What can burn out a pot?

[motterpaul]

I am building a Diefet Deizel VH4S

http://tagboardeffects.blogspot.com/2014/06/diefet-diezel-vh4s-simulator.html

I just plugged it in and the "presence" pot went up in smoke.

It has an ICL7660S charge pump followed by two 5817 diodes. Coming off the ground is a 9v zener going directionally to pin 1 of the IC. In the comments someone wrote "you can skip that diode, it is just to protect the IC from the wrong power supply." I skipped it because I didn't have one (I have an 8.1v or a 12v, but no 9v zener).

So, I finished the build, put it in my breadboard and connected the OneSpot DC adaptor (not a battery). I have used this adaptor for every build with no problems yet.

BUT - this time the "presence" pot literally went up in smoke. I have never seen a pot go up in smoke before. (I've felt ICs get real hot, and I have seen J201s explode)

Any thoughts on what happened? Do you think it could be related to the absence of this 9v zener? (all my large caps are correct polarity)

[induction]

Pots will go up in smoke if they get too much current. The presence pot should have no DC voltage across it, so that shouldn't be a problem here. Even if you accidentally connected it between 18V and ground it would see less than 1 mA. Are you sure you used 22k and not 22R?

In any case, I suspect a mistake in your build. Double-check your component values and placement.

[motterpaul]

That is not a crazy question - but I did check and I used 20k - was all I had.  What if, in this case, you see it says 1&2 on one spot, and 3 on another. I tied 1 & 2 together at the pot and only ran a lead from 1, then I also had the lead to 3 on the next row down. Did I mess that up and should have made sure I had a separate lead from 2, or rather should have run the lead from 2 instead of 1?

[induction]

What you describe sounds fine. That's a completely reasonable way to wire a variable resistor. 20k should be just fine as well.

Thinking about it a little more, I realized that if you turn the presence pot all the way up, lugs 1 and 3 are shorted. If you happen to have DC voltage across the pot when it's dimed, the lack of resistance will create excessive current that can easily let the smoke out of the pot. I think that may be what's happening here. I notice that the row with lug 3 is at ground, while the row just below it will have a DC voltage very near 18V. If you mistakenly transposed your presence connections down one row, then you would easily burn out the pot at high presence settings. Check the placement of those pot lug connections.

A shorted cap, whether defective or the victim of a solder bridge, could have the same effect, if it was in just the right place. But I'd check the placement of those pot lugs first. That seems much more likely to be the culprit.

[motterpaul]

Well, I don't think I transposed them. I already replaced the pot but the wires are in the right place (I used the same color code, 1,2,3 for all pots in the build).

oh, I just found a missing cap, 470P near pin 5 of IC2. That is probably the problem. It connects to that pot. This is a dense build and I missed it.

Thanks for your help. I would have just plugged in again if you didn't suggest misplaced caps.

[induction]

That cap being missing shouldn't cause your pot to burn up. It's part of a low pass filter. Leaving it out might change the tone a little or might cause oscillation, but it won't damage anything. The only reason it connects to the presence pot is because they both connect to ground. I'd keep looking.

If you fire the circuit up again, make sure the presence pot is set to minimum. Then check for DC voltage across the pot with a DMM.

[motterpaul]

I'll do that

[motterpaul]

Dammmmit --

I just had a chance to try this again with a new pot, so I did as you said and set it full off and turned it up slowly. Now - I didn't even have transistors or ICs installed. I was getting steady reading of 9v from having it up about 30% all the way up to 95% - so I thought I was good. But then when I did the final 5% (to full on) - it flashed and burned out.

I think I just found the problem - the 100uF below IC 2 seems to be too high by a row (it is hard to see in there, it's a big cap). That would do it, though, I am guessing.

[Frank_NH]

Yeah - big caps can be dangerous.  I had a similar incident recently.  I was testing an IC-based (op amp) overdrive and noticed the IC was burning hot!  I quickly turned it off and realized that I had inserted the IC upside down!    Don't do that.  I didn't check to see if the IC was any good - just popped in a fresh one (which is why we purchase items in bulk, right?  )

[motterpaul]

Well, I stemmed the pot problem - reseated that cap and put in a new pot. But guess what, plugged in the power and it smoked again. This time I found a tiny solder bridge where a cut was not deep enough on the +power copper strip. I don't usually have cut problems but I noticed this vero seems to have the copper a little to one side of the holes. I had to fix a number of them a few days back, but I missed this one.

So, now the pot didn't burn out, but with my first test, still no audio, so I have to come back later.

Yeah - backwards mounted (polarity) caps can also cause ICs to get real hot. But in some cases they still work if you get them out right away. So can mixing up + for - when you plug wires into a breadboard.

[induction]

I should have been more clear in my instructions. If the circuit is working correctly, there should be no DC across that pot. I meant for you to fire up the board with the pot all the way down,  measure for DC, and report back. If there is appreciable DC across the pot, it absolutely will burn out if you turn it all the way up. Sorry I was unclear. Next time you fire it up, keep the pot at min and check for DC across it. If you find any, unplug the battery or adapter from the circuit.

The misplaced cap wouldn't cause the pot to burn out, but it will kill the cap because there was voltage across the pot, and electrolytics do not like to be reverse-biased. You should replace that cap. Even if it isn't dead yet (it probably isn't because if it was you wouldn't have measured voltage across the pot), it will die soon. When it does, it will create a short circuit between V+ and ground, which could kill your PSU or other components on the board. At the very least, the circuit will just stop working.

The copper bridge was probably the real culprit. The question now is whether anything else got damaged in the process.

Replace that cap and see if that fixes things. (Don't forget to put the IC's and transistors back in first.) If it's still not working, it's time for photos and voltage measurements.

[motterpaul]

I did replace the cap (I should have been more clear) - and you can see I found what was burning the pot. No harm done.

Such a big build - it's a lot to take on. I don't recommend trying this build when you are feeling rushed or tired.

[Frank_NH]

Although we don't think about it, there IS a lot of power (for a short duration) in those 9V batteries we use in guitar effects.  For example, putting 9V across a one ohm resistor gives P = I^2 * R = V^2/R = 81 W of power!   And if you short it completely with ~ 0 ohms - poof!  Enough to start a fire...

[Beaker]

As I found out to my cost when I stuck a fresh 9V battery in front jeans pocket, along with a capo many years ago. I've still got the scar on my leg to prove it.