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Help with smoking pots

Discussion in 'General Electronics Chat' started by mtv22, Jul 8, 2013.

  1. mtv22

    Thread Starter New Member

    Jul 4, 2013
    First of all: Yes, pun intended.

    Okay so this sort of relates to my previous thread. There's a schematic that another user posted that I was able to get to work fairly well. I have since removed that circuit and now I'm trying to get it to work again. I have attached to the base of the transistor pair the left audio input from my phone/laptop. The forward voltage of the LEDs is 3v each. The first pot I used was a 10k Ohm 1/2 Watt with linear taper. That one started smoking and glowing red when I had the audio jack plugged into my phone but with no music playing. I turned the power supply on and started turning the pot until the lights would be a little dim. I noticed that sometimes if I jostled the audio cord the LEDs would flash or be fully on when the pot was relatively low and other times they wouldn't come on until the pot was turned up fairly high. This is the same thing that happened with the second pot (which I just spent an hour driving to radioshack to buy) except this time the audio was from the laptop and again, no music was playing. Here's a drawing of my working schematic:

    It's kind of cutoff but the Vcc is 17 volts and that is from a 12v 400mA wall-wart.

    Here's what the breadboard looked like when I smoked the second pot:
    You can't see the tip31 transistor because its off the screen but you can see its legs.

    Anybody got any ideas as to what's going on here?
  2. wayneh


    Sep 9, 2010
    By inserting the audio connection from your laptop, you connected grounds of your laptop's power supply and wherever your Vcc is coming from. In other words, the problem may be what does NOT appear in the schematic, a current path from one power supply to another.

    When your pot is turned to one end or the other, there's nothing to protect it from excess current.
  3. mtv22

    Thread Starter New Member

    Jul 4, 2013
    I forgot to include the ground of the audio in the schematic but you can actually see it in the top right corner of the picture of the breadboard; it's on the same bus as the ground of the power supply. Does this clear up what you're saying or could this be the problem? Sorry I'm not entirely sure if you meant that having them connected is causing the problem.
  4. strantor

    AAC Fanatic!

    Oct 3, 2010
    Man I got excited when I saw the title.. I wondered if you might be geographically close enough.
    Metalmann and shortbus like this.
  5. tindel

    Well-Known Member

    Sep 16, 2012
    Your audio output is low impedance to ground or the positive rail - if your trim pot is close to Vcc then you will get a lot of current flow because your resistance is low going to ground... and similiarly when your trimpot is near ground and your audio output is high. Add a discrete resistor about 1k+ between your pot and ground and between your pot and Vcc.

    What is it you're trying to do? Maybe there's a better way to do it?

    BTW - Smoking pots is legal in CO!
  6. Relayer

    New Member

    Jan 1, 2013
    You can try inserting a 10k resistor between the pot and ground.
    That should prevent the pot from passing Vcc directly to ground when you turn it to its lowest setting.
    Relayer :D:D:D

    EDIT: Tindel beat me to it... lol
  7. BillB3857

    AAC Fanatic!

    Feb 28, 2009
    Install a capacitor between your laptop signal source and the circuit shown.