Author Topic: Help! APC B1M pot catching fire!!  (Read 1583 times)

npx

Help! APC B1M pot catching fire!!
« on: October 27, 2008, 10:10:12 AM »
Well I certainly didn't think my first post here would be about the following situation!

I've been lurking here for a couple of months... a couple weeks ago I built an Atari Punk Console from this schematic:



The bold dots are body contacts, plus I also added a normally closed momentary switch, and a switch to toggle between normal (constantly on) and momentary modes.

I had the circuit working fine and I put it away. Today I etched the enclosure, and tonight I've been assembling the little beast. I got everything in the box, and plugged in the battery to make sure I had the on/off switch pointing the correct way. Great, it works! I unplugged it and took it into the next room. Then I looked down and the 3rd lug of one of the 1M pots (the one to the right of the 9v+ on the schematic) spontaneously caught fire! Needless to say, I was a little surprised. It subsided, and I figured the pot must now be burned out... so I plugged it in again to see if it still worked, and to my surprise, it did... I turned the pot and it sonically did what it was supposed to... and then ignited again:-\

I'm confused. Can anyone tell me what is going on??

Thanks for any help!!

DougH

Re: Help! APC B1M pot catching fire!!
« Reply #1 on: October 27, 2008, 10:16:36 AM »
You need a fixed resistor between your 1M pots and ground to limit the current when the pot is set to minimum resistance. When the pot is set to minimum, there is a dead short across the battery. That's what is burning up the pot.
"I can explain it to you, but I can't understand it for you."

npx

Re: Help! APC B1M pot catching fire!!
« Reply #2 on: October 27, 2008, 10:27:25 AM »
Wow, thanks for the quick reply! You're right, that pot was set at minimum when it happened...

So what value resistor/s should I use? And where exactly should I put it/them? (I'm still new at all this!)  :)

DougH

Re: Help! APC B1M pot catching fire!!
« Reply #3 on: October 27, 2008, 11:00:46 AM »
A 1k 1/4W should be more than enough for 9v. For longer battery life, use as large of resistance value as you can get away with without changng the control in a way you don't like. (Try 10k-20k for example as that limits the current draw to 1ma or less.)  Break the connection between the pot and ground and insert the resistor there.
"I can explain it to you, but I can't understand it for you."

npx

Re: Help! APC B1M pot catching fire!!
« Reply #4 on: October 27, 2008, 11:11:02 AM »
Thanks Doug, I'll try that in the morning.  :)