Home & workshop Electronics

Series resistor calculator

Arrangement

Resistor values in ohms

Applied voltage

Total resistance 1,020 Ω

R = R₁ + R₂ + … over 3 resistors

In kilohms 1.02 kΩ

If wired in parallel instead 103.056 Ω

Current through the chain 4.902 mA

Total dissipation 0.0245 W

Resistors 3

R = ΣRₙ · same current through all

Resistors in series simply add: R = R₁ + R₂ + … A 220, 330 and 470 ohm chain totals 1,020 ohms. The same current flows through every resistor, and the voltage divides between them in proportion to their values.

How to use a series resistor

1 Enter each value in the chain.

2 Read the total resistance.

3 Add the supply voltage to see the current — the same current flows through all of them.

4 Check each resistor dissipation against its rating.

A series resistor is the standard way to drop voltage or limit current, and its value follows from Ohm’s law on the difference. If a 5 V supply feeds something that wants 3.3 V at 20 mA, the resistor has to drop 1.7 V at 20 mA, which is 85 Ω. The catch is that the drop depends on the current: change the load and the voltage changes with it. That is why a series dropper is fine for an LED, whose current you control deliberately, and unsuitable for a circuit whose consumption varies.

Questions

How do I add resistors in series? −

Just add the values. Three resistors of 220, 330 and 470 ohms total 1,020 ohms.

Does the same current flow through all of them? +

How do I choose a dropping resistor? +

Which resistor gets hottest in series? +

Does tolerance add up? +

Was this tool any good?

INTERNAL SIGNAL ONLY · WE USE IT TO FIND TOOLS WORTH REBUILDING