Homemade amp-meter

[Dillzio]

Hi all,

I was wondering if it would be possible to take 2 multimeters, each of which are rated to 10 amps, and hook them up in parallel to measure a current of about 15 amps? I did some surfing on the subject, and came across an Electrical Engineering Training reference that had a section on how to measure the current of circuits with very low resistance accurately:
www.tpub.com/content/neets/14193/css/14193_23.htm

It says that:

The accuracy of current measurements depends on the internal resistance of the meter as compared with the resistance of the external circuit. If the total circuit current is decreased by increasing the load, then the percentage of error will decrease. Therefore, greater accuracy is obtained if the meter resistance is considerably less than the load resistance. A method of obtaining greater accuracy of current measurement is to decrease the total internal meter resistance with respect to load resistance. This is accomplished by connecting two ammeters in parallel with each other and in series with the circuit in which the current is being measured. Additional ammeters may be connected in parallel in the same manner for increased accuracy. This method also increases the range of measurements that can be taken. The arithmetical sum of the indications of all the parallel meters represents the total current flow in the circuit. You should note that this is not a common test method and that your test equipment may be damaged if connected incorrectly



So, according to this, it CAN be done. Has anyone ever tried it before? I already have one multimeter (one that still measures amps anyway), one more and I'll have myself a 20a multimeter!

[flydiver]

$3 Wattmeter
www.rcgroups.com/forums/showthread.php?t=949923

Also look at the link in post #3.

[Dillzio]

Thanks for the tip.

I've been going over the circuit trying to figure out how it works, and it's just making my brain hurt.

I guess it's shunting more power 'around' the ampmeter, instead of through it, then you calibrate it to read 1/10 what it should. I still don't get HOW it works though.

EDIT:
Just went over that post #3 again, thanks for the tip on that one, ingenious!

So, I take it that the resistance of 12 inches of 10-gauge copper wire is exactly 0.001ohms, and you divide voltage drop by resistance to get amperage. If you measure in millivolts, it's 1/1000 of a volt, and if the resistance is .001 you're multiplying by 1000, so the two cancel each other out.

According to the information here:
www.epanorama.net/documents/wiring/wire_resistance.html
The resistance of #10 copper wire is .204ohms per 100 ft, which is .00204 ohms per ft (12 inches). That's twice as much as the above instructions assume, so all your readings would be out by a factor of two.

I guess the only way to be sure is to get a long length of wire, like 20ft, and measure the resistance yourself, then cut a piece to size to make the .001ohm shunt.

[Dillzio]

...but I did work out an easy way to make a .01 ohm shunt... buy 10 0.1ohm wire-wound resistors and hook them up in parallel. Read the voltage drop across the shunt in milivolts and divide by 10 to get the amps.
Wouldn't be too hard to check the calibration of it either by measuring a light bulb of known resistance, or measuring the output of a battery charger... or I could just buy the $25 Turnigy one from HobbyKing.


So this thread has THREE ways of making your own high-current ampmeter. A tight-arsed RC fanatic's dream!

1) Hook two ampmeters together in parallel
2) Measure the voltage drop across a shunt
3) Modify your multimeter, to have the wires for measuring current attaching DIRECTLY to the shunt, and set their spacing so that the ampmeter reads a fraction of what it should (2, 5 or 10 times).