(Newbie) Doepfer USB 64 based MIDI controller build - Smoking pot!

[suarezmiranda]

Hi all,

Apologies for the newbie question.

I purchased one of these -- http://www.doepfer.de/usb64.htm -- and was testing it.

I think I understand it as I wired it up correctly to work with one potentiometer (10k which was specified in the manual) and I received the correct MIDI message into my computer. Although when I had the pot at maximum resistance i.e. 0 on the 0-127 scale the pot started smoking! I duly disconnected!

Why would this happen?

I'm going to purchase pots and toggle switches for this build, do I need to buy a certain type of pot?


Thanks in advance for your help!
Suarez

 

[pucho812]

you can measure the pot with an Ω meter and if it measures o.k. then you can use it but I would replace it when you can. You may have a spot in the movement where it is open circuit as the resistive strip burned off. I have seen smoke before out of a pot but the pot was rated at the wrong wattage and ultimately burned it out. I can't say that is what is happening in your case and I doubt that would be the case as your not dealing with huge amounts of voltage...

 

[Harpo]

I think I understand it as I wired it up correctly to work with one potentiometer (10k which was specified in the manual) and I received the correct MIDI message into my computer. Although when I had the pot at maximum resistance i.e. 0 on the 0-127 scale the pot started smoking! I duly disconnected!

Looking at the pot shaft, pins facing down, leftmost pin of your pot connected to 0V at pin10 of J1, rightmost pin of your pot connected to +5V at pin9 of J1 and the pots center pin connected to pin1 (_or 2...8_) of J1 ?
J2 ... J8 are connected the same for ports 9..16 up to 56..64.
The pot is just a voltage divider, varying the voltage at wiper between 0V and +5V in respect to 0V reference voltage and the 10K lin pots power rating will be sufficient (5V/10K=close to nada 0.5mA).

edit: smiley response when typing '8' followed by ')'

 

[suarezmiranda]

Thanks both for your very swift replies!

I think I understand it as I wired it up correctly to work with one potentiometer (10k which was specified in the manual) and I received the correct MIDI message into my computer. Although when I had the pot at maximum resistance i.e. 0 on the 0-127 scale the pot started smoking! I duly disconnected!

Looking at the pot shaft, pins facing down, leftmost pin of your pot connected to 0V at pin10 of J1, rightmost pin of your pot connected to +5V at pin9 of J1 and the pots center pin connected to pin1 (_or 2...8_) of J1 ?
J2 ... J8 are connected the same for ports 9..16 up to 56..64.

Yes. I don't have it connected anymore but I used Pure Data to monitor the CC data and I was getting a clean MIDI control change as I moved the pot with no craziness so I presume the everything was wired up fine.

The pot is just a voltage divider, varying the voltage at wiper between 0V and +5V in respect to 0V reference voltage and the 10K lin pots power rating will be sufficient (5V/10K=close to nada 0.5mA).

edit: smiley response when typing '8' followed by ')'

- Okay it's possible it was a log taper pot. As the CC stayed in the 120's for half the turn of the pot!?

- Also when I bought the pot it was advertised as a high quality audio type.

- Also it definitely was a stereo pot (I used the front three pins. This was the only pot I had lying around!).

Would this new information explain the smoking?!?

 

[Harpo]

- Okay it's possible it was a log taper pot. As the CC stayed in the 120's for half the turn of the pot!?

data would be crammed at one side, but electrical it wouldn't make a difference.

- Also when I bought the pot it was advertised as a high quality audio type.

so the log taper would better be used for maybe attenuating audio instead of tweaking in a DC voltage varying on a for usual linear scale.

- Also it definitely was a stereo pot (I used the front three pins. This was the only pot I had lying around!).

so the backside pot (with same wrong taper) would be still alife when you burnt the frontside pot.

Would this new information explain the smoking?!?

No. Most likely you mistakenly might have mixed up the pots +5V outer connection with the wiper connection, either at pot side or jumperblock side.

 

[suarezmiranda]

Okay thanks so much for clearing those things up! I can't rule out that I may have wired it up wrong for the test.

I don't really care about that burnt pot but I'm going to purchase new pots for the build.

Will I need to get specially rated ones or will any 10k lin ones do?

 

[Harpo]

Will I need to get specially rated ones or will any 10k lin ones do?

Any lin pot I know of will take 5V^2/10Kohm=0.0025W easily if wired correctly.