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Zener circuit not working as expected
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Topic: Zener circuit not working as expected (Read 1231 times)
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@rt
Super Contributor
Posts: 1059
Zener circuit not working as expected
«
on:
March 05, 2020, 03:12:15 am »
False alarm.. mods please delete.
Transistor was shorted collector to emitter.
«
Last Edit: March 05, 2020, 04:02:47 am by @rt
»
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greenpossum
Frequent Contributor
Posts: 408
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Re: Zener circuit not working as expected
«
Reply #1 on:
March 05, 2020, 04:03:44 am »
Even so, you need to calculate the series resistor so that you're running the zener on the slope, not where a little leakage current will turn on the transistor if it has enough gain. A zener is not a perfect threshold switch.
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Zero999
Super Contributor
Posts: 19536
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0999
Re: Zener circuit not working as expected
«
Reply #2 on:
March 05, 2020, 10:00:54 am »
And a zener probably isn't required in this application, just use a higher value base resistor. The relay is also missing the important flyback diode across the coil, to protect the transistor against the back-EMF, when it turns off.
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JoeyG
Regular Contributor
Posts: 117
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Re: Zener circuit not working as expected
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Reply #3 on:
March 05, 2020, 10:35:32 am »
No diode snubber across the coil to protect the transistor???
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@rt
Super Contributor
Posts: 1059
Re: Zener circuit not working as expected
«
Reply #4 on:
March 05, 2020, 01:11:09 pm »
No, the transistor was out of a junk box and already no good.
My circuit does have the protection diode, caps for the reg, and another diode for reverse polarity protection.
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