Ohms Law

Ohm's Law

In the controls world, we don't run into Ohm's Law very often, but it does come into play working with control signals.

The formula is I = V /R or V = IR

I is the current in amps

V is the voltage

R is the resistance in Ohms


Let's say you have a 2-10 vdc input on a controller but have a 4-20mA transmitter. You have probably heard that you can use a 500 ohm resistor to "convert" the signal. Let's run this through Ohm's law to explain what is happening.

4 milliamps = .004 amps

Using the V = IR form above, if you take the .004 amps times 500 ohms, you get 2 volts measured across the resistor.

20 milliamps = .02 amps

Again using the V = IR form above, if you take the .02 amps times 500 ohms, you get 10 volts measured across the resistor.

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