This is a somewhat unusual, although hopefully not too irresponsible, question (I also hope it falls within the remit of this site, I can't think of anywhere else online where I'd get a trustworthy answer). I'm making a short film in which a simple LM386N-4 power amp chip explodes and catches fire. It will appear to be in the audio amp circuit like the one shown here, but need not in reality be connected to anything (the magic of film...)

(source: hobby-hour.com)

By accident, I've found that when the chip is in the circuit above and is supplied with 13.2v, and 13.2v is then applied to pin 5 (output) it dies immediately and a small plume of smoke appears.

My question is this: taking the obvious safety considerations as a given (I'll be wearing safety goggles, will be standing a good distance from the chip, next to the mains switch, and I have a CO2 fire extinguisher for if things get serious), what voltage should I apply, and to which pins, to get some smoke, a few flames and maybe an initial flash?

I'm guessing that putting a full 240v between pin 5 and ground (switched using a relay for safety) might be overdoing it. The datasheet for the LM386N is here and I'm using the -4 type, which takes a maximum voltage of 18v.


First of all I am glad safety is a real concern to you. What I am going to suggest will require a ton of it and it's good that you already are aware you could get hurt.

Your suggestion is good: putting an high voltage at the output will exceed the bottom NPN \$V_{CE_{max}}\$ and it will probably fail as a short. That guy down there, together with the other NPN just above is the biggest transistor you can find on the die: plenty of Si to carry your deadly current and to heat the whole die. If you connect terminal 6 and 4 together and apply an high voltage at terminal 5 your result might even be better.

The question is: what kind of power supply are you using? Don't think to put the mains across that thing: your protection switch/fuse/whatever will probably go puff, plus it's not healty for your wires. But what if you use some capacitors? If you happen to have some old ATX psu lying around you can find some 10-100uF 400V caps inside. Hook them all in parallel, charge them to rectified mains voltage and then discharge them on your chip. That will literally blow it up, but the current spike probably won't start a fire. If you have some spare LMs you can give it a try.

Please note that 100uF@400V is 8J of energy. That's more than enough to kill you, capacitors have no mercy. Please, please, please be extra careful if you want to try the capacitor way. To discharge your bank do not use some flying wires, you will need a fat switch or a fatter mosfet. And add a bleeding resistor in parallel to the bank just to be safer.

You can also tear apart an old microwave oven, there is a step up transformer, an HV diode and cap, that should be some 1uF at about 1kV. If you manage to discharge this in the poor LM it really will blast it to pieces. Just be aware that again the charge on the cap is deadly, plus in a microwave oven you can find a magnetron, if you turn it on bad things happen like you becoming blind or impotent.

I am purposely not adding many details hoping to keep the average teen far from this. I once discharged the flash cap of a throwaway camera with a finger and it was quite unpleasant.

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  • \$\begingroup\$ And some people have a bank of dozens of flash caps for their coil gun fun... anyways, since there is nothing time critical in charging the cap, the equivalent of a mosquito zapper, the cap(s), a diac, voltage divider and triac will make this a small portable thing that can run from batteries, so no direct mains connection required (i.e. a little bit of extra safety). In any case starting a fire with the LM is probably hard, but a old tantalum cap might be a better replacement that burns easily and brightly at much lower voltages. \$\endgroup\$ – PlasmaHH May 3 '15 at 21:50
  • \$\begingroup\$ Yeah I experimented a bit with coil guns \$\endgroup\$ – Vladimir Cravero May 3 '15 at 21:54

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