The most common LEDs need to have current through them limited to no more than 20mA while forward biased. Usually a resistor is what is used to limit that current.

Current (I) through the resistor is the voltage (V) across it divided by it’s resistance (Ω). I=V/R . Current in amps, voltage in volts, and resistance in ohms.

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There are three main factors in determining how much current will flow through a forward biased LED protected by a resistor.

Voltage across resistor and LED

Voltage drop (forward voltage) of the LED

Resistance of the resistor, which has the total voltage minus the LED voltage drop across it.

LEDs are a type of diode. They have to be inserted in the circuit in the forward biased direction to conduct. They will just block current, up to their breakdown voltage, while reverse biased. If breakdown voltage is exceeded (not much more than 9V from my testing) they will be destroyed.

Forward biased (FB) – Anode more positive than cathode.

Reverse biased (RB) – Cathode more positive than Anode.

Diodes primarily intended to block higher voltages while reverse biased are called rectifier diodes, or just diodes.

Voltages across a resistor and forward biased LED/diode

How voltage goes across LEDs based on forward voltage drop schematic demonstrated and multimeter



The resistor should also have twice the wattage rating of the power dissipation expected of it. Most through hole resistors that are commonly used are rated for 1/4W (0.25W).

Also check out.

Common voltages for powering LEDs

There are 3 supply voltages across the protective resistor and LED schematic circuits in the diagram near the top of the page. They are common supply voltages.

Common supply voltages are 5V, 9V and 12V.

Common LED forward voltage drops

The LED you are protecting will have a forward voltage. This is the amount of voltage needed to get the forward biased (FB) LED conducting. That will be the amount of voltage built up across the LED and therefore removed from series components.

Forward voltage of LEDs is commonly around 2V-3V at 20mA. At lower currents, the forward voltage will be slightly lower.

A multimeter can be used to measure the voltage across an LED while it is being powered and protected by a resistor. Doing so will give you the actual forward voltage of that particular LED at that particular current.

Series LEDs will have their voltage drops add up.

Setting LED current with a series resistor

After the LED has enough voltage to start conducting, the resistor will limit current based on the supply voltage minus the voltage dropped by the LED from reaching the resistor.

Vcc – diode forward Vdrop = voltage across resistor.

Current (I) through the resistor and LED will be set by the voltage (V) across the resistor. The amount of voltage across the resistor will be the total voltage minus the voltage dropped by the LED. Take that voltage and divide it by the resistance (R) of the resistor. I=V/R. Current is usually given in milliamps (mA). Sometimes the schematic diagram of the circuit will show the calculated current amount in a circle in series with the components. I did so in the example circuits near the top of the page. A multimeter measuring current can be inserted in series like that to get an accurate measurement of the actual circuit current.



Resistor wattage

Resistive components generate heat while limiting current. Heat is their form of power, which must be dissipated because it is waste energy unless your primary goal is to use it to heat something. The most common resistors are rated for 1/4W (0.25W) of power dissipation. They should however, still only be expected to dissipate half of their wattage rating over extended periods of time. Aim for 1/8W (0.125W) or less.

Power (P) in watts (W) is the voltage across a component times the current through it. P=VI.

In the examples above, it should be clear that higher voltages heat up resistors a lot more than lower voltages. So be extra cautious as voltages rise.

Bonus video and diagram

Voltages of series LEDs plus current and color variation effects electronics demonstration

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