Hiccup protection: why does the board go 'tick tick'?

You plug the board in and hear a regular 'tick... tick...' or chirp, and the standby LED blinks. The board isn't dead; it's protecting itself and trying to start again and again.

What does it do? Hiccup means the supply runs briefly on an overload or short, stops for a long time, then tries again. In most flybacks this happens by itself: the start-up resistor charges the VCC capacitor and the IC starts; but with the output shorted the auxiliary winding can't feed VCC, VCC drops and the IC stops. The cycle repeats. The average power stays small, so the MOSFET and diode don't burn. Some ICs do the same with a dedicated timer.

Tap the dashed parts for what they do, how to measure them and how they usually fail. Numbered points are scope measurements.

Phase 1/3: 1. VCC charging

The IC is off. The start-up resistor slowly charges Cvcc. Nothing happens until VCC reaches the turn-on threshold (e.g. 16 V on a UC3842): this wait is the silence between the 'ticks'.

1. VCC (IC supply)

A sawtooth: rises slowly to 16 V, drops quickly to 8–10 V while the IC runs, then charges again. If you see this shape, the board is hiccuping.

On the scope

Multimeterswinging between 8 and 16 V

🛠 Fault scenarios: pick a symptom and see what to measure, in order, on the schematic

🔍 How to spot it on a board

•A regular 'tick tick' or chirp (the transformer vibrating on each burst)
•A blinking standby LED, or an output that comes and goes for a moment
•A sawtooth on VCC swinging between 8 and 16 V

🔧 How to measure it

•Unplugged, measure Ω to ground on each output rail; the one near zero is suspect.
•Unplug the boards being fed (mainboard, LED bars): if the ticking stops, the fault isn't on the power board.
•Scope VCC; a sawtooth means hiccup.
•Watch the output: if it reaches full value on each burst, there's no short; suspect overload, feedback or over-voltage protection.

⚠ Common faults

•A shorted output diode or output capacitor: the most common cause.
•A dried-out VCC capacitor: the board hiccups even with a healthy output; the cheapest fix.
•An open auxiliary winding diode or a burnt series resistor: the IC stops every time Cvcc runs out.
•A current-sense resistor that has drifted up: even normal load looks like overcurrent.

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Go further

Concept: start-up and auxiliary winding Concept: current sensing Concept: series bulb How it works: TV power board The real flyback circuit