Buck converter output voltage drops when the load starts

If the output sags only under load, the converter, the input supply or the wiring cannot give the current.

What the message means

A converter needs enough input voltage above the output, a source that holds its voltage and a rating above the real load current.

Fix it in this order

Measure the input voltage at the converter while the load runs.
Check that the input is at least a couple of volts above the output.
Compare the load current with the converter rating, and leave margin.
Use thicker wires and good connectors on the input and output.
Add a capacitor on the input and output if the board has none.
Check the converter for heat, since many cut back when hot.

Why it happens

A buck converter needs an input that stays above the output plus a margin. When the load draws more current, the input supply can sag, the converter can reach its current limit or heat up, and the output falls.

Measure the input and the output while loaded, use a converter with a higher current rating, add a capacitor close to the input, and use a supply that can deliver the current.

If you see this, do this

You see	Do this
Output falls when the motor starts	Current peak. Add capacitance or a bigger converter.
Input voltage falls too	Source is weak.

set the voltage first, battery sag.

Last reviewed: November 2026. These are the usual causes and the order that finds them fastest. Test on your own board; clones and cables differ.

Why this happens

A buck converter takes power in at a higher voltage and delivers it at a lower voltage, so the input current depends on the output power. Input current is output power divided by efficiency and by input voltage. A lower input voltage means a higher input current for the same load, and that current flows through your battery and wires.

Three things can collapse the output under load. The source may sag. The wiring may lose too much voltage on the way. Or the converter may reach its own current limit or temperature limit and reduce its output. The converter also needs some headroom: its input must stay above the output by its dropout margin.

How to confirm the cause

Measure the voltage at the converter’s input terminals, not at the battery, while the load runs. A drop there means the source or wires are weak.
Measure the output at the converter and again at the load. A difference means wire loss on the output side.
Use a meter with a minimum hold function to catch short dips when a motor starts.
Try a resistor load that draws a known current. If the output holds at that current but not with the motor, the motor start current is the problem.
Feel the converter after a minute. Very hot suggests it is overloaded.

Worked example

Input current for a 5 V, 2 A load, assuming 85 % efficiency. The efficiency is an assumption; check your converter.

P_out = 5 V * 2 A = 10 W
P_in  = 10 / 0.85 = 11.76 W
I_in at 12 V  = 11.76 / 12  = 0.98 A
I_in at 7.4 V = 11.76 / 7.4 = 1.59 A

wire loss at 7.4 V: 1.59 A * 0.1 ohm = 0.16 V lost before the converter

Common mistakes

Setting the output close to the input, which leaves no headroom.
Adjusting the output with no load and not checking it with the load connected.
Trusting the peak current rating when the load is continuous.
Using thin or long wires between the battery and the converter.
Running servos and logic from the same converter without extra capacitance.

If this did not fix it

The source is the real limit: a low battery, a weak adapter, or a connector with high resistance.
A poor solder joint or a loose screw terminal on the input or output.
A converter that cannot deliver the current its label claims. Test it with a known load.