Skip to content
LearnEE
Sign inGet started

Electronics

How to wire LEDs in series

LEDs in series share one current, so a single resistor can set it for the whole string. Subtract every LED's forward voltage from the supply, then divide what's left by the current you want: R = (Vsupply − n × VLED) ÷ I, where n is the number of LEDs.

One path, one current

In a series circuit every part sits on the same path, so the same current flows through each one. Charge can't pile up between two LEDs or leak away, so whatever passes the first LED passes the last.

The voltages are different. Each LED keeps its own forward voltage, about 2 V for red and 3.2 V for white, and the resistor gets the rest. Round the loop, the drops add up to the supply.

Live experiment

Add a resistor to the string

A 9 V battery, a row of 220 Ω resistors and two red LEDs, Pip and Dot, all on one path. Add a resistor and both dim together, and the two current windows always agree. Take every resistor out and see what holds the current back.

Try it yourself: free lesson

“Series circuits” is lesson 1 of 7 in Junction City, Electronics World 2. Free with an account. Lessons unlock in order, so you’ll do the 7 before it first.

Size the resistor

Three white LEDs on a 12 V supply, at 20 mA:

R = 12 − 3 × 3.2 over 0.02 = 2.4 over 0.02 = 120 Ω

120 Ω is a standard E12 value, so it fits exactly. Two red LEDs on 5 V from USB, at 15 mA:

R = 5 − 2 × 2.0 over 0.015 = 1 over 0.015 ≈ 67 Ω

Round up to the next standard value, 68 Ω, so the current lands just under the target. The resistor's power is small: 2.4 V × 0.02 A = 0.048 W in the 12 V string.

Why strings are efficient

The same three white LEDs could sit in three parallel branches, each with its own resistor. Then each branch draws 20 mA, 60 mA in all, and each resistor burns 12 − 3.2 = 8.8 V. The series string draws 20 mA and burns only 2.4 V. That's why LED strips and backlights wire their LEDs in short series strings.

The catch

A series string has one path, so one broken LED stops the current everywhere and the whole string goes dark. Fairy lights used to fail exactly like this. And the LEDs must all fit under the supply voltage: on a 9 V battery, two white LEDs (6.4 V) work, but three (9.6 V) won't light properly.

Free lesson

Now build it yourself

Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.

Try it yourself: free lesson

“Series circuits” is lesson 1 of 7 in Junction City, Electronics World 2. Free with an account. Lessons unlock in order, so you’ll do the 7 before it first.

Or start from the very first lesson, no account needed

Work it out

Practice questions

Three quick questions from the lessons. Pick an answer to check it.

  1. Question 1 of 3Three white LEDs from 12 V. Which wastes least?
    Show the answer

    Answer: One string with one resistor. The string draws 20 mA instead of 60 mA, and its resistor only burns 2.4 V. Branches with no resistors would burn out.

    • Why not “Three branches, no resistors”? With no resistors, nothing limits the current, so the LEDs draw far too much and burn out.
    • Why not “They all waste the same”? The string draws 20 mA and wastes 2.4 V of it; three branches draw 60 mA and waste 8.8 V each. They're nowhere near equal.
    • Why not “Three branches, each with its own resistor”? Each branch draws its own 20 mA, 60 mA in all, and each resistor burns 8.8 V of the 12. That's far more heat than one string.
  2. Question 2 of 3A 6 V pack of four cells drives 100 mA. The current through the second cell is…
    Show the answer

    Answer: 100 mA. One loop, one current. Every cell in the chain carries the same 100 mA, so each drains at the same rate.

    • Why not “400 mA”? Current can't grow along a chain: charge doesn't pile up or appear from nowhere, so one loop has one current.
    • Why not “25 mA”? 25 mA is 100 mA shared four ways, which only happens to cells side by side. In a chain, each cell carries the whole current.
    • Why not “Zero”? Every cell in the chain is on the one path, so the full 100 mA flows through each of them.
  3. Question 3 of 3Why does an LED need a series resistor?
    Show the answer

    Answer: To limit the current. Once an LED is on, a tiny rise in voltage causes a huge rise in current. The resistor sets a safe current.

    • Why not “To store charge”? Storing charge is a capacitor's job. A resistor stores nothing: it limits the current and turns its share into heat.
    • Why not “It doesn't need one”? An LED's current rises steeply with a small rise in voltage, so without a resistor it draws far too much and burns out.
    • Why not “To change its colour”? An LED's colour comes from the material it's made of. A resistor only changes how much current flows.
Keep practising: free lesson

“Series circuits” is lesson 1 of 7 in Junction City, Electronics World 2. Free with an account. Lessons unlock in order, so you’ll do the 7 before it first.

Questions people ask

How many LEDs can I put in series?

Add up their forward voltages and keep the total a volt or two below the supply, so the resistor has something to work with. On 12 V that's three white LEDs (9.6 V) or five red ones (10 V). Four white LEDs need 12.8 V, more than 12 V can give.

Do LEDs in series need a resistor each?

No. One current flows through the whole string, so one resistor sets it for every LED. LEDs in parallel are different: each branch needs its own resistor, or the LED with the lowest forward voltage hogs the current.

If one LED in a series string fails, do the others go out?

Usually, yes. Most LEDs fail open, which breaks the only path, so the current stops and the whole string goes dark, like old fairy lights. One that fails shorted leaves the rest lit, a little brighter.

Does it matter where the resistor goes in the string?

No. At the start, the end or between two LEDs, it carries the same current as everything else on the path. Its resistance sets that current, not its position.

Related topics

Every topic