Alex Wiebe: December 2009 Archives

I'm not dead yet!

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Well, seems that while I did do something stupid yesterday that did some damage, turns out the damage was not completely fatal to my project. The caps on the 7812 regulator could not handle something (voltage, current, power?) and let the magic smoke out with a vengeance! The prototyping board I was using was attached at the time (lesson learned - verify voltages and stability before connecting sensitive bits!). It stopped working. When I connected it to a stable source that was within tolerances, it decided to blow up as well!

Blown1

Reverse view...

Blown2

Thinking I was hooped, I set it aside and pondered my next move. Today I testing the LCD display hoping it was far enough removed to have survived and it had! Cool. While connecting it to the safe PC 5V supply I thought about the prototype board. The cap that was blown was related to the onboard 5V regulator. I wonder if the rest of the board worked -just not the regulator? See this board can accept 6-15V (not sure how much it took yesterday, but apparently more than 15) and then provides 5V to simple low power devices like the LCD display. I bypassed the regulator and supplied 5V direct from my PC to the board, connected the LCD and IT'S ALIVE!

Blown3_Alive

Back to hacking!

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.