Basic Multimeter function

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Basic Multimeter function

#1

Post by Whiskerfish »

I know many times I have read about guys delving into electrical stuff for the first time. Many have never used a Multimeter. It is not hard. If you think about it in the bigger picture Electricity is very similar to Plumbing, very simply you have a supply and a return and controls that tell it what to do. These Meters just help us see what is going on inside the pipe.

The 2 Functions we as bike Mechanics need 95 % of the time are measuring resistance (ohms) and measuring pressure (Volts).

This is a Basic Fluke, Very common meter. I think I paid about 60 bucks at a pawn shop for it. Many others out there but the symbols will mean the same thing
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Some meters will have each of these functions and have scales to select for each function. For example 1-20 Volts, 20-100 Volts, 100-1000 volts etc etc.. You want to make sure you select a range high enough not to harm the meter. So if you are working on a 12 volt system you want to select a 20 volt range or better. This meter and some others do that automatically.

Off is self explanatory I hope. The first position to the right of Off is used to measure AC (Alternating Current) Volts. Notice the Sine Wave above the V. The next position is used to measure DC (Direct Current) Volts, the third position is used to measure DC Mili-Volts. That is very Low voltage below 1 Volt. The fourth position (currently selected) is used to measure Resistance Ohms. Electricity for our purposes is measured using Volts and Resistance is measured using Ohms. Everything we will do with Volts will be below 100 Volts so there should not be any other unfamiliar abbreviations.

Ohms (resistance) is measured 1-999 then at 1,000 it is referred to as 1 K (thousand) Ohms. 1,600 ohms would be referred to as 1.6 K Ohms. It goes on that way up to 999 K Ohms and then at 1,000 K Ohms it is referred to as 1 Meg (million) Ohm. That is why you see the M on the display above. So basically the K and M are just abbreviations for the next upper scales in resistance. Rather than writing out 1,600,000 ohms I can just say 1.6 M or meg. Resistance also has a few unfamiliar terms. If you put the probes together and the meter drops to a very low reading that is called reading "Continuity" or "shorted" (see pic below). Once you release the probes then the circuit no longer has continuity and is referred to as "Open".
One quick note about reading Resistance especially in the upper scales. Do not allow your fingers to contact the probes and this will skew readings.
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Here you can see I am measuring the resistance of a jumper wire and it measures 4 tenths of an ohm. Not great but for most applications it will do.
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On our Bikes that only time you would need to use the AC (Alternating Current) scale is when checking the output of the Stator via the three yellow wires. Everything else on the bike operates from DC (Direct Current).

And then here I am testing a battery for my drill using the DC function.
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Re: Basic Multimeter function

#2

Post by Toehead »

Great work Whisker!


Continuing the plumbing analogy:
Voltage and Resistance are also related to the amount of electrical current (Amps) that flows through the system, much like flowrate in pluming (GPH) is related to pressure and resistance (pipe diameter).

They are related as follows:
Voltage = Current in amps X Resistance in ohms, or V=I*R

A load that has a a 10 ohm resistance will draw 1.2 amps on a 12V system.


Power is the amount of work that can be done by the system (similar to horsepower).

The power (units of Watts) is related to the current and resistance as follows:

Power (watts) = Current squared * Resistance.

Since we know that V=I*R, I=V/R. We can substitute that into the power equation to get:

Power= V^2/R

This is important for wire sizing. If you have ever worked on a 6 volt system, you may have noticed that the wire sizes on these systems are much larger. This is because for 6Volts, you need to flow twice the current (amps) to do the same amount of work (Watts)
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Re: Basic Multimeter function

#3

Post by 77Gowing »

Common Electrical Units and scientific notation

Mega: 1,000,000 or 1 x 10^6 or 1M
Exm: 3.2M VDC, 3,200,000 VDC and 3.2 x 10^6 VDC and reads 3.2 Mega Volts DC
Exm: 5.1M ADC, 5,100,000 ADC and 5.1 x 10^6 ADC and reads 5.1 Mega Amps DC
Exm: 7.5 MΩ, 7,500,000 Ohms and 7.5 x 10^6 Ohms and reads 7.5 Mega Ohms

Kilo: 1,000 0r 1 x10^3 or 1k
Exm: 3.2k VDC, 3,200 VDC and 3.2 x 10^3 VDC and reads 3.2 Kilovolts DC
Exm: 5.1k ADC, 5,100 ADC and 5.1 x 10^3 ADC and reads 5.1 kilo Amps DC
Exm: 7.5 kΩ, 7,500 Ohms and 7.5 x 10^3 Ohms and reads 7.5 kilo Ohms

Milli: .001 Or 1 x10^-3 or 1mili
Exm: 3.2 mVDC, 0.0032 VDC and 3.2 x 10^-3 VDC and reads 3.2 Millivolts DC
Exm: 5.1 mADC, 0.0051 mADC and 5.1 x 10^-3 ADC and reads 5.1 Milliamps DC
Exm: 7.5 mΩ, 0.0075 Ohms and 7.5 x 10^-3 Ohms and reads 7.5 Milliohms

Micro: .000001 Or 1 x10^-6 or 1micro or 1µ
Exm: 3.2 µVDC, 0.0000032 VDC and 3.2 x 10^-6 VDC and reads 3.2 microvolts DC
Exm: 5.1 µADC, 0.0000051 µADC and 5.1 x 10^-6 ADC and reads 5.1 micro amps DC
Exm: 7.5 µΩ, 0.0000075 Ohms and 7.5 x 10^-6 Ohms and reads 7.5 micro Ohms


Now what about the unit of Joltz or Joltage? (Wet yer fangers and put em in the light socket...Joltage!)
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Re: Basic Multimeter function

#4

Post by Liam »

The Fluke 73 above is a very good multimeter. They are not cheap, but they are good enough to last you a lifetime.
I am on my second one which I purchased in 1991 to replace one that was stolen out of my toolbox, which I got in 1983.
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Re: Basic Multimeter function

#5

Post by desertrefugee »

Well, as a long-time electrical engineer, I'll throw out another very handy automotive use for the basic DMM. Using the Fluke 73 above to measure current in a car (or motorcycle) can quickly isolate offending, but elusive battery drains.

Before beginning, make sure your charging circuit is not at fault. I'm assuming here that this confirmation has already been made.

1) To run the DMM in current mode, the Fluke (and most others) will need the positive lead moved to the DC Amp jack.

2) Current measurements MUST be made with the instrument in series with the circuit. The best way to start with an unexplained battery drain is to confirm a discharge from the battery.

3) Make sure the main switch is OFF and any other live switches are OFF. Disconnect the POS battery terminal and connect the corresponding POS DMM lead to the battery and the NEG DMM lead to the cable removed from the battery.

4) Make sure the leads are configured as noted in 1) above. Switch the DMM to DC Current (Amps) mode. (Not the one with the sine/alternating current symbol).

5) A reading of greater than, roughly 5 to 10mA (.005 - .010) will indicate a low level drain. If it's an Aspencade, this might be a digital clock or stereo head. 20 - 40mA (.020 - 040) will begin to indicate more troublesome issues. Greater than 50mA is a level that will begin to pull a motorcycle battery down after a week or two. 100mA will drain it in a hurry.

6) So, we've established a magnitude of drain (#5 above), but where is it coming from? It can get tricky here, but most circuits can be isolated by reading the meter configured as directed in #5 and, one-by-one, pulling fuses to the various circuits. Once you see the current drop, you'll know the fuse you just pulled is the "bad acting" circuit.

At this point, you're on your own finding the actual fault. Lights circuits could be bad sockets or switches. A bare wire shorting to frame can be the issue on any current drain. You can even employ the DMM in either resistance or voltage mode to help zero in on the problem.

But, that would be another installment...
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