Mains transformer only outputting 2V instead of 22V

If the capacitor is bad, there'd be some drop in output voltage, but not that much.

What you may be seeing is simply whatever voltage remains on the capacitor, and there is no voltage going into it.

Check the AC connector for continuity. If it's not a relatively low resistance, there's either a bad connection or a blown fuse, or a switch if there's a switch (assuming there's no other circuitry on the primary). And in some low end equipment, the fuse is inside the transformer, making it problematic to fix.

If the primary seems okay, then check continuity on the secondary. If it too is a relatively low resistance, then fire it up and measure the voltage going into the bridge rectifier. There should be somewhere close to 22volts AC there. If it's not, but the primary is okay, then there would seem to be an open secondary on the transformer.

Unless there's actually more circuitry than you say, this can't be much simpler a circuit.

Michael

OK, thanks.

There is no other circuitry on the primary side and the primary side is open circuit.

No, that's all there is on the secondary side: four diodes and a capacitor.

On Fri, 22 Sep 2006 08:15:57 +0100, Art Deco Has Frothed:

Open primary or secondary winding. Check continuity.

thats the prob then. Basic chargers like that are simple to replace, just enure the replacement is not rated at higher current, since its primarily the transformer R that controls the current in this case.

NT

The primary side should be around 2,000 ohms (wide variation)

The secondary side should be around 10 ohms (wide variation)

If no continuity on either it's toast.

-- _____ _ _ |_ _| | | | | | | __ _ _ __ ___ | |__| | ___ _ __ ___ ___ _ __ | | / _` | '_ ` _ \ | __ |/ _ \| '_ ` _ \ / _ \ '__| _| |_ | (_| | | | | | | | | | | (_) | | | | | | __/ | |_____| \__,_|_| |_| |_| |_| |_|\___/|_| |_| |_|\___|_| __ ____ / _| | _ \ ___ | |_ | |_) | ___ _ __ __ _ / _ \| _| | _ < / _ \| '__/ _` | | (_) | | | |_) | (_) | | | (_| |_ \___/|_| |____/ \___/|_| \__, (_) __/ | |___/

Homer J Simpson spake thus:

But the OP reported low voltage. A transformer is like being pregnant: either you is or you ain't. As you point out, either open or not. If open, zero volts. How do you get 2 volts?

I wonder if the OP was measuring correctly. (Don't laugh: the other day I measured something with a very low voltage; turned out I was trying to measure an AC source with my (analog) multimeter set on DC.)

It's real easy to measure an erroneous voltage with a DVM...phantom voltage.

jak

A DMM is crap for checking a lot of things - high impedance. A good old 5K per volt unit is often better.

jakdedert spake thus:

Explain "phantom voltage", please. Never heard of it.

Sounds scary.

D "woooo ooooo oooooo" N

A high impedance meter will often pick up a few volts out of the air. Place one probe in a live wall outlet without making a complete circuit and you'll likely see a volt or two on the meter.

James Sweet spake thus:

Aha, so it's a matter of high sensitivity to ambient EMF. Gotcha.

I'm glad I still have my trusty old analog VOM.

Put any DMM on the lowest AC voltage range, hold the leads in your hands and look at the reading. Move the probes around. It won't read zero.

-- _____ _ _ |_ _| | | | | | | __ _ _ __ ___ | |__| | ___ _ __ ___ ___ _ __ | | / _` | '_ ` _ \ | __ |/ _ \| '_ ` _ \ / _ \ '__| _| |_ | (_| | | | | | | | | | | (_) | | | | | | __/ | |_____| \__,_|_| |_| |_| |_| |_|\___/|_| |_| |_|\___|_| __ ____ / _| | _ \ ___ | |_ | |_) | ___ _ __ __ _ / _ \| _| | _ < / _ \| '__/ _` | | (_) | | | |_) | (_) | | | (_| |_ \___/|_| |____/ \___/|_| \__, (_) __/ | |___/

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DVMs have a very high input impedance and will give strange readings due to capacitive etc coupling. An old fashioned moving coil meter with an impedance of 1000 ohms per volt AC is often more useful for this sort of testing.

As an addition to a DVM, though, I hope? ;-)

Possibly the voltage I was reading was the residual voltage on the capacitor, as someone suggested. In the event, it turned out to be open circuit on the primary side.

If you had the meter on the ac range (as you should have to read the voltage on the transformer), you picked up a stray voltage...or not. If not, then more investigation is necessary. I believe somewhere in the thread, you stated the transformer was open on one side or the other. If that's the case, you picked up a stray reading...phantom reading which is indeed real, but not useful with respect to a determination of your problem.

jak

OK, but why would I put the meter on the AC range when the transformer is supposed to be outputting rectified voltage?

Art Deco ha escrito:

A transformer doesn=B4t output rectified voltage. It outputs AC voltage, which is rectified and filtered by the diodes and capacitors following the transformer. That=B4s why there are diodes, capacitors and voltage regulators after the transformer: to convert the AC unregulated voltage of the transformer into regulated DC.

OK, so why would I put the meter on the AC range to check the regulated DC voltage coming out?