I have a single transistor relay amplifier driven by GPIO output of Raspberry PI. The amplifier works reliably and is not causing any immediate problems. Still, if I leave the relay flipping on/off every second, the computer board somehow gets reset after an hour or about. I suspect that my relay still generates too much noise and I need to take more measures then just a flyback diode which I already include.
simulate this circuit – Schematic created using CircuitLab
Would adding a capacitor between base and emitter of the transistor help? By idea, when GPIO level changes, this capacitor should cause gradual closing/opening of the transistor rather than abrupt. This would give much more time for the flyback diode to engage. I understand this would slow down the switching, but 200 ms or about delay is acceptable for this relay.
If you think this would work, that would be a recommended capacity of this capacitor?
Please also tell if could do something else can be done to make this amplifier really quiet. I cannot easily get rid of the relay because the circuit it controls is totally undocumented (no clue about the voltage, current, polarity, requirements, just that < 12 V and this is a signal circuit so unlikely a lot of current). I only have a single power source to power up both computer and amplifier.
I currently have no oscilloscope in my disposition so my possibilities to investigate the noise spikes are limited.
Best Answer
Yes, it can help.
I suggest to use capacitor 1...10uF here, it will make time constant (R*C) about 2...20ms, but the real OFF time will be about 10x shorter, because only starting part of capacitor unloading curve will be effective.
The voltage on capacitor will change between 0V and about 0.7V - so you can use electrolytic capacitor for any voltage.
BTW: if the flyback diode is not enought, it means that you probably have problems with ground wiring (to thick wire, too long wire etc).