Unauthorized thermal event

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Yikes! Here's what not to do.

I am trying to build a monitor / data logger for my furnace. And I want it powered off the 24VAC thermostat power supply. I had a 7812 voltage regulator wired with a 25V (input side) and 16V(output side) caps. Initially this was connected to a 30VDC power supply and all worked just fine.

Next step was to regulate the 24VAC, which resulted in a nice smooth 35VDC. What's an extra 5V? Well, to be sure I checked a data sheet and the max voltage for the 7812 is 35V. It is close, but should work, yes?

3 seconds later...

Unauthorized Thermal Event

The answer was a resounding NO. Both caps blew. The report was impressive - especially for the second cap. I found it 10 feet away after it had ricochet off a wall, around a computer and through a door...

Back to the old drawing board for this project.

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