Zener Diode Testing

Went out this morning and the ammeter was showing charging of around 6 amps which is very high and I thought the Zener Diode had failed. Got home and looked for guidance on testing a Zener which all looked complicated and needing special equipment. I boiled all this info down to a simpler test.
I hooked up an old 20v power supply unit ( I store all old units from mobiles, printers etc. ) to the Zener ( +ve to ground ) and then tested the voltage across the diode and got about 14.4v which I believe is correct. The other test was to leave the connection for about a minute and felt the Zener which was hot indicating it is passing current. The final test was to connect up a another old 12v power supply unit and got only the 12v across the Zener and it didn't get hot.
So I am happy the Zener is OK and believe the high charging was caused by the battery fluid being low, hopefully anyway !

I had a boiled battery, the zener was good but the alloy heat sink has corroded under the zener and excess current had stopped flowing until the heat sink was cleaned.

@TopgunPeter

Your test protocol was clever and all that could be done without special equipment. From what you report, the Zener is working normally.

I am not sure your excessive charge rate is due to low battery electrolyte. Top up the electrolyte and report back again.

@kommando

Good point regarding heat sink.

Slick

kommando said:

I had a boiled battery, the zener was good but the alloy heat sink has corroded under the zener and excess current had stopped flowing until the heat sink was cleaned.

Click to expand...

Correct and apply some heatsink compound under the zener and the area of contact with alloy heat sink,this will also help inhibit corrosion.