Power Transformer - Am I Doing Something Stupid?

[CraigGa]

As I've mentioned elsewhere, I'm on with my first build and I have had issues with getting the correct transformers previously but I was eventually sent the correct ones.

This afternoon I completed the mains to PT primary wiring so I decided to test it by measuring the secondary voltages.
None of the secondary wires are terminated anywhere yet and they are insulated in individual Wago connectors except the HT centre tap which is grounded.

I applied power through my limiter with a meter on the 5V heater tap and measured 4.9V so I flicked the bypass on my limiter and it went to 5V exactly - good start.
I switched it off and put my meter on the 6.3V tap, this time there was nothing, the mains fuse had blown and I must have left my limiter bypassed for that to blow?

Anyway new fuse and now my limiter is lighting up bright everytime, my plug in power monitor says it's drawing 230mA or 54W through a 60W bulb.

I've tried the neon and battery test and that works
The primary measures 7.1 Ohm DC, The heater secondaries are around 0.2 Ohm but pass the battery test and the HT's are 45 Ohm to the CT.

Am I doing something wrong here?

Thanks in advance
Craig

[Stevem]

If you lift the 6.3 volt heater ground are you still seeing the bulb light up brite and pulling big time current?

What color is that heater ground wire by the way?

[CraigGa]

There isn't a heater ground, just an ht ct ground.
The heaters are orange 5V and yellow 6.3V

I could try lifting the HT CT ground?

Craig

[R.G.]

Have you tested resistance from winding to winding? Should be near-infinite. How about resistance from each winding to the core or mounting bolts?

Does the transformer blow fuses with the secondary winding open?

[CraigGa]

Have you tested resistance from winding to winding? Should be near-infinite. How about resistance from each winding to the core or mounting bolts?

Does the transformer blow fuses with the secondary winding open?

Thanks R.G

I've not tested those, I'll do that and report back.
I've not let it blow another fuse but it lights my limiter with no secondaries connected except the HT CT to ground

I guess I'm going to have to disconnect that to prove it.

Craig

[CraigGa]

It did work at first though, how likely is it that something failed after ten seconds with no load on it?

Craig

[blackeye]

Craig, sorry about the troubles with the PT. Which make and model are you working with?

[CraigGa]

It was wound to my spec

320-278-0-278-320V and 5V and 6.3V heaters

Craig

[blackeye]

It was wound to my spec

320-278-0-278-320V and 5V and 6.3V heaters

Craig

Ah, I see. It has only one set of primary leads then?

[CraigGa]

Ah, I see. It has only one set of primary leads then?

Yes, one 240V primary

[R.G.]

I've not tested those, I'll do that and report back.
I've not let it blow another fuse but it lights my limiter with no secondaries connected except the HT CT to ground

I guess I'm going to have to disconnect that to prove it.

Mother Nature is whispering at us. AC mains frequency currents can only flow at high currents through resistive connections to any significant extent. If the limiter glows full with only the secondary CT connected, then there must be a current path from the primary wires to the secondary CT. No other way for current to flow (discounting wiring mistakes, etc that you've already checked for). I suspect that an ohmmeter reading from the primary to secondary CT will show low ohms.

In one of your posts you ask if it's possible that it could fail after only about 10 seconds - well, yes, it can. Not the most likely thing, but possible.

[blackeye]

How about resistance from each winding to the core or mounting bolts?

If there is somehow a short between the HT secondary and core or mounting bolts, be careful when you do the test with the HT CT un-grounded! I wouldn't want you to become the ground path. Definitely keep that limiter in use. Good luck... It doesn't sound like you are doing anything wrong to me.

[martin manning]

This afternoon I completed the mains to PT primary wiring so I decided to test it by measuring the secondary voltages.
None of the secondary wires are terminated anywhere yet and they are insulated in individual Wago connectors except the HT centre tap which is grounded.

I applied power through my limiter with a meter on the 5V heater tap and measured 4.9V so I flicked the bypass on my limiter and it went to 5V exactly - good start.

Everything was hunky dory at this point.

I switched it off and put my meter on the 6.3V tap, this time there was nothing, the mains fuse had blown and I must have left my limiter bypassed for that to blow?

Anyway new fuse and now my limiter is lighting up bright everytime, my plug in power monitor says it's drawing 230mA or 54W through a 60W bulb.

It was working fine on full line voltage before you moved the meter connection. Is it possible that some other leads had shorted together?

[CraigGa]

Thanks guys,

I'll do all those tests tomorrow, today after no gigs for 19 month my band has 2 in a day so it's a going to be a bit full on but to answer Martin, no there's absolutely no chance that I shorted anything out, that's why I'd put one of theses on every secondary wire,

Wago.jpg

Craig

[Stevem]

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.