- To calculate the value of a resistor (in ohms) to be inserted between a voltage supply and some electronic part/device, subtract the voltage the device expects (target voltage, in volts) from the supply voltage (in volts), then divide the result by the current the device expects (target current I, in amps).
For example, to power a 2V / 35mA LED (light-emitting diode) by a 9V battery, insert a (9V - 2V) / 0.035A = 200Ω resistor between the positive terminal of the battery (supply voltage) and the anode (positive pin) of the LED. Finish the circuit by connecting the cathode (negative pin) of the LED to the negative terminal (ground) of the battery.
If the result of the formula is a value of a resistor that does not usually exist, e.g. 95.3 Ω, use the next *higher* value available, e.g. 100 Ω (or else you will blow the LED or other device).
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