whiterabbit
- Sep 14, 2013
- 90
- Joined
- Sep 14, 2013
- Messages
- 90
Hey!
So, yesterday I had two 10k pots burn out at [I think] the exact same time, but I don't understand how/why and was hoping some of you might have some insight.
The project is a midi controller based on the Teensy 3.1, powered by USB. I have two banks of five 10k pots (L/R) that are being read by a 74HC4067. There is a ceramic cap [.1uF] between the Teensy and the IC [and each of the banks] across +3.3v > Gnd to provide a small bit of isolation. All are connected to +3.3v and AGnd on the Teensy.
Each side of the controller also has an illuminated 3-pin rocker switch [Vin, Vout, Gnd] which I have set up to apply power to the base of an NPN when it is activated. The NPN connects a Teensy pin with a pullup resistor to ground. I am unfortunately failing to remember clearly offhand if I have the switch in the same loop as the 5 pots, or if I separated it so it is connected directly to the +3.3v and Gnd (not AGnd) of the Teensy. I will check that tonight to be certain.
Anyways, I had everything hooked up and put together and all inputs were reading properly and with the full range of values for about two weeks. I had started into the programming phase of the project on Friday and was playing with the controller all weekend just working on getting the software to behave as I want and at one point I noticed that two of the pots were only reading in the range of 0-3 instead of 0-127. I don't know if they went out at exactly the same time or just very close in time, and because I was working on unrelated code I wasn't checking the pots' output each time I uploaded a modified sketch- so I don't know at exactly what point they went out.
I can easily swap both of them out with spares tonight, but I'm hesitant to do so without knowing the cause of their failure. I don't want to end up with four dead pots instead of just the two I have now. Apart from manufacturer defects, the only way I know of burning out a pot is to run current into the wrong pin or run too much current through the correct pins. I don't see how I could have done the former, since the related code has been working consistently as expected this whole time and returning accurate values, and all the wiring was thoroughly tested and then hot glued together to keep anything from shifting or shorting. But I also don't see how it could have been the latter, either, because there are three more pots on the exact same rails connected millimeters away and they're all fine- so if there was some sort of unexpected overload, wouldn't they be burnt out too?
The reason I mentioned the LED rockers is both because they may be in the same power loop and are located directly next to the pots, and also because early in development one of the first rockers I was using would cause the Teensy to reset when the switch was toggled on. The weird part was that it was dependent on how you turned the switch on. If you flipped it fast and it had more of that 'snap' into the on position- reset. If you eased it carefully from off to on, no reset. I chalked that up to defect since it was only that one switch and just replaced it- which solved the reset problem.
As a side note, I measured and found that the LED in the switch pulls 6mA when hooked up to a 6v battery (meaning R = 1000ohms; 3.3mA = 3.3v/1kohms ...right?) so even if the switch was causing a spike it wouldn't be very large, would it?
The attached schematic isn't the full circuit but just wanted to give a small visual on the parts I've mentioned.
Any thoughts / links / suggestions / help very welcome. Thanks!
So, yesterday I had two 10k pots burn out at [I think] the exact same time, but I don't understand how/why and was hoping some of you might have some insight.
The project is a midi controller based on the Teensy 3.1, powered by USB. I have two banks of five 10k pots (L/R) that are being read by a 74HC4067. There is a ceramic cap [.1uF] between the Teensy and the IC [and each of the banks] across +3.3v > Gnd to provide a small bit of isolation. All are connected to +3.3v and AGnd on the Teensy.
Each side of the controller also has an illuminated 3-pin rocker switch [Vin, Vout, Gnd] which I have set up to apply power to the base of an NPN when it is activated. The NPN connects a Teensy pin with a pullup resistor to ground. I am unfortunately failing to remember clearly offhand if I have the switch in the same loop as the 5 pots, or if I separated it so it is connected directly to the +3.3v and Gnd (not AGnd) of the Teensy. I will check that tonight to be certain.
Anyways, I had everything hooked up and put together and all inputs were reading properly and with the full range of values for about two weeks. I had started into the programming phase of the project on Friday and was playing with the controller all weekend just working on getting the software to behave as I want and at one point I noticed that two of the pots were only reading in the range of 0-3 instead of 0-127. I don't know if they went out at exactly the same time or just very close in time, and because I was working on unrelated code I wasn't checking the pots' output each time I uploaded a modified sketch- so I don't know at exactly what point they went out.
I can easily swap both of them out with spares tonight, but I'm hesitant to do so without knowing the cause of their failure. I don't want to end up with four dead pots instead of just the two I have now. Apart from manufacturer defects, the only way I know of burning out a pot is to run current into the wrong pin or run too much current through the correct pins. I don't see how I could have done the former, since the related code has been working consistently as expected this whole time and returning accurate values, and all the wiring was thoroughly tested and then hot glued together to keep anything from shifting or shorting. But I also don't see how it could have been the latter, either, because there are three more pots on the exact same rails connected millimeters away and they're all fine- so if there was some sort of unexpected overload, wouldn't they be burnt out too?
The reason I mentioned the LED rockers is both because they may be in the same power loop and are located directly next to the pots, and also because early in development one of the first rockers I was using would cause the Teensy to reset when the switch was toggled on. The weird part was that it was dependent on how you turned the switch on. If you flipped it fast and it had more of that 'snap' into the on position- reset. If you eased it carefully from off to on, no reset. I chalked that up to defect since it was only that one switch and just replaced it- which solved the reset problem.
As a side note, I measured and found that the LED in the switch pulls 6mA when hooked up to a 6v battery (meaning R = 1000ohms; 3.3mA = 3.3v/1kohms ...right?) so even if the switch was causing a spike it wouldn't be very large, would it?
The attached schematic isn't the full circuit but just wanted to give a small visual on the parts I've mentioned.
Any thoughts / links / suggestions / help very welcome. Thanks!