Testing lighting circuit with multi-meter

[garrymum]

Hi

I am wanting to test for voltage in a lighting circuit. Basically, I have added a new bedroom light and it is not working. It's the sort with a built in transformer. I have had to shorten the steel cable (which supports the light from the ceiling) and also the cable from the light to the transformer. I am wanting to test for voltage FROM the transformer.

Transformer output is 150W (says 11.5 VAC MAX 150W).

I have the following multi-meter, just want to know what setting I need to check for the required voltage. Do I also need to put the red probe plug into the left hand 10A max socket?

Need to get this light working before it gets dark.
Many thanks.

 

[hairyben]

Many meters can't "read" the voltage coming out of a lighting transformer, evan if it's good. Best way to test it is to use a known good 12v lamp with a pair of tails.

 

[mattylad]

DO NOT put the red probe into the 10A MAX socket to test a voltage.

You will introduce a short across the terminals you are measuring.

That socket (10 max) and the COM one are for measuring current/amps.

As Hairyben says, you may not be able to measure it as it may well be at a high frequency rather than just plain old DC.

Is there mains at the input of the transformer?
Although its often referred to as a "transformer" it is not one, it is actually a switched mode power supply and usually does not have a transformer in it

Have you tried another lamp?

 

[ban-all-sheds]

Although its often referred to as a "transformer" it is not one, it is actually a switched mode power supply and usually does not have a transformer in it

Really?

 

[plugwash]

mode power supply and usually does not have a transformer in it[/i]

I find that pretty unlikely. Afaict SMPSUs that don't contain a transformer don't provide isolation.

However the high frequency transformer in a SMPSU can be much smaller than a mains frequency transformer handling the same power.

 

[TicklyT]

Some switch mode power supplies won't even power up unless there is a load connected to the output.
The load it's looking for, of course, is the lamp(s), which would then light up when you switch the thing on, which means you wouldn't need to check the output because you can see it is working.

Sort of chicken and egg problem.

Maybe better checking you can see the load connected to the power supply output, using a low resistance range.

With all lamps in place, I would expect a reading of about 1 ohm. With all the lamps removed, it should approach infinity.
If removing the lamps doesn't make any appreciable change, then the lamp wiring is suspect.

 

[mattylad]

OK then I'll rephrase that, in comparison to what a DIY'r may think a transformer looks like.
I.E. http://upload.wikimedia.org/wikipedia/commons/7/7a/Transformer-hightolow_smaller.jpg

The majority of SMPS's do not have anything near as big as them to enable the "unit" to be called a transformer (although they do have a small one).
Calling the power supply for halogen lights a "transformer"is IMO incorrect.

Y'all like to be pedantic of such small issues so I'm just joining you

 

[ban-all-sheds]

The majority of SMPS's do not have anything near as big as them to enable the "unit" to be called a transformer (although they do have a small one).

It wouldn't matter how big, or small the transformer in the power supply was, the fact that it contains one does not mean it is one.

Calling the power supply for halogen lights a "transformer"is IMO incorrect.

I agree.

Y'all like to be pedantic of such small issues so I'm just joining you

It's not pedantry...

Although its often referred to as a "transformer" it is not one, it is actually a switched mode power supply

That's quite correct.

and usually does not have a transformer in it

That isn't.

 

[mattylad]

Cool, mixing up quotes to make it look like it was one comment.... aren't we being clever now.

Care to help the OP test his lights?

 

[garrymum]

Thanks all. All ok now. Turns out it was the switch, a dimmer. I swapped it over for a standard switch and it works now. Instructions say suitable only for a halogen compatible dimmer - guess mine isn't!

 

[Stoday]

As this thread is so pedantic, perhaps I can be excused for pointing out that the running resistance of a filiament lamp is normally over ten times its cold resistance.

With all lamps in place, I would expect a reading of about 1 ohm.

Make that less than 0.1 ohm