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Abou The Zener Diode Tester

By Jestine Yong on October 7, 2008

Got question regarding the zener diode tester from a blog reader/ERG Member:

“Jestine:

It’s always a pleasure to read your monthly newsletter and blog. Thanks for bringing the human side to the repair business!

I have a question about the zener diode tester design. Could I substitute four 9V batteries in series for the entire power supply? Since the tester is used for short amounts of time, the batteries should last quite a while. I realize the batteries would put out 36VDC versus 32VDC (after rectifier) but would that be a problem? Batteries would simplify construction by eliminating high voltage wiring, the transformer, bridge rectifier and filter cap.

Thanks,

Kirk”

Hi Kirk,
The maximum voltage the LM317T IC could withstand is 37 Volt and it is safe when the input is 36 volt. However you may have to change the panel meter to 40 or 50 vdc range otherwise the 36 volt would cause the 30 vdc panel meter pointer to kick to max which is very bad to the panel meter. If you take a 50 vdc panel meter then when you want to measure low voltage zener diode , it may be hard for you to see the small scale as you do not know whether it is 2.4 volt or 2.8 volt (assuming you are checking on the 2.4 volt zener diode).

I suggest that you use three 9 volts battery instead of four. If your intention to use the meter for a short while yes, you can put in the batteries instead of transformer and the bridge rectifier plus the e-cap. The batteries current may drain off fast due to the usage from the big panel meter. If can, you may use a smaller size panel meter, eliminate the LED to save some current.

Jestine

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