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# Ohm's Law and your BMS

Updated: May 2

"Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship." Thank you WIKI.

This law is very useful for basic building automation and should definitely be in your tool box.

We can probably get away with saying that Ohm’s law maybe very foundation of how Automation can get done. The simple relationship of Voltage, Current, and Resistance makes controlling a building with a computer possible.

Ohm’s Law is stated as: V=I X R

R=V/I

I=V/R

V=voltage is represented in Volts

R = resistance is represented in Ohms

I = current is represented in Amps (note that 1000 milliamps = 1 Amp or 1 mA = 0.001A)

One common use of Ohm’s Law is signal conversion. Using a 4-20mA signal to operate a device controlled by a 2-10V signal is a fairly common occurrence in Building Automation and Energy Management. Here is how it works:

V=I X R

V = 4 mA X 500Ω

V= 2

V=I X R

V = 20 mA X 500Ω

V = 10

Applying a 500Ω resistor in parallel to a 4-20 mA signal creates a 2 – 10 Volt control signal.

Another application to consider is how does a Building Automation or Energy Management system sense temperature? Ohm’s Law, that’s how. Thermistors and RTDs are devices that have resistances that change with temperature. Since resistance can be calculated by applying known voltage and current values, computers can “sense” temperature using Ohm’s Law (R = V/I). Once the resistance is known the temperature can be associated via either a calculation or look up table.