Ohm's law describes how voltage, current, and resistance in a circuit are related. Ohm's law can be written in three equivalent forms; the formula you use depends on the quantity you need to determine.

1. Ohm's Law can be stated as follows:

I= V/R

For a constant resistance – If the voltage applied to a circuit board increased, the current will increase; and if the voltage is decreased, the current will decrease.

For constant voltage – if the resistance in a circuit is increased, the current will decrease; If the resistance is decreased, the current will increase.

2. Ohm's law can also be stated another equivalent way:

V=I*R

With this formula, you can calculate voltage in Volts if the current in amperes and resistance in ohms are known

3.There is a third equivalent way to state Ohm's law:

R=V/I

This form of Ohm's law is used to determine resistance in ohms if the values of voltage in volts and current in amperes are known.

Example

My track voltage is 13Volts and the bulb I am about to use is a 1.5V 15mA device. I need to find the voltage limiting resistance value.

13V – 1.5V = 11.5V (the bulb voltage was subtracted from the track voltage) 15mA = 0.015A Using the R=V/I Ohm's law form we got following:

R=11.5/0.015 R= 767 Ohms Choose closest resistor value R=780 Ohms.

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