On Fri, 26 Nov 2004 10:32:55 -0500, "Glenn Ashmore"
wrote:
I should know this but I just can't get my brain around it.

I have a data logger that will accept 4-20 mA or 0-2.5VDC. I want to track
the current going to the refrigeration compressor which I figure will be in
the range of 0 to 25 amps (to be safe for start ups).

What is the best way to accomplish this?

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As others have mentioned a current shunt is one way to do this.

However, since you're data logger can also be voltage driven, I think
it would be cheaper and easier to build a voltage divider with a
couple of resistors in series across the compressor circuit.

A voltage divider divides in proportion to the two resistors, i.e., if
you put a 9,000 ohm resistor is series with a 1000 ohm resistor, the
voltage across the 1000 ohm resistor will be 1/10 of the voltage
across both. Keep the total resistance high enough to minimize
current flow.

Assuming a 12 volt compressor and a desired output of 2 volts for your
data logger, you need a resistor ratio of approximately 1/5 in series
with each other. A 1000 ohm and 5000 ohm would work OK assuming your
data logger has a high impeadance input with respect to the resistor
bridge.