I am using a 7805 voltage regulator to provide +5 volts to a Basic Stamp processor and a matrix LED driver chip. On the input side of the regulator, I have a .33uF 35-volt tantalum capacitor and on the output, I have a .1uf 35-volt tantalum capacitor and a 10uF 25-volt electrolytic capacitor.
I am using a 5 pin connector on my circuit board that provides unregulated +9 VDC (actually more like 12VDC) from a plug-in AC adapter. Also on the cable to this connector are RS-232 Rx, Tx and DTR lines. The power ground and the RS-232 ground signal share a common ground that is tied together.
My circuit was working fine (for a few hours) when plugged into the serial port of my computer used for development and testing. The circuit had problems when plugged into the serial port of another device--it would not run properly, but when unplugged from the serial port, it would work. However, there was no serial data transmitted and the port was idle when it was plugged in. Strange that when plugged into an idle serial port nothing would happen.
I plugged the serial cable back into my notebook computer and it worked fine again. Finally (after several attempts), when plugged into the other serial device, it would then start properly, but after a few moments, some of the smoke got loose. I unplugged it before all the smoke escaped but the circuit was working while it was having a smoke.
The strange thing--it was the .33uF cap on the input of the voltage regulator that smoked. There is no connection between the serial lines and the input power other than sharing the same cable and connector (and the common ground), but there were no shorts obviously since everything worked fine while connected to the PC. The other serial device only had pins 2, 3, and 5 connected so no other pins could possible be shorted by the device.
My wildest thought is that the route of the cable was different when plugged into my serial device. There was an AC power strip mounted under the counter top and a monster of a UPS on the floor below that. Could voltage have been induced into the cable to cause this problem? If so, how can the circuit be protected? I am not certain that is my problem, but I am baffled what else it could be. I was not able to take any voltage readings before I had to leave.
Does anyone have any ideas? I had to leave the equipment at a remote location, but will build a duplicate of the circuit for debugging. What should I look for?
Thanks,
Dale
I am using a 5 pin connector on my circuit board that provides unregulated +9 VDC (actually more like 12VDC) from a plug-in AC adapter. Also on the cable to this connector are RS-232 Rx, Tx and DTR lines. The power ground and the RS-232 ground signal share a common ground that is tied together.
My circuit was working fine (for a few hours) when plugged into the serial port of my computer used for development and testing. The circuit had problems when plugged into the serial port of another device--it would not run properly, but when unplugged from the serial port, it would work. However, there was no serial data transmitted and the port was idle when it was plugged in. Strange that when plugged into an idle serial port nothing would happen.
I plugged the serial cable back into my notebook computer and it worked fine again. Finally (after several attempts), when plugged into the other serial device, it would then start properly, but after a few moments, some of the smoke got loose. I unplugged it before all the smoke escaped but the circuit was working while it was having a smoke.
The strange thing--it was the .33uF cap on the input of the voltage regulator that smoked. There is no connection between the serial lines and the input power other than sharing the same cable and connector (and the common ground), but there were no shorts obviously since everything worked fine while connected to the PC. The other serial device only had pins 2, 3, and 5 connected so no other pins could possible be shorted by the device.
My wildest thought is that the route of the cable was different when plugged into my serial device. There was an AC power strip mounted under the counter top and a monster of a UPS on the floor below that. Could voltage have been induced into the cable to cause this problem? If so, how can the circuit be protected? I am not certain that is my problem, but I am baffled what else it could be. I was not able to take any voltage readings before I had to leave.
Does anyone have any ideas? I had to leave the equipment at a remote location, but will build a duplicate of the circuit for debugging. What should I look for?
Thanks,
Dale