I'm doing an access control system, a few months ago I had my system mounted on a protoboard and it worked correctly.
A few days ago I made the pcb for the system, then finished soldering each component, performed several tests and worked well.
Yesterday gave me a problem, it turns out I was doing tests, when the arduino commanded to turn off the electromagnetic lock, it did not turn off, it was on. Then disconnect the terminal that connects the Arduino with the optocoupler, to see if it was the arduino of the problem and the lock remained on.
Then remove the optocoupler to see if it was the problem, and nothing,
The electromagnetic lock was also on.
In short, the Mosfet constantly switched (on), so that it could not be turned off. And as a consequence did not turn off the electromagnetic lock.
I found it strange because the alarm circuit was exactly the same and worked properly.
So I measured the resistance of 330 ohm (of the alarm circuit) that is in the emitter of optocoupler, and average 326 Ohm. Then measured the resistance (330 ohm) of the electromagnetic lock circuit and average 91 ohm
I have a couple of questions:
Do the resistors wear out internally when working at high power, so that they lose their value?
Is the problem that the mosfet is not turned off, it is because of the resistance is bad, and that is why the mosfet and the lock is constantly On?
How do I detect a problem of the components, if my circuit is already soldered in a pcb?
Thank you very much
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