How do you test 12v spotlights?

[Rednadnerb]

I was trying to figure out whether a 50w 12 volt spotlight bulb had blown a nd used the multimeter to see what voltage there was at the contacts, nothi ng registered, I tried it on another known working spot and still nothing. I thought the multimeter might be faulty so tested it on the 12 car battery . It registered 12.7 volts so it is OK. What is going on?

Thanks

Brendan.

[Chris Bartram]

Put the meter on a resistance or continuity range and measure the bulb out of circuit. It should read quite low when cold.

[Graham.]

I am guessing you had your meter set to measure DC volts instead of AC volts. Why don't you pull the bulb out and test it with the resistance or continuity range?

[ARW]

It happens.

Life is a learning curve.

[Dave Plowman (News)]

You were checking the socket to see if there was 12 volts present? If it is a SMPS rather than transformer, it may not switch on unless the minimum load is present, and a DVM presents virtually no load.

However, it's easier to check the bulb - simply set the DVM to resistance. You will get a very low reading on a good bulb - perhaps 5 ohms or so.

[harryagain]

Thanks

Brendan.

Flat battery in your multimeter. Needed to measure resistance but not voltage.

[Rednadnerb]

Thank you everyone.

I know how to test the bulbs for continuity, that bit is not a problem. The answer that it is SMPS rather than transformer seems the most likely explanation for why I cannot test the connectors. I am assuming that it is 12v DC not AC but I think I tested for AC as well.

[Muddymike]

I was trying to figure out whether a 50w 12 volt spotlight bulb had blown and used the multimeter to see what voltage there was at the contacts, nothing registered, I tried it on another known working spot and still nothing. I thought the multimeter might be faulty so tested it on the 12 car battery. It registered 12.7 volts so it is OK.

Test the bulb on the car battery, simple!

Mike

[The Natural Philosopher]

12V AC at several KHz (electronic transformer) or 50Hz (real toroid) wont register on a DC meter

[Rednadnerb]

I guess my question should have been 'How do I use a multimeter to test that there is power at the place (bayonet?) where you insert the spotlight bulb pins?'

[John Williamson]

If it's supplied by a SMPS, then you may need a load to circumvent any safety cut-off. So, I would suggest first checking the bulb, and using that or another identical one to confirm power. Otherwise, you need to gain access to the back of the socket.

You *could* possibly make up an extension lead with the right plug and socket combination and use that as a break-out to check for power with the bulb connected. It is, though, possible that the frequency of the output on the SMPS is too high to register on your meter, even on the AC ranges. Try putting a transistor radio on AM or FM nearby to check for RF interference.

Or a damp finger... Give Darwin's law a chance. :-?

12 volts *shouldn't* do any damage.

[John Rumm]

and even with the meter on AC you might get odd results on those modern SMPSU style "transformers", since the waveform out is unlikely to be very sine like.