Low voltage lights.

Try switching it to AC... :-)

Seconded.

Adam

If it's an electronic transformer, you still won't get an accurate reading, as test meters aren't designed for ~20kHz AC and a grotty waveform. You should get something somewhere between 6 and 18V though.

Doh!

What a pillock I am. Automatically assumed 12v would be DC.

You aren't alone! :((

I read an re-read your post wondering what could be wrong. Gave up in the end.

(Anybody want to buy a well used DMM?) (With AC scale)

You've also got to bear in mind that some electronic transformers are of a design that requires there to be a reasonable load before any output occurs. This is just a characteristic of switch mode power supplies, not a design aim.

Rob

Yes - I tried measuring the output voltage of a single to three phase invertor with a DVM and did guess that 600vac, or so, wasn't quite right. A more modern DVM gave me 280vac and a traditional AVO 8 gave

Rob

Not entirely... worth noting that many lighting "transformers" these days are in fact switched mode power supplies. Some of these will shutdown in the absence of a load, so measuring the voltage without a bulb present may also get you nothing even if using the correct range.

You may not get an accurate reading with some DVMs on the AC range either as SMPS run at a much higher frequency than mains. And may not give any output at all in the absence of a load.

Just for guidance SMPS are quite light and usually shaped so they'll fit through the downlighter mounting hole. A true transformer will be much heavier - and likely larger too. Most these days are SMPS.

Thanks Dave (& John).

That makes perfect sense, I used the word 'transfomer' almost in a generic way. Seems that SMPS are described as transformers even by TLC

To further my education :-) how does an SMPS work in simple terms? I understand how a transformer works more or less, how does an SMPS achieve a similar thing?

What about wall warts? Is a Nokia phone charger an SMPS?

But it makes some sense to call them all transformers because they do the same job. And most won't be interested in any differences internally.

The electronic ones, yes.

Like for like, transformers gets smaller as the frequency increases. So an SMPS converts the 50Hz mains frequency to something like 30,000 Hz. So a much smaller and cheaper transformer can be used.

Most are these days. When you make things by the million the material costs are important and the saving in expensive copper for the windings etc can easily exceed the complexity of making the thing. As well as making it smaller for a given output. It's also simple to make an SMPS that will work with virtually any input voltage and give a constant output

- not so with a normal one.

Quite.

Aha! I'm getting there!

Thanks, now I understand.

Going back to testing with a DMM, all I need to know is if the SMPS is working or not. Given that the reading isn't likely to be accurate, do SMPS's either 'work/not work' or do they gradually deteriorate?

If the former, would 'a' reading indicate that its OK and 'no' reading indicate that its fubered?

Thanks for this Dave, I appreciate your help.

Ah. So they need a 'known to be working' bulb (sorry lamp) to be there?

I'm just looking for a simple & reliable way of knowing if I need to change the SMPS.

Like all electronics any fault is possible. ;-)

The best way to test one is with a known good lamp. Because as has been mentioned some won't start up without a load - and a DVM doesn't present this.

Low voltage lamps are quite robust so should stand a fair amount of being chucked around in the van etc, so you could make up one with a couple of test lead prods to it - Maplin etc sell such things.

Eazy. Test the 240V side of the transformer with your multimeter, if there is 240V there and a known working lamp will not work then swap the transformer.

Adam

Yup, too small and light to be an iron cored transformer.

There are quite a number of different designs, but the general principle generally goes something like rectify the mains and use it to drive an oscillator producing an output at many times mains frequency. Use this in a much smaller step down transformer (transformers gain efficiency as the frequency rises). In the case of a lighting transformer there is no need for much else - you can use the LV HF waveform directly.

Almost certainly. This is one of the factors that makes the "switch off your mobile phone charger to save the planet" arguments kind of pointless.

Note with the TLC transformers, these will give a voltage output with no load (I tested one of the individual lamp 50W ones the other day. The reading was also a good approximation to 12V using my DMM (although this was a "true RMS" clamp meter).

If you want a reliable way to test an electronic transformer you might find it worth making up a small test jig using some high power wire wound resistors. You could then use them in series with your DMM on its current measuring range (assuming it will read at least a couple of amps). That ought to be enough to ensure the transformer starts ok.

True, but many will not realise that much of the control circuitry of an SMPS is at mains potential, so represents an unexpected hazard.

I doubt you'll find many pro sparks who'd even bother looking inside - even if you could - given the low price of these things.

You certainly won't find me poking about inside them :-)