amperage

[AdamT]

From post #396


A) A simple method for measuring large currents is to place a
very low value resistor (of appropriate power rating) in series with a
high current. You can then measure the voltage change across the
resistor (which could be positive or negative depending on the current
flow) and calculate the current using Ohm's law. The reason for the
text about the low impedance input being ok is that, technically, you
are very slightly changing the output, and text just above it says
that you can't directly affect the outputs. Basically, we are giving
specific permission to directly affect the outputs in this
insignificant manner. The same goes for high impedance voltage
monitoring inputs.

> If so, I assume we are allowed to measure
> motor voltage with respect to ground as well?

A) Yes. Keep in mind, however, that the motors run in two directions,
so you'll need to design a circuit that can either measure positive
voltages relative to ground on both motor leads, or design a circuit
that can measure both positive and negative voltages across the two
motor leads.

> If not, would it be
> acceptable to use a long length of wire to *act* as a series
> resistor? Thanks for clearing this up!

A) Using a long wire rather than a precision resistor of known value
is allowed but not recommended. Keep in mind that, for current
measuring applications, you will likely want a sub-1 ohm resistor in
order to avoid burning up your circuit. Many multimeters cannot
accurately measure resistances that low, and therefore it would be
difficult and perhaps even dangerous to manually calibrate the wire's
resistance.