I bet it's cheaper to do an LED plus something or other than to secure a reliable supply of neons. I haven't tried to buy NE2s in over a decade. Still working on my last baggie of them.

[martin manning]

Actually, now that I'm thinking about it, you don't have to test all the windings. Just the primary or HT will do. An internal short ought to clamp all the turns to the same voltage, so a single test on the primary or ht will tell you whether you have a shorted turn.

I do wonder if the hardware store line tester is actually a neon these days.

As I understand the neon bulb test, if one can get a flash on any winding, then there are no shorted turns on any winding.

The voltage tester is indeed a neon bulb and a resistor, which I determined to be a 220k by curve tracing.

[martin manning]

Would you mind running that test again, leaving the hardware store neon across the primaries? I suspect that loading and opening the 5V and 6V windings will cause a flash then.

Should and does, with a bright flash. This is akin to an IC engine battery-coil ignition.

I was trying to replicate the OP's test:

Does it pass the shorted-turns test with a battery and an NE-2?

Using this method I get a tiny flash across the primary but nothing across an of the secondaries.

That result was a bit ambiguous. I would say passing is a good strong flash on either the primary or the HV secondary.

An unloaded primary current test is more exact, but I don't know how much to expect for different transformers. Another possible indicator is open secondary power factor, which in my case was 0.16. If that gets high, it would indicate a failed transformer. A small 12V CT transformer I checked draws 62 mA at PF of 0.32.

[CraigGa]

My replacement transformer just arrived!
It's been on test for 24 hours at the manufacturer so hopefully I'll have better luck this time.

Craig