Small PCB transformer - typical measured values?

Question

Hi - I took a punt on a non-working ebay second-hand document shredder (£

20).

It's a Rexel, very decently built pro model with substantial mechanicals.

The problem seems to be no +5V to the control-board PCB. This control-board has a traditional regulated power supply, transformer/br idge-rectifier-diodes/caps and regulator to run the control circuitry only (the shredder motor is mains ac). The control board transformer is PCB mounted, no markings - I'd guess somet hing like 8V secondary.

I *can* see measure mains ac voltage across the transformer primary. I *cannot* see ac voltage across the secondary. I *cannot* see dc voltage after the rectifier diodes, across the supply rai ls.

I can also see lo-ohm on the transformer secondary (in circuit). I'm getting a very high resistance on the primary - probably just the suppl y filtering.

So my prime suspect is this little PCB mount transformer. I've just ordered a solder pump so I can remove it.

What sort of resistance value would you expect on the primary?

Scott M · Accepted Answer

>rectifier-diodes/caps and regulator to run the control circuitry only  (the shredder motor is mains ac). How high? "Very high" sounds open circuit to me; few k-ohm would be  normal. Some transformers have built in thermal fuses which can go phut.

Dave Plowman (News) · Answer

Just measured a 6 VA one - 800 ohms approx.

dom · Answer

Measured value about 390K. I didn't want to "lead the witness" by putting my measured value in the first post. Wasn't aware of internal thermal fuses. Thanks

dom · Answer

Thanks. Exactly what I wanted to know!

John Rumm · Answer

a few tens of mA once you allow for some inductive reactance....

meow2222 · Answer

You also need to know what voltage it is. 5v, if the reg needs 2.5v thats 7.5v, the diodes drop 0.8v ea that's 9.1v, and allowing for 10% mains variation that's 10v peak = 7.1v ac at least.

Those numbers may need tweaking depending on the circuitry and components

NT

Harry Bloomfield · Answer

pretended : A few hundred ohms at a guess. Probably the thermal fuse in the primary  has popped. Sometimes you can dig them out of the winding and replace  them, sometimes not.

Brian Gaff · Answer

Yes thermal fuse is my gues due to a duff bridge rectifier and now probably  caput capacitors in the supply.  Try a protected  supply of dc in just after the bridge.  Brian

Andrew Gabriel · Answer

IME, the regulation of small mains transformers is so bad, you have to get your hands on a sample to try. I have some which produce twice their output voltage off-load. This can make an enormous difference when you are using one to drive a small load through a linear regulator. Often you want to go for a lower output voltage one than you might expect.

If you know what regulated DC voltage the circuit needs, I would be tempted to buy a modern cheap switch mode wall wart, and use the guts (suitably mounted/enclosed). If it's 5V, then these can be found for £1, if you don't already have several spare phone ones around.

dom · Answer

ts 7.5v, the diodes drop 0.8v ea that's 9.1v, and allowing for 10% mains va riation that's 10v peak = 7.1v ac at least.

Ok - thanks to all for the advice. More news once I lift the transformer fr om the PCB.

Dave Plowman (News) · Answer

Quite. Bought a couple of PCB mounting 6VA types off Ebay. Not cheap, not expensive. Two 20 volt windings. Off load voltage 36v.

Small PCB transformer - typical measured values?

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