Using a multimeter

Just purchased a Clarkes digital multi meter.

I want to check whick if 3 lighting cables coming through to ceiling rose is the switch cable.

Any ideas what I set the dial to?

Confused; never used one before.

If you've got 3 black and red pairs (or blue and brown) then I'd do the following:

1) Isolate the supply. Either turn off the mains to the whole house (preferable, in my opinion), or the light circuit.

2) Separate out the three different pairs

3) Set your Multimeter to measure Ohms. If it has got a continuity tester buzzer, set it to that instead. Touch the test leads together and check they read about zero (or buzz)

4) Double Check 1)

5) Hold it on a pair of wires (one test lead on each wire). It will either say a very low number, or say 1 (or buzz or not buzz). Get someone to flick the switch and see if that changes from the low number to the 1, or vice-versa

6) Keep on doing it until you find one that does.

I've ignored the earth in all the above.

Ohms. (also called resistance/continuity)
With the power to the circuit off.
First, put the meter on an ohms range, if the probes do not touch each other, the reading will be whatever the meter shows for no connection (inf, infinity, a big 1 or what ever - remember this )
Then touch the probes togther, the reading should become a low number (0, 1 or 2 sort of thing.) That is the resistance of the probes and meter internals.
Now you know what your meter says for 'well connected' and 'not connected' respectivly... (an intemediate number is the resitance of the object - when bored measure a light bulb for a laugh.) A switch should be only either a very low resitance when on, or practically infinite when off.

Now hold the probes between the metal ends of the wires that you hope might go to the switch.
If correct, when a colleague operates the switch, (or you on your own if you like running up and down steps or have either a small room or arms like a gorrilla), the resistance between the pair of wires will change between the two states (connected and not connected) as the switch is operated (connected for switch is on, unsurprisingly).
If no change when switch is operated you have the wrong pair of wires... roll the dice and try again. (or the switch is short of course)
Thats it.

Some meters have a 'beep' setting, so that you know the connection has been made without looking - can be handy as you can leave the meter probes jammed in the terminals while you operate the switch from afar.

Please remember to remove all meters and tidy up before putting the power back on.
come back if this makes no sense after you have had a play...

PS only works with proper switches - clever dimmers and things need to be replaced with a normal switch temporarily.

Thanks for that.

I have the following numbers in the ohms range on my multimeter:

200
2000
20k
200k
2000k

Which do I select?

Any one should do, as you are going to be detecting the difference between a short circut (couple of Ohms) and an open circuit, but may as well go for the 200.....

seconded. Dont want the resistance of those shaking hands to confuse things - on the higher ranges (20k plus) you will be able to see your own body resistance if you grip the probe tips tightly. To the unwary this can look like a real reading.

I am not black and crispy and hanging from the ceiling.

So thanks to all the speedy help.

Its sorted.

mapj1

when bored measure a light bulb for a laugh

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When really bored hold the probes between your fingers note the reading, then wet your fingers & do it again. You will see why electricity loves water.

If you're really bored measure the resistance of a light bulb then use ohm's law to work out its power rating.

Current = voltage / resistance

Power = voltage x current

You'll get quite a shock. You'll double check your maths then you'll check it again but the answer will still be ridiculous. Want to know why? Tune in next week.

PS: With a bit of practice you can learn to vary your body resistance by mind control alone. Don't ask me how but I've done it. It's a bit like wiggling your ears or dilating your pupils to order. Does anybody else have any unusual party tricks before this gets moved to general chat?

I can make one hand colder than the other by imagining it is under running water. Odd, remember trying it at university, and being almost as sceptical as those who witnessed it. Two thrmometers, one in each hand, and wait, one goes up, one down,then swap thoughts, and wait it swaps too...
I presume the body is sweating more or less in response to the imagined feeling of hot/cold?
Does the body resistance work on the same principle I wonder..

Take ye multimeter back the shop, get ye money back and go and by some cable tracers, i would be lost without them.
My multimeter is in back of van gathering dust....

felix said:

Does anybody else have any unusual party tricks before this gets moved to general chat?

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Yes, but I have to be quite drunk to do them, and I can't describe them before the children's bedtime.

But I can recount two fascinating ones I've seen.

a) Someone who'd worked out a way to keep 4 running totals going in his head used to win bets with this one. Take a pack of cards, and remove one, any one. Then deal them one at a time face up onto a pile in front of him, at a fair old clip (less than 1 second per card). As the last card went down he'd tell you what the missing one was.

b) The brother of someone I worked with was ambidextrous, and he could write with either hand. He could also write with both hands simultaneously, and produce two different texts. This was widely regarded as very disturbing, although as he always pointed out when a pianist reads two lines of music and does different things with each hand nobody finds that upsetting.

BenStiller said:

Take ye multimeter back the shop, get ye money back and go and by some cable tracers, i would be lost without them.
My multimeter is in back of van gathering dust....

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I tried to use cable tracers, never worked, it seems that it is working for you, can you recommend one the works?

Albert, sorry mate i cant, i have been asked that one before, and like i said then, I inherited them off an old workmate, this was a few years ago, dunno what ones are on the market now, but i might pop down maplins see what they have in stock...

BenStiller said:

Albert, sorry mate i cant, i have been asked that one before, and like i said then, I inherited them off an old workmate, this was a few years ago, dunno what ones are on the market now, but i might pop down maplins see what they have in stock...

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Thanks for the reply, If you find something let me know, I have 2 different ones, and tried before 2 others and they never seem to be reliable. If you don't want to use the forum, mentioning makes and names you can you can send me a personal message, if you can't find it, no worries, thanks for trying.
Albert