Multimeter Tutorial

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Oscar
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Multimeter Tutorial

Post by Oscar » Mon Feb 05, 2007 12:50 pm

Tom already knows that I've blown up my digital multimeter. Now I'm using the analogue one, but only in the most basic way possible ("Oh look, the needle's moving - that means there's current. Now what?")

It would be great if one of the more skilled electrical engineers could post up a quick tutorial on how to use a multimeter, or point to a web resource.

Any offers?

Oscar
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M

Post by M » Mon Feb 05, 2007 3:04 pm

I bet you tried to measure currant (amps)in parallel rather than series!

Measure Volts (ac/dc) in parallel (ie meter between live item and earth (DC) or neutral (if measuring AC)

And currant (Amps) in series - ie between thing you are measuring and the next part of the circuit - not to earth / neutral - that way lies the demise of the test meter (probably an internal fuse).

Resistance (ohms)is always over the top of the item (ie between the in and the out of whatever you are measuring)

Continuity is useful to tell you if there is a break in a wire or if it is shorting to earth.

There is a handy little gizmo in this months CM magazine advertised which is a car fuse blade current meter - remove fuse, put in meter and it displays what current is being drawn by that circuit.

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Post by Oscar » Mon Feb 05, 2007 3:31 pm

That's the sort of thing Marty. Nice one.

Now what about all those different scales and numbers? And different holes for the jacks?
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Post by ellevie » Mon Feb 05, 2007 3:56 pm

There's loads of stuff out there. As ever the problem is to find something at the right level.

I like this site "How Stuff Works"
What are amps, watts, volts and ohms?
http://science.howstuffworks.com/question501.htm



Using a Multimeter
http://www.doctronics.co.uk/meter.htm

The function I find most useful is "continuity checking" using the lowest setting of the resistance (Omega) function. This is useful for checking if switch contacts are making poor or no contact. Only check with the power switched off. A "good" contact will read almost as low as the reading you get with the two probes shorted together.
David

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M

Post by M » Mon Feb 05, 2007 3:59 pm

If your using it on the car:

Volts - DC 20V is probably your best setting
Amps - DC 30A again is probably the best - unless measuring current drain in milliamps as nothing is likely to be pulling more than (in reality about 20A - unless its a heater eliment, possibly 25 - 30) This will give you scope to measure things pullung more than they should be. Again - if your measuring the CCA of the battery - select a suitable scale.

Anything mains related
Volts AC-250V
Amps AC - 20A (unless you are fiddling with big huge heater eliments for hot water cylinders or showers)

If in doubt - dont go near the mains with anything sharp or pointy and metallic :twisted:

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Post by ellevie » Mon Feb 05, 2007 4:14 pm

Marty, that blade fuse gizmo you mention sounds really useful. I wonder if you could make a home-made one using a blown blade fuse ?
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Post by Way2go » Mon Feb 05, 2007 4:37 pm

ellevie wrote:Marty, that blade fuse gizmo you mention sounds really useful. I wonder if you could make a home-made one using a blown blade fuse ?
Yes, I've done that BUT remember that most cheap multimeters wont take the current that cars use so you will also need to use a SHUNT for it to be useful without damaging the meter. :D
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Post by ellevie » Mon Feb 05, 2007 5:01 pm

...sssh ! dont mention "shunt" to Adam ...

My cheapie meter can measure up to 10A. Is there anything stronger than that going through the fuse box ?

I've answered my own question. There seems to be a few 25A fuses for the rear window heater etc. And there are even a few 30A ones.
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Post by jeremy » Mon Feb 05, 2007 6:09 pm

Many cheap meters have protection circuits for everything byt the 10 amp scale - so it will bow easily - and accidentally - as happened to me.

In fact you don't really need that scale - so the best advice is don't use it. You very seldom need to make an accurate current reading - you just need to know its flowing or look for variations in it.

A shunt is simply sharing the current - so that some goes through a resistor and some through the meter. This means that the meter is connected in PARALLEL with the shunt resistor - which can be something like an indicator bulb if that's what is handy.

If you're tracing leakage currents - put the bulb in series and it'll protect your meter (they'll conduct about 2 amps alright - a bit less) if you get it wrong - then if you're certain its OK - take it away and measure. If the bulb is lighting - then there's a heavy current in the circuit - and you may damage the meter.

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Post by ellevie » Mon Feb 05, 2007 6:40 pm

That's a :idea: bright :idea: idea Jeremy ! :lol:
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Post by tom » Mon Feb 05, 2007 6:58 pm

The only thing you need a DMM for is measuring charging voltage.
Everything else can be done with the battery or sensor disconnected when you need to know about resistances or with a test lamp if you need to know if voltage is present or current is being drawn. DMMS are so good and cheap for hobbyist use that people tend to use them regardless of the situation but often a test lamp will save you a lot of time.
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Post by ellevie » Tue Feb 06, 2007 11:12 am

Marty wrote:There is a handy little gizmo in this months CM magazine advertised which is a car fuse blade current meter - remove fuse, put in meter and it displays what current is being drawn by that circuit.
Marty, what does CM stand for ?

Ah ! I've found out that it means Car Mechanic.
The gizmo is actually a special offer at £9.50 and there are 10 to give away in a prize draw.

Probably something like this 220077222311 on eBay.
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Post by Way2go » Tue Feb 06, 2007 3:31 pm

tom wrote:The only thing you need a DMM for is measuring charging voltage.
Everything else can be done with the battery or sensor disconnected when you need to know about resistances or with a test lamp if you need to know if voltage is present or current is being drawn. DMMS are so good and cheap for hobbyist use that people tend to use them regardless of the situation but often a test lamp will save you a lot of time.
Everyone will have a favourite way of doing things but the advantage of measuring current is that it is non-invasive and will give you the current state (excuse pun) as functional switches etc are operated in the circuit under test. :D

Also it is wise to measure the total battery drain current when the car is switched off (parked state) because if this is anything signficant and the car is parked up for a period then the battery can go into deep discharge from where you can not bring it back to life. (Happened to me on a good battery in a Volvo where it was parked up fo a month! New battery only solution then.)
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Post by ellevie » Tue Feb 06, 2007 7:52 pm

Actually I have just such a fault in my 2nd BX which I will have to sort out eventually. With everything switched off it will drain the battery if I don't remove a particular fuse. Its a bit intermittent but I've seen it measure about 800mA which varies as I waggle the passenger door, so I suppose it might be a broken wire in the door harness.
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Post by tom » Tue Feb 06, 2007 7:58 pm

"The advantage of measuring current is that it is non-invasive."
Unless you are using a clamp ammeter, it is not non- invasive and a clamp ammeter is of little use on a car. In order to measure current, the meter must be in series with the object being measured. How can this be done in a non-invasive way?
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