Tech Tips 9# Power Amps

Tech Tips
9th Edition, 2008
Volts, Power...resistance etc?
I often get asked about the relationship between Volts, Amps, Ohms and Power so this week I thought I
would cover just that. This relationship is defined by ohms 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 that Voltage = Current multiplied by Resistance ie V=IxR This can also be stated as
Voltage divided by resistance equals current ie V÷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 and R=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 states Power
= Current multiplied by Voltage ie P=IxV. This can also be stated as Power divided by Voltage = Current ie P÷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.
Chris Henseleit
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Fax: (07) 3891 7722
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