Series bulb: a safe first power-up after a repair

You've replaced the blown MOSFET and fuse. If you plug straight in and there's another short, the new parts blow too. An incandescent bulb prevents that.

What does it do? An incandescent bulb in series with the device limits the current to the bulb's rating on a short and takes the voltage itself. Its brightness also shows the device's state: dim means healthy, steadily bright means there's still a short.

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

Phase 1/2: Healthy: the bulb dims

At switch-on the bulb flashes (while the capacitor charges), then dims. The device draws little current, so almost all the voltage goes to the device and it runs.

1. Bulb current with a healthy device

Rises for a moment, then drops to the device's normal current (below a few hundred mA).

On the scope

Multimeterbulb dim

🔍 How to spot it on a board

•A simple test box made of a lamp holder, a socket and a plug
•An LED bulb won't work; it must be incandescent or halogen

🔧 How to measure it

•A 40–60 W bulb for small devices, 100 W for TV boards and PC supplies.
•With the bulb dim, measure the main voltages (bulk capacitor, standby); then take the bulb out.

⚠ Common faults

•Steadily bright: there's still a short, switch off at once.
•Flashing on and off: the device is protecting itself, or the bulb is too small (big load).
•Doesn't light and the device doesn't run: an open in the input (fuse, NTC, filter) or the bulb is blown.

🔒 This content is in the repair pack

The repair pack unlocks all interactive content in one go: diagrams and current flows, the Repair Workshop, simulations, concepts, IC guides, the Component Library and comparison guides. One sample in every section is free.

Repair pack: 5,000 ₺, members get 10% off → 4,500 ₺. Or 1-week access for 1,000 ₺.

Go further

Concept: input protection Repair Workshop: TV power board Repair Workshop: PC power supply