using a multimeter?

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dan.2cv
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using a multimeter?

Post by dan.2cv » 19 Apr 2005, 01:17

Sorry that this has no relevance whatsoever to Citroëns, but I'm trying to fathom out how to work a multimeter I got for Chrismas a couple of years ago. It's a fairly standard digital multimeter with a big dial and about a million settings. I can work out where to plug the probes in according to what I'm trying to measure, but don't know what setting to put the dial on: ie, if I want to measure DC volts on a car, I have a choice of 200m, 2,20,200,1000. Is it that 12v is a 'tens' number so choose 20? If I was testing mains voltage would I set it to 200 being in the hundreds?
What if you dont know the amps of what you're trying to test?
finally, how do I test for continuity? For example if I wanted to test a component?
Thanks guys, anyway I need to do some electrics on my car, so I'm not miles off topic!!

Stinkwheel
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Post by Stinkwheel » 19 Apr 2005, 02:10

Right, pretty easy this really.
yes for testing car it will be 20volt range. You shouldnt have anything more than about 14 volts anywhere and it will read right down so you are covered. Continuity is another easy one, its usually marked as a buzzer or something, touch the 2 wires together you will get a reading in ohms on the screen (as near as 0.0 for open circuit continuity) and a buzzer on continuously. Also marked will be an ohms test, same as above but without noises.
Dont bother trying to measure amps, 99% of multi meters only read up to 20 amps max. your car is putting more than that through all the bits you need to measure current (starter, alternator etc) You can measure these things but need a seperate 'amp clamp' that reads through the milivolts section of your meter.
That any clearer? If not just ask.

andycarter
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Post by andycarter » 19 Apr 2005, 02:15

Hi Dan,
The range setting is the maximum value that can be displayed, using an LCD display with elements in the format "188.88" e.g. it can display values from 00.00 thru 199.99
On DC Volts 200m means 200mV and so on, although after 199.99mV it would probaly say oL or something to indicate an overload.
You are correct to choose 20V range for 12V car electics, for mains voltage you would want to select AC Volts and range 1000.
To measure amps, your meter presents a low internal resistance, whereas for voltage it has a high internal resistance, this is why you need to plug the leads into different sockets. Trying to read amps with the leads in the voltage sockets will do nothing, BUT if you put the leads in the amps socket and try to measure mains voltage, or even your car battery you'll get a nice surprise and probably destroy the meter.
If you don't know the amps range, start with the highest ans work down until the display reads something sensible, same applies to voltage. Note the meter may have differnt sockets for say 10amps and 200mA.
For continuity you want the lowest resistance range, if its a posh meter it may also include a buzzer so you don't need to look at the display when checking continuity.
Andy

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Kowalski
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Post by Kowalski » 19 Apr 2005, 02:19

To measure current flow (amps) the meter has to be connected in series with whatever you're measuring the current flow through. Be careful when measuring current flow, most multimeters will only do 10A and even then mine is only rated at 15sec per minute at this flow, 10A is not enough to measure the current flow through a glow plug for example.

dan.2cv
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Post by dan.2cv » 19 Apr 2005, 02:37

Cheers guys, that is all so helpful!! I've never heard the macjine buzz before until just now!!
Before I go prodding around all my delicate switches and instruments, can I cause any damage to the component?
What I mean specifically is testing a switch with 4 pins and I dont know which pins are 'power in' and 'power out' can I do any damage?
thanks again.

andycarter
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Post by andycarter » 19 Apr 2005, 02:51

A straightforward 'switch' is just two pieces of metal that touch, or don't touch. You won't cause any harm to it by testing for continuity.

dan.2cv
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Post by dan.2cv » 19 Apr 2005, 03:29

Well the switch is fine. It's actually the rear wash/wipe switch for the BMW. The relay is a complex affair which is quite rare 2nd hand, so I think I'll have to try and get it tested as I don't have any friends with a similar car I can try one off of. I dread to think what the MD would want for one.

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Kowalski
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Post by Kowalski » 19 Apr 2005, 04:12

You have to be careful using the resistance meter around air bags and seatbelt pretensioners but I can't think of anything else off hand that you can damage with a meter.
All you have to be careful of is shorting stuff with the ammeter, when you're using the ammeter think of it as a length of wire, i.e. you wouldn't try connecting positive and negative on the battery together with it would you? Lots of heat would follow and the magic smoke would come out of the meter.

bxbodger
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Post by bxbodger » 19 Apr 2005, 22:29

I'm no auto electrician, or any sort of electrician for that matter, but I do know of a useful way to use the amp setting.
The Amps range can be of some use for pinning down a faulty component. If your battery is constantly going flat with no apparent reason you can put the ammeter in series between the + battery terminal and the + lead and look for any current reading. You can then isolate the cause by pulling and replacing fuses one by one until the reading drops-i.e. the faulty item is no longer connected.
It will obviously change when things like the clock or radio are disconnected, but its odd things you are looking for which shouldn't be live with the ignition off.

jeremy
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Post by jeremy » 24 Apr 2005, 02:53

You can safegaurd the meter on amps settings by connecting it in series with a 21 watt bulb which will limit the current to about 1.8amps. If necessary you can always detect fluctuations by measuring the voltage drop accross the bulb.
In fact rarely is it necessary to measure current accurately on a 12 volt system. If you do and don't have the appropriate meter for the likely current - you can always use Ohms Law - just don't ask me to work out how to calculate shunt resistances.
jeremy

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Kowalski
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Post by Kowalski » 24 Apr 2005, 05:11

Shunt resistances?
V=IR
Thats all you need to know...

tomsheppard
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Post by tomsheppard » 24 Apr 2005, 15:30

However R=V/I may be more useful.

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Kowalski
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Post by Kowalski » 24 Apr 2005, 20:43

How about I=V/R too.