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Old 08-13-2009, 10:42 AM   #1 (permalink)
MetroMPG
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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):

Quote:
Hello,

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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