Testing Open Primary Power Transformers

[FrankB]

Here is a service tip I have had great success with in the past when I had my own servicing business.

I used to get in consumer electronics with open primaries in them due to lightening strikes or the PUD (Public Utilities District) making an "Oops" (And doing things like dropping a live 10KV line on a 220V house feed).

I would take a transformer, like a filament transformer, and back feed voltage into the secondary of the transformer with an open primary. Normally, I fed it into the 6.3V filament or pilot light winding.

This will allow you to make some very quick tests to see if anything else was "fried" during the power surge, lightning strike or whatever.

I do not recommend running the set like this for long, but its a good enough quick test that you can see if anything in the power supply or output stages was damaged. In this way, I was able to give the customer a quick estimate for the repair of the set.

Also on many of the Asian imported sets, they had a fuse in the primary under the outside covering of the power transformer, as they wound the primary as the outside winding.
If one was lucky, they would have the fuse readily accessible just by opening up the covering on the transformer. One could then wire in a standard fuse and holder, re-insulate the opened up area, and make the set work and safe to use again. If the primary was wound close to the core and the fuse inaccessible, a new transformer was needed.

[rswanborough]

Originally Posted by FrankB

... Also on many of the Asian imported sets, they had a fuse in the primary under the outside covering of the power transformer, as they wound the primary as the outside winding.
If one was lucky, they would have the fuse readily accessible just by opening up the covering on the transformer. One could then wire in a standard fuse and holder, re-insulate the opened up area, and make the set work and safe to use again. If the primary was wound close to the core and the fuse inaccessible, a new transformer was needed.

Thanks FrankB, that idea of powering with the secondary voltage is brilliant! I'd also lack to add to your ending suggestion...

Most of the power supply transformers of recent manufacture have a temperature operated fuse buried under the paper in the primary windings.

Whenever someone presents me with a 'dead' power supply of ANY size, simplicity or complexity, I check the primary winding for continuity with my ohm-meter. If it's open circuit then I remove the transformer and carefully peel back the paper covering the primary windings until I find the fuse. It usually looks like a little (3 or 4mm square) bit of black plastic. Carefully short it out or take a connection to an external fuse and retry the continuity. 99% of the time the problem is solved!

I have repaired dozens of power supplies this way, particularly when a power supply has been misused or has overheated for some reason.

[broadgage]

Bypassing a failed thermal fuse and fitting an external fuse may render the equipment if not actually dangerous, then at least less safe then it was originally.

A thermal fuse should operate before the windings become hot enough to represent a fire risk.
An external fuse will pass enough current for long enough to start a fire.

Consider a transformer rated at ay 10 watts, I doubt that an external fuse could be sized smaller than about 250ma lest it blow on the inrush current.

If the transformer develops shorted turns then it will draw extra current from the mains and get very hot.
A 250ma external fuse will allow about 60 watts to be dissipated indefinitely, and perhaps 100 watts for some time.
That is almost certainly enough to ignite the insulation or immediate surroundings.

For equipment under skilled supervision this may be an acceptably small risk, but I can't recommend it for anything left unattended (unless outdoors or in a reliably non flammable area)

[Refugee]

Thermal fuses can be obtained in several packages from the larger component suppliers we use on a regular basis.

[rswanborough]

I repeat here what I wrote under "Testing Open Primary Power Transformers'. I have done this in case others may miss it, I have found it very useful.

Most of the power supply transformers of recent manufacture have a temperature operated fuse buried under the paper in the primary windings.

Whenever someone presents me with a 'dead' power supply of ANY size, simplicity or complexity, I check the primary winding for continuity with my ohm-meter. If it's open circuit then I remove the transformer and carefully peel back the paper covering the primary windings until I find the fuse. It usually looks like a little (3 or 4mm square) bit of black plastic. Carefully short it out or take a connection to an external fuse and retry the continuity. 99% of the time the problem is solved!

I have repaired dozens of power supplies this way, particularly when a power supply has been misused or has overheated for some reason. The reason for the overheating should be established before trusting this as a permanent repair.

HOWEVER, broadgage made a valid addition:-

"Bypassing a failed thermal fuse and fitting an external fuse may render the equipment if not actually dangerous, then at least less safe then it was originally.

A thermal fuse should operate before the windings become hot enough to represent a fire risk.
An external fuse will pass enough current for long enough to start a fire.

Consider a transformer rated at ay 10 watts, I doubt that an external fuse could be sized smaller than about 250ma lest it blow on the inrush current.

If the transformer develops shorted turns then it will draw extra current from the mains and get very hot.
A 250ma external fuse will allow about 60 watts to be dissipated indefinitely, and perhaps 100 watts for some time.
That is almost certainly enough to ignite the insulation or immediate surroundings.

For equipment under skilled supervision this may be an acceptably small risk, but I can't recommend it for anything left unattended (unless outdoors or in a reliably non flammable area)."

To which Refuge added:-

"Thermal fuses can be obtained in several packages from the larger component suppliers we use on a regular basis.

[G6Tanuki]

This approach may be fine for initial testing, but if it shows the rest of the device is OK, rather than messing around bypassing thermal-fuses in transformers and then releasing the result back to an end-user (with all the public-liability-insurance issues this involves) I'd ***always*** lob any suspect transformer/supply in the trash and order-up a replacement power-supply from the original manufacturer/distributor.

If you're billing your time at commercial rates (and if not, why not?) it'll probably work out cheaper overall too.

[rswanborough]

Originally Posted by G6Tanuki

...I'd ***always*** lob any suspect transformer/supply in the trash and order-up a replacement power-supply from the original manufacturer/distributor....

Sadly, many repairers out there, like me, live in a country where it is all but impossible to "order-up a replacement power-supply". If I COULD order one it would never arrive (the Post Office staff steal everything that looks remotely valuable) or I would have to use couriers that take long bucks and long time.

I don't charge for repairs so I just warn the owner that they'd better keep an eye on the heat... I wish I lived where you do.

P.S. Quite by coincidence, I was brought a clock/radio for repair from a friend who had bought it in the USA about a year ago. The 6v transformer was open circuit and I asked her how long it had lasted. She said about a year and I checked the little transformer. It was a traditional power supply (not electronic) and rated at 120v AC 60Hz. She had had it by her bed at 230v AC 50Hz for almost a year before it died. The thermal fuse was open circuit but I couldn't let her use it again and was able to replace it with a 230v transformer from my scrap box. Quite amazing it lasted that long...

[Julesomega]

Originally Posted by Refugee

Thermal fuses can be obtained in several packages from the larger component suppliers we use on a regular basis.

Yes, just solder a new one in

I agree, these thermal fuses are a (necessary) nuisance

[nicam49]

Yeah, but any tips on how to replace/renew the paper/card that the thermal fuse is buried under so as to keep the new thermal fuse in sufficiently close contact to the windings to be effective?

[AC/HL]

I don't think it's critical, it's a long term thermal overload device, not quick blow.

[julie_m]

Originally Posted by rswanborough

P.S. Quite by coincidence, I was brought a clock/radio for repair from a friend who had bought it in the USA about a year ago. The 6v transformer was open circuit and I asked her how long it had lasted. She said about a year and I checked the little transformer. It was a traditional power supply (not electronic) and rated at 120v AC 60Hz. She had had it by her bed at 230v AC 50Hz for almost a year before it died.

Did your friend not notice the clock losing four hours every day?

[FrankB]

NTE makes thermal fuses that open up under a variety of temperatures.
I only did the fuse bypass when I was unable to get replacement OEM transformers. (Try to find a transformer here for some little old ladies low end, late 60's Morse Electrophonic or Readers Digest stereo she was in love with. Unobtanium.)
When you are dealing with 30-50 yr. old equipment original transformers are pretty hard to come by. A thermal fuse that blows under heat, in series with a regular fuse always worked fine. I use what electricians call "Fish paper" to replace on the transformers. Here they use it as an insulator behind the main breakers on circuit breaker boxes. It comes in a roll, and is typically grey, or red in color. About 7.50/ roll (USD)

[rswanborough]

Originally Posted by julie_m

Did your friend not notice the clock losing four hours every day?

It was a quartz clock, not mains frequency clock. But well spotted.

[nicam49]

Originally Posted by FrankB

I use what electricians call "Fish paper" to replace on the transformers. Here they use it as an insulator behind the main breakers on circuit breaker boxes. It comes in a roll, and is typically grey, or red in color. About 7.50/ roll (USD)

Now that IS interesting! Bet it's not available over in the UK, but I'll try and obtain some...thanks for the tip!

[Boater Sam]

I use automotive gasket paper for re-covering transformer windings. It is a fireproof fibre material.

[trobbins]

Not a commercial repair option, but I have a Keithley 197 DMM with one of its two 120VAC primaries open circuit. Lab grade instrument PT's are somewhat special, with excellent screening, so using any form of replacement other than original was not a great option to consider. So it now gets powered through a step down transformer at 120V applied to the one good primary winding. Four years additional operation so far

[Herald1360]

Originally Posted by nicam49

Now that IS interesting! Bet it's not available over in the UK, but I'll try and obtain some...thanks for the tip!

Sounds like a US version of Elephantide- a sort of bluey grey stiff card like substance.

http://www.ukinsulations.co.uk/flexi...on/elephantide