Zener in series with incandescent bulb

Thread Starter


Joined Jan 24, 2011
I recently decided to undertake a small project to convert a very high quality headlamp which uses an incandescent bulb to an LED one. While opening it up I discovered that it contains a Zener (bzx55c3v9 a 3.3v 500mw) in series with the 4 1.5v batteries (cells) and a 3.5v 2A incandescent bulb. Zeners are not used often and I am far from an expert but I do know that they have a breakdown voltage which when the input voltage rises above this voltage they begin to conduct and reverse the current back into a resistor. The explanation is from memory. The order of parts are Bulb-cell-cell-zener-cell-cell-switch-(back to bulb). Theoretically (in my mind) it would be like placing a battery in backwards which causes problems.

I am mainly confused as I have never seen them used in this way (without being in a voltage divider with a resistor) and can not find any mention of them being used like this. What bit of obvious information am I missing?

Thank you


Joined Jul 17, 2007
I believe they may have done that to prevent certain types of rechargeable batteries from discharging too deeply. Once the voltage of all of the four cells went below the LED threshold voltage + the Zener breakdown voltage, the LED would turn off.