I have a really weird problem. I have a soldered PCB that uses 20 duplicated circuits. Everything works except for one of them. I have "fixed" the problem circuit three times and each time I fix it the problem reappears the next day. It has been off and nobody has touched it overnight.
I believe the problem has to do with the resistance dropping over an extended time period?! I have replaced all five components in the circuit with new components and the problem disappears. Then the next day the problem is back.
I'm getting a higher output voltage than on the rest of the circuits. I fixed it by replacing two resistors. It worked for 4 hours straight turning it on/off while testing. When I come in the next day once again the same circuit is outputting a higher voltage than the night before.
Is it possible that a resistor was damaged during soldering and after a period of hours or days it's resistance is dropping? Is there anything else that could be causing it? I'm really at a loss here as to what is happening.
Any thoughts or insight would be helpful.
Thank you
Do you have a multimeter to test the resistors after you desolder them? They should come out the same as they went in. It's difficult to change resistance values without totally burning out the component.
How much is the voltage changing, and what's connected to the resistor(s)? Maybe you can show us a schematic?
It's very unlikely that you have unstable resistors. I've come across a non-linear resistor once (in over 30 years) and I may have seen an unstable "noisy" resistor once.
Of course, in 95% of circuits you'd never know if a resistor is non-linear or unstable ...And I don't buy reject parts off eBay, and neither does the company I work for.
P.S.
What are the resistor values? Very-low values (in the ballpark of 1 Ohm) will be affected by any series resistance, such as the resistance in connectors, wires, or poor solder joints. Very high values (Maybe 1M or more) can be affected by parallel "leakage" resistance, such as contamination, or perhaps a leaky capacitor, and high impedance circuits are also subject to noise pickup.
I attached the schematic. Yes this thread is the same project as the other thread but a different problem.
Friday I noticed a trace on the PCB was partially burned out for this particular circuit. I soldered a copper wire in place to ensure a better connection. It was fixed after that but once again when I came in today (Monday) the problem is back.
I replaced a resistor and tested the resistance of the old one with a multi-meter and the resistance appears correct (510ohms).
As for the 10k base resistor I have not calculated it (nor do I know how to). We tried it and it worked so we went with it. Everything works and the AC dimmers perform as expected besides this one problematic circuit. It's worth noting I've hooked this circuit to different dimmers and get the same behavior and I've isolated the circuit design on a breadboard and the problem does not occur so it seems to be a problem with the soldered circuit.
As for the burned out trace I resoldered it again today (with a copper wire) and once again the problem goes away. I'm just not certain whether it will return. The problem connection is between the emitter of the transistor and the 510ohm resistor. See attachment.
Some thoughts... could the problem be occurring due to corrosion over night or something like that? I'm just baffled as to why when I resolder the trace it works all day, then the problem reoccurs the next morning.
