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Old 08-13-2009, 09:42 AM   #1 (permalink)
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How to use a digital multimeter to measure current/amps

I had someone PM me this question (edited to protect the innocent):


Sorry to bring up your old thread - http://ecomodder.com/forum/showthrea...olts-8908.html

I'm planning on putting together a solar panel/deep cycle battery array/alternator delete in my car. I'm trying to figure out how big a panel/batteries i would need to keep up with the electrical needs of my car.

I'm kind of a novice to a DMM, I can do the basics volts/continuity measurements, but could you please explain how i would use the DMM to measure the different amps usage for different electrical components in my car?
I'm no DMM expert, but all the ones I've seen have three connections for the pair of red/black leads:

1 - ground or common (black)
2 - voltage/ohms/mA/continuity (red)
3 - current/Amps (red)

Most of the time your red lead is on #2 for reading voltage, etc.

To read current, you need to switch the red lead to the #3 connection.

At this point, note the maximum current draw for your meter - it's probably about 10A. If you try to run a higher current than rated through it you may either blow the internal fuse if there is one, or damage the multimeter if there isn't a fuse (I've only seen one meter that was unfused).

To get a current reading, you connect the multimeter inline - in series - with the load you want to measure.

EG. imagine a basic light bulb circuit with a battery. Normally batt (-) is connected to one side of the bulb, batt (+) on the other side of the bulb, and the bulb lights up.

To measure current of that circuit, you could disconnect batt (+) from the bulb and connect it to multimeter (+). Now when you touch the multimeter (-) lead to the bulb, the circuit is complete and power will flow through the meter, showing current and turning on the light.

When I measured the current draw of the accessories in the car, I disconnected the main battery (+) cable, and put the multimeter (+) on battery (+) and multimeter (-) on the battery (+) cable. Then turning on any accessories will show their current.

Obviously the engine wasn't running to do this. And you can't use your starter with the DMM in the circuit (too much current - it'll blow).

Note you'd have to measure and subtract the "base load" of switching your key to "ON" for accessories that require it.

If converting to watts, you should also measure the system voltage at the same time switch each accessory on - the more current it draws, the lower voltage will sag. Watts = volts * amps. So 5A at 12.7v = 63.5 watts, but 5A at 11.8v = 59 watts.


Don't forget to switch the DMM leads back to the usual connection when you're finished measuring current! Otherwise the next time you go to measure battery voltage (for example), you'll put full short circuit current through the DMM and blow the fuse or fry the meter if it's unfused!

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Old 08-13-2009, 02:33 PM   #2 (permalink)
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Thanks this will be very helpful some users on here .

Oh btw,
Originally Posted by MetroMPG View Post
If converting to watts, you should also measure the system voltage at the same time switch each accessory on - the more current it draws, the lower voltage will sag. Watts = volts * amps. So 5A at 12.7v = 63.5 watts, but 5A at 11.8v = 59 watts.
Does this mean, put the DMM back to reading voltage and battery back to normal, then taking the voltage reading from the battery with the particular accessory on?

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Old 08-13-2009, 07:18 PM   #3 (permalink)
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jedi, short of having two meters to use at the same time, you would need to re-set the meter (including moving the red lead from the one socket to the other, and setting the function of the meter to reading volts in the roughly 0-20V range) in order to check the voltage.

So yes--you're right.

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Old 08-14-2009, 12:25 AM   #4 (permalink)
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you can also use an inductive current probe
like the Fluke 80i110 or other offerings .
one place to get them is at AESwave.com

they output 10mv per amp or 100mv per amp so you need to do the math to see what you have , inductive current probes can measure hi current values without damaging the DVOM or DSO.

another way is
obtain a 1 ohm 10 watt resistor from radio shack , connect it in series with the load to be measured
connect the DVOM across the 1 ohm resistor, using ohms law E=IR
voltage equals resistance multiplied by current , but resistance will be 1 so therefore voltage will equal current .

so set the meter to DC volts and the value read will be the current measured .
with high loads , the resistor will pop , but test quickly and it works .

1 ohm resistors are much cheaper than fuses for fluke DVOMs .
.................................................. ...................
fuel pump current s for the main pump and transfer pump using two inductive current probes and Pico 6 .
the fuel Pumps are bad.
a DVOM would show average current if connected to either inductive current probe ... not as valuable a measurement

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