Tech Tips: Volts, Power... Resistance Etc?

I often get asked about the relationship betweenVolts, Amps, Ohms and Powerso this week I thought I would cover just that. This relationship is defined byohms law.

The symbols and abbreviations used are as follows:-

V= Voltage, measured in volts
R= Resistance, measured in ohms
I= Current measured in amps
P= Power measured in watts
mA= milliAmps. 1000mA = 1 Amp

Ohms law states thatVoltage = Currentmultiplied byResistanceieV=IxRThis can also be stated as Voltage divided by resistance equals current ieV÷R=I

This means that if we know the voltage and the resistance we can calculate the current draw of a device. For example, we have a 12v supply driving an electric strike with a coil resistance of 50 ohms and need to know the current draw.

In this example:-

V= 12 andR= 50 so our equation can be written as 12÷50=0.24. Therefore the electric strike will draw 240mA.

Another common application of ohms law is in power calculations. The equation for power statesPower = Currentmultiplied byVoltageieP=IxV. This can also be stated as Power divided by Voltage = Current ieP÷V=I

Using the above information we can calculate the current draw of a device, like a camera, if we know its Power consumption (Watts) and supply voltage. A typical figure for a camera is 3 Watts.

Assuming our supply voltage is 12v then the camera will draw 3(W) ÷ 12(V)=0.25A, or as it is more commonly written 250mA.

Considering our above lock example its power requirement is 0.24A x 12v = 2.88 Watts.