Biltema Power Drill Charger 17-277 Repair

Published 8 February 2020

Safety warning: This charger is connected directly to 230 V mains. Troubleshooting and repair should only be carried out by someone familiar with mains-powered switch-mode power supplies and the associated shock and fire hazards.

A short repair note for a completely dead Biltema 17-277 power-drill battery charger.

Symptom

The charger was completely dead.

Components to check

In my case, the following parts were worth checking or replacing:

Fuse: 2 A slow-blow (2AT).
Mains-side bridge rectifier: replace with a suitably rated rectifier for 230 V AC mains use.
ZD2: check whether it has failed short-circuit or has physically blown off the PCB.

ZD2 gate protection

ZD2 appears to protect the gate of the switching transistor. The gate-driver circuitry can apparently produce a voltage higher than the transistor's maximum permitted VGS, so the Zener clamps the gate voltage.

I did not determine the original ZD2 value with certainty. I had a small 18 V Zener available and used that successfully in the repaired charger.

Important: 18 V is the value that worked in this particular repair; it is not confirmed to be the original manufacturer's specification. The replacement value should be checked against the switching transistor's VGS(max) and the actual circuit.

The Zener cathode should face the switching transistor gate.

Quick functional check

With no battery connected, the red LED should illuminate. If it remains completely dark, the fault is likely still in the charger's primary supply or control circuitry.