First Project: Noisy Cricket amp with exploding capacitor

[cheshire-cheese]

I set one up on a prototype board following this schematic...

...using ceramic capacitors in place of the polyfilm ones and used a 2N5458 transistor.  Wired into a 6ohm speaker and a 9v battery, it worked really well.
Then I tried it with a DC power adapter (12V max 10VA) and capacitor C1 exploded.  The capacitor was rated at 16V.

I'm new to electronics; what went wrong?

[gcme93]

I can't give you a precise breakdown of what happened, but the recommended thing is to use caps that are rated at least double the voltage you're using

A very likely thing that might have happened is that you had a DC plug with the positive and negative the wrong way around - you'll notice that many plugs have the middle pin (the 'tip') postive and others have the tip negative. Make certain which you need before you try plugging it in again.

Have a google of "polarity protection" to find out how to use a diode to block current going the wrong way around your circuit

Good luck!

[JRB]

As gcme93 already mentioned a 16V cap is pretty close to your 12V, if you take tolerances in the equation it can get even closer to the 12V. First thing I suggest you do is make sure that all the caps are rated well above the 12V and make sure there is no short in the circuit.

[Tony Forestiere]

I am guessing C1 blew. C1 and C8 are polarized (see the "+" symbol?). C1 is for power conditioning. If put in the circuit backwards, it will explode.

[cheshire-cheese]

Thanks for the replies.  I'm fairly sure I got C1 the right way around, the -ve side connected to earth.  As for the power in, I used a continuity tester to check which solder tag was which, so I'm fairly sure the polarity was the right way round.  I'm guessing that it's down to the closeness of the maximum voltage rating.  In which case, if I replace C1 with the same again, I'm probably best sticking to a 9V battery or using a lower rated power adapter.

[Pyr0]

There's also a chance that your power supply was putting out a lot more than 12v, did you try measuring it's output with no load ?

[cheshire-cheese]

Ashamed to say that I had the thought and the means to check it, but didn't 

[cheshire-cheese]

  I've just had a thought.  Although I took great pains to make sure I knew which solder tag on the dc socket was connected to which part of the socket, I made the assumption that ground was the sleeve.  Now I find that my iPod is knackered, I have come to the view that the center of the dc jack is negative and the outer part is positive (as gcme93 suggested) , so I ran 12V through my iPod 

bugger

[petey twofinger]

dont feel so bad .


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