Author Topic: Don't want Smoking Pot  (Read 308 times)

Phend

Don't want Smoking Pot
« on: January 31, 2021, 11:15:22 AM »
Hello:
This has most likey been hashed over and over, but simple question here.
Are the two circuits in the pic the same or should I say will produce the same results.
I once read, where?, that the connections on the left diagram are made to satisfy spice or the like.

« Last Edit: January 31, 2021, 11:43:28 AM by Phend »
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Ya sure you Betcha.

Fancy Lime

Re: Don't want Smoking Pot
« Reply #1 on: January 31, 2021, 11:36:56 AM »
They are the same as long as the pot works as intended. However, if the pot fails by loosing the connection between the wiper and the trace, which may happen with prolonged use, then the left variant will have the maximum resistance of the pot and the right variant will be an open circuit. In most cases you want to avoid an open circuit if the pot fails.

HTH,
Andy
My dry, sweaty foot had become the source of one of the most disturbing cases of chemical-based crime within my home country.

mozz

Re: Don't want Smoking Pot
« Reply #2 on: January 31, 2021, 12:09:21 PM »
" If the pot fails" has been said many times, i don't think it's a common problem (not withstanding damage). Now, if it was a bias adjustment for a fixed bias tube amp, i would want it failproof.
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ElectricDruid

Re: Don't want Smoking Pot
« Reply #3 on: January 31, 2021, 12:18:16 PM »

While it might not be common, it does happen. The 'scratching" that you get on old pots is often the wiper making then losing then making the contact lots of times as it moves across the track surface. If something bad happens if the thing goes open circuit, that's enough.
The other possible way it can occur is if the wiper goes off the end of the track. I've seen this a few times too.

It might be true that your CAD package would prefer the circuit on the left. Many packages will throw an error (or at least a warning) when you try and make a PCB with a component that doesn't have all its legs connected.

I'd go with the one on the left because it means the circuit will still work even if the pot fails. That's building in more reliability. Aside from that, there's no reason *not* to, and I can't see any advantages to the one on the RHS.


antonis

Re: Don't want Smoking Pot
« Reply #4 on: January 31, 2021, 12:32:44 PM »
IMHO, left circuit reflects a working rheostat (pot wired as variable resistor) which is exactly the right one..

+1 to what Andy said about max shunt value vs open circuit..  :icon_wink:
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

GGBB

Re: Don't want Smoking Pot
« Reply #5 on: January 31, 2021, 12:48:11 PM »
" If the pot fails" has been said many times, i don't think it's a common problem (not withstanding damage). Now, if it was a bias adjustment for a fixed bias tube amp, i would want it failproof.

Potentiometers are notoriously susceptible to dust and oxidation failures - I would consider that "common" - certainly moreso than "damage" IMO. A "dirty" pot can disconnect and reconnect the wiper as it is being turned. Better to jump between X and Y resistances than X and open.

Fail-safes always make sense when they are virtually free (this one costs one solder joint and a wee jumper).
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iainpunk

Re: Don't want Smoking Pot
« Reply #6 on: January 31, 2021, 01:42:26 PM »
i have had one pot fail due to over current, they wil stay working at high resistance, but dialing it down makes the current higher and higher until it started to smoke and it burned the track cleanly off... its now a switching pot, haha and one of the terminals has only a function as a switch when you travel past the dead spot. the outer terminals read 381M ohm, while it used to be a 10k

the lesson here: use a stopper resistor and don't play with high current sources when you are tired.

cheers, Iain
half man - half snail - 6 feet to scale - Snail man's - not frail - He's been - to jail - snail man is fuccing real
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