How can I measure amperage with a digital multimeter that says 200mA max?
I have heard that you can add an external resistor to the positive in line, after estimating what the amps might be, to the meter and than use a formula which incorporates the ohms of the resistor and the amount of amps that the meter displays. Basically what is the formula for this and how do I go about it?
Yes: The resistor is called a “Shunt”. The shunt is connected in series with the load (the device or circuit being measured) and the resistance of the shunt developes a voltage across it directly proportional to the current passing through it. For example a 1-Ohm (one Ohm) shunt with 1 Amp flowing will develope 1 volt across its terminals. Almost any low resistance can be used as a shunt, a long coil of wire can be used as a shunt if you measure it’s dc resistance accurately. (wire coils do not work well for AC circuits due to inductance but are fine for DC circuits)Keep in mind the accuracy of your Amp calculation is only as good as your measurement of the shunt value.
Therefore set your meter to measure “VOLTAGE”
Place the Shunt resistor in series with the load,
turn on the device or load.
Read the Voltage across the shunt.
You Know the Voltage now,
You Know the resistance .
to determine the current:
Divide the Voltage reading on your meter by the Olm value of the shunt” That number is the value of the current in amps.
Amps= voltage/resistance (olms Law) 101
example: shunt value = .5 Ohm (1/2 Ohm)
Voltage reading across the shunt = 2 volts
Divide .5 into 2 = 4 Amps
How-To use a digital multi-meter
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