Small PCB transformer - typical measured values?

[dom]

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]

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)]

Just measured a 6 VA one - 800 ohms approx.

[dom]

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]

Thanks. Exactly what I wanted to know!

[John Rumm]

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

[meow2222]

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]

[e-mail] 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]

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]

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]

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)]

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

[meow2222]

hats 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.

Normally you base min transformer V required on full load rated voltage. If you go below that you're risking brownout on the LV rail, with severe hum if its regulated. You can do as you suggest if you're speccing the psu for less than the transformer's rated i_out.

As has been said, inserting a complete wallwart is probably easier

NT

[dom]

bridge-rectifier-diodes/caps and regulator to run the control circuitry onl y (the shredder motor is mains ac).

ething like 8V secondary.

ply filtering.

ed a solder pump so I can remove it.

Transformer removed, and found to have open-circuit primary.

Replacement (ebay, £4.50) soldered in, and shredder now fully working.

Thanks to all for advice.