Question about transformers - techi types!
Turbo Mini Forums

I have a transformer that has a 15vDC output - or rather is meant to have according to the spec on the transformer and the label on the thing the 15vdc is meant to be fed into....

Instead, the transformer was putting out 18.2v when the thing it was driving crapped out (or rather I measured the output afterwards and noted this high voltage under no load).
I immediately assumed this unit was crapping out, but someone else on a forum (for this tool) claimed that transformers will over-read on no-load.

So, do they over-read by over 20% under no-load?

Next - I left this thing plugged in (intentionally) and it's now reading over 21v (40% over-spec) now it's all toasty warm. Admittedly, i haven't stuck a lamp or something in there to give it a 'load' but this all sounds to me like a coil-wound transformer that has the insulation breaking down and is crapping out...

This forum expert (I'm thinking it should be shit-spurt, not expert) is telling me now this is normal...

Either way, I've sourced a nice switch-mode power supply from an old toshiba laptop that will always give out 15v no matter what,,, I just have to repair my base unit that has screwed up.
I'd just like to have someone who knows transformers better than I do to clarify my own thoughts... :)

when i used to work in the electronics lab, you would expect some fluctuation in voltage from a normal power supply, but not 5-6 volts, maybe 1-2....the switch mode psu will also have a constent voltage due to its design...

"Cars are a lifetime of pain"

I immediately thought "Optimus Prime". Dissapointed!

Depends on what it really is... It can't be a transformer if it's supposed to give 15V DC.

Transformers are AC.

If it's a power supply of some sort, it depends on whether its a basic transformer/rectifier/linear regulator (= heat) or switched mode (less heat).

More info required !!!

But, back to basics, transformers do NOT get hot with no load unless they have shorted turns, then they get hot (the shorted turns just being P=I^2R) and that cooking leads to more shorted turns until total failure.

Schrödinger's cat - so which one am I ???

Yeah - my bad - should have guessed someone would pull me on the terminology!

It is a Power Supply.
110v 60hz in
15vdc out

Is an old-school transformer that has a primary and secondary windings around a soft-iron core, with diodes rectifying the Ac to DC output.

As i said - it is lablelled at 15VDC, the unit is powers, says 15VDC, but when cold it gives out over 18vdc under no load, and when left on, it gives out over 21vdc.

Right,

If it's basic transformer and diodes (probably setup as a bridge rectifier) and the voltage changes WITHOUT load being applied as it warms up, shorted turns are the most likely cause.

Have you measured the output voltage on the AC secondary side direct (before the diodes) - if this goes up as the thing warms up, shorted turns without a doubt.

EDIT - unless (highly unlikely) there is a faulty diode causing something strange. If possible, disconnect the secondary from the diodes and measure the secondary voltage alone. if this goes up as the thing warms up, shorted turns without a doubt.



Rod.

Edited by Rod S on 25th Jun, 2008.

Schrödinger's cat - so which one am I ???

Rod, I'm not intending on usiong this power supply as i have a superior 'switch mode' unit to do the job. All i was really wanting to know is can 20% over-voltage in a no-load situation exist in a healthy power supply as i described, or is this 'out of spec' from your experience (neglecting the fact that this one is clearly toast from the voltage varying as it warms up)...

Cheers,
D

Well, there is no absolute answer to that.

For a basic transformer/rectifier/possible smoothing capacitor and voltage regulatore chip (or even zener doide and power transistor) then, yes, voltage will vary with load.

For such basic "linear" power supplies, there is no absolute answer.

Switched Mode gets around this (and doesn't produce so much heat) but I thought your problem was things getting hot with NO load.

With a linear power supply, voltage will vary, depending on how good the zener or chip are at regulating it, and there wll be heat, but it's determined by load.

At no load, a linear power supply should not get hot, nor should its voltage vary signifactly from design, say +/- 10% at the most.

That's just my experience, the manufacturer should have a +/- percentage quoted by design.

Schrödinger's cat - so which one am I ???

also note that the voltage coming out of your wall socket will not be 115VAC. At various times of the day you will measure different voltage, it isn't well regulated. Your power supply has no regulation either, so its output voltage amplitude will vary with the input voltage amplitude. I suggest you start by checking the diodes in your rectifier, see if they are still diodes, and I would replace the output filter cap. If it still gets hot with no load, just the rectifiers and cap, then get another transformer.