Bench Power Supply Transformers

[Musicmanager]

Hi Guys

Remembering I'm a novice can someone please help me with some advice.

A friend has given me a bench power supply, looks to be homemade - no manufacturers marks or numbers - single output 0 - 30vdc; there appears to be current limiting that works OK, analog volts and analog amps display but neither work.

It powers up OK and the voltage pot will adjust between approx. 1.5 and 33 volts and the amps pot will limit current with an LED indicator OK. When I opened the case neither analog meter is attached to anything which explains why they don't work and that's a project for later maybe. A good look over the PCB, all looks very clean and tidy and not very old - there is still some flux residue about in parts.

My immediate concern is the transformer - after the power has been on for a few minutes there is a considerable amount of heat - enough to make a slice of toast - which I don't think should be there (there is a heatsink on the main chip which gets warm but nothing outrageous). It is giving 24 vac out of 2 separate secondary sets ( only one of which is in use ) I'm guessing it's going to go belly up very soon so I need to find a replacement. There is a label on the frame but the language is outer mongolian or similar so no clue to identification from that.

Using a multimeter, I've measured maximum output of volts at 33.2 vdc which coincides with the scale on the analog volts display @ 30 vdc; I don't know how to assess the max amps limit, but the analog amps display scale goes up to 3 amps so I'm guessing that's right.

I've calculated my new transformer would need to be 100 VA based on max volts @ 30 x max amps @ 3 = 90 VA

Are there many flaws in my plan ?

Thanks

 

[tunedwolf]

Are there many flaws in my plan ?

Yes, you have yet to determine if the original transformer is faulty or just being loaded down by a faulty circuit or poor design. If it is the latter, simply replacing the transformer will not cure your issue. Disconnect one of the transformer secondary wires going to the board, watch to see if the core temperature rises sharply, if not, then you have an issue on the board that you'll need to look into

 

[Musicmanager]

Ah, I hadn't thought of that ! I'll get on it tomorrow and come back to you. Thanks

 

[Musicmanager]

Hi

As you advised, I've removed the 'load' from the transformer with somewhat startling results ! When I powered it up the T was OK and cool for 2 or 3 minutes, but then began to sing like a kettle, gradually getting stronger. I went to grab the multimeter to measure for an output but before I got there the noise had got worrying so I switched it off. Lots of heat ! I let it cool and then powered on again but no response. Fuse had blown. Replaced fuse and powered same events. Turned it off - not going there again !

Don't think there's any doubt transformer doesn't want to play properly. Are my calculations in first post correct ?

 

[tunedwolf]

Your calculations are correct VA would be around 100W. However, you are not taking regulator losses into account, I would go with a 120VA transformer.
Interesting that the transformer began singing. It would suggest that the laminate stack is loose, however that doesn't explain the heat. I suspect that the primary winding is also breaking down.

 

[Musicmanager]

Hi Tunedwolf

Thank you for your help, much appreciated.
I have now removed the transformer from the case and it's only an uneducated impression but I would say it is much older than the rest of the package - I recognise some gauze type fabric and varnish on the outside. Nothing much to see that would indicate a problem but it smells like a gorillas armpit ! I've dropped and broken my camera so I can't do a pic at the moment but I'll save it and send you one as and when.
Something I need to learn ' not taking into account regulator losses' ?? Something I can read to understand the principle ?

Thanks again