Electronics > Electronics

Why did my capacitor in my transmitter blow

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Mastermime:
Hey everyone, as I was testing my circuit with a 12v SLA 7.5 Ahr battery (dont have the actual batteries yet), a capacitor in my transmitter blew and smoke.  I have no idea why.  If there is a flaw in my circuit below, could you let me know.  I dont know if this was coincidence or not, but right as I was testing voltage at transmitter line the cap blew.  The caps in the transmitter are 470uf 16v.  Please help. 

Thanks

Edit: I wonder if I accidentally touched the two probes together when checking the voltage.. hmm maybe.  If you guys dont see anything else wrong, I bet thats what happened

ebrady:
Assuming you did not inadvertently touch anything with your probes...

#1 - Did you connect the battery backwards?  That might blow the 470uf capacitor in front of the 7812 if it is polarized.

#2 - Did you build this circuit?  If so, do you have either of the 470uf caps in backwards?

There might be other things, but those are the two that come to mind immediately.

waltr:
What is the voltage rating of the cap that blew?
Over voltage and reverse voltage (cap in backwards) are the two easiest ways to blow an electrolytic cap.

Mastermime:
Well the caps are rated for 16v.  So that shouldn't have been an issue. And I didn't put them in backwards.

waltr:
Looking at your circuit the pass transistor is backwards and should be an NPN.
Look at the example circuits in a National Semiconductor data sheet for a 78xx regulator with a pass transistor.
A smaller, 0.1uF, cap on the 7812's output pin is also recommended. This helps prevent the regulator from oscillating.

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