Zener diodes not working

[Mark Korvin]

I'm having problems getting a couple of different 12 volt zener diodes (1 watt & 5 watt) to work. When connected to an unregulated power source of 16-18 volts with different series resistors (75, 470, 1k ohm) the voltage measred across the zener is in the range 13-15 volts - the breakdown doesn't seem to be kicking in.
Any suggestions on what could.be going wrong?

 

[(*steve*)]

What voltage are you seeing? Less than a volt or the 16 to 18 volts?

Are you measuring across the zener? Or across the zener and the resistor? Or something else?

 

[Mark Korvin]

What voltage are you seeing? Less than a volt or the 16 to 18 volts?

Are you measuring across the zener? Or across the zener and the resistor? Or something else?

Across the zener.
Following a suggestion from another thread I tried hooking up a couple of 9 volt batteries in series with a 10k resistor to the zener diode. Supply voltage was about 23 volts, resistor 9 volts and zener 14 volts. Same outcome with a replacement zener...

 

[Audioguru]

Were the zener diodes very cheap and were sold on ebay?
Was your voltmeter very cheap and was sold on ebay?

 

[(*steve*)]

With a 10K resistor, the voltage across the zener is likely to be slightly lower than the specified voltage. The fact that you're seeing a higher voltage is suggestive that your zener is not a 12V device.

 

[Audioguru]

9V/10k= 0.9mA, a very low current for a power zener diode. A 1N5349B 12V/5W zener diode is tested at 100mA. It might be 11.5V at 5mA.

 

[Mark Korvin]

Put a new battery in the multimeter and the problem looks to be resolved. Didn't seem to be complaining about low battery but definitely made a difference. Nothing worse than 'fake' readings.!

 

[(*steve*)]

Hahahaha, I have a multimeter like that. The low battery indication is that it becomes very inaccurate. :-(

 

[Audioguru]

Doesn't your multimeter have a "low battery" indication before its calibration fails?

 

[(*steve*)]

In my case, no. And you can imagine how much use it gets now.