PCB Design
Daisy chain vs star power distribution
A daisy chain feeds several loads from one trace that runs past each in turn. A star gives every load its own trace from a single point at the supply. In a chain the early segments carry everyone's current, so the voltage drops stack up, the last load gets the least, and its voltage moves whenever a neighbour switches. In a star each load's voltage depends only on its own current.
The drops stack
Take a 5 V rail feeding three 100 mA loads through three 0.5 Ω segments of trace:
| Segment | Carries | Drops |
|---|---|---|
| T1 | 300 mA, all three loads | 0.15 V |
| T2 | 200 mA, the last two | 0.10 V |
| T3 | 100 mA, the last one | 0.05 V |
The last load sits 0.3 V down, at 4.7 V. (The loads sag a little, so each draws a touch under 100 mA and it really gets 4.71 V.) The first segment also runs hottest: heat is I² × R, so carrying three times the current makes nine times the heat of the last.
Neighbours brown each other out
Switch LOAD1 off and T1 now carries only 200 mA, so LOAD3 comes up by about 46 mV. Switch a motor on near the supply end and everything after it dips. Loads on a chain modulate each other's supply, which is how a relay clicking on can reset a microcontroller further down the same rail.
Live experiment
Chain the rail, or star it
Three 100 mA loads on a 5 V rail through 0.5 Ω traces. In a chain the drops pile up and the last load gets 4.71 V; switch a neighbour off and it rises. Flip to a star, or widen the copper to 0.05 Ω, then switch the neighbours again.
“Daisy chain or star” is lesson 1 of 6 in Solder Strait, PCB Design World 2. Free with an account. Lessons unlock in order, so you’ll do the 7 before it first.
A star instead
Run a separate trace from the supply to each load. Each trace carries only its own 100 mA:
Switch a neighbour on or off and nothing else moves, because no trace carries anyone else's current. Thin traces are fine when each one is yours alone. The star point must be the supply itself: a trace that branches from another load's trace is a chain again.
Or use more copper
Making every segment ten times wider, 0.05 Ω, cuts the chain's total drop to 0.03 V and the last load gets 4.97 V. Multilayer boards go further and give power a whole copper layer, a power plane, so nothing shares a thin trace at all.
Which to choose
A chain is fine for a row of low-current parts. Use a star when loads draw heavy or switching currents, or when one load, like an ADC's reference or a sensor, must not see the others. The same thinking applies to the return path: that's star grounding. Long LED strips show the chain problem in daylight: fed from one end, the far end runs dim, so installers feed them from both ends.
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.
“Daisy chain or star” is lesson 1 of 6 in Solder Strait, PCB Design 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 neededPractice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: 50 mV: its own 100 mA through 0.5 Ω. Only its own current flows on its own trace: 0.1 A × 0.5 Ω is 50 mV, and no other load can change it.
- Why not “Nothing”? Every trace has resistance, so 100 mA through 0.5 Ω still drops 50 mV.
- Why not “0.15 V”? 0.15 V is what a chain's first segment drops carrying 300 mA. A star trace carries only 100 mA.
- Why not “0.3 V”? 0.3 V is the total stacked drop at the end of a three-load chain. On a star no trace carries more than its own 100 mA.
Show the answer
Answer: Comes up about 46 mV: LOAD1's current is off T1. T1 was carrying LOAD1's current as well as the others'. Without it T1 drops less, and everything after T1 comes up: 4.71 V becomes 4.76. On a star LOAD3 wouldn't move at all, because its trace never carried anyone else's current.
- Why not “Gets the full 5 V”? T1, T2 and T3 still carry LOAD2's and LOAD3's currents, so some drop remains.
- Why not “Drops further”? Less current through T1 means less drop, so LOAD3 rises rather than falls.
- Why not “Stays exactly the same”? On a chain, T1 carried LOAD1's current too. Take it away and T1 drops less, so every load after it rises.
Show the answer
Answer: 150 mV: its own 50 mV plus LOAD1's trace, now carrying both. LOAD1's trace now carries 200 mA, 0.1 V, and LOAD2's own trace adds 50 mV on top: 150 mV. Back to a chain.
- Why not “0.3 V”? 0.3 V is three stacked segments carrying 300, 200 and 100 mA. Here only two loads share one segment.
- Why not “Nothing”? Both traces have resistance and carry current, so LOAD2 loses 0.1 V plus 50 mV.
- Why not “50 mV”? That's LOAD2's own trace only. LOAD1's trace now carries 200 mA and drops another 0.1 V before it.
“Daisy chain or star” is lesson 1 of 6 in Solder Strait, PCB Design World 2. Free with an account. Lessons unlock in order, so you’ll do the 7 before it first.
Questions people ask
What is a daisy-chained power connection?
One wire or trace runs from the supply to the first load, on to the second, and on to the next, so every load hangs off the same line. It's simple and uses the least copper, but the first segment carries every load's current.
Is star power distribution always better than a daisy chain?
No. For small currents, or loads that don't mind a little sag, a chain is fine and simpler to route. A star earns its extra traces when loads switch heavily or a sensitive part, like an ADC's reference, must not see anyone else's current.
How do I calculate the voltage at the end of a daisy chain?
Work segment by segment. Each segment drops its own current times its own resistance, and its current is the sum of every load beyond it. Add the drops and subtract them from the supply.
Related topics
- Star grounding explainedGive each load its own return to one point, so a big current never flows through a sensitive circuit's ground.
- How to calculate voltage drop in a wireA wire is a resistor: how many volts a cable steals, the heat it makes, and why the grid runs at high voltage.
- PCB trace width for currentCount the squares: how a trace's width sets its ohms, its voltage drop and its heat, and how wide to draw it.
- Kelvin (4-wire) sensing explainedMeasure a shunt's voltage right at its own pads, so the copper either side can't add to the reading.