I have been asked to determine why a 12 volt Pizo SonaLert continues to fail. Upon investigation this pizo is wired in parallel with a 1/2 hp 12 volt DC motor. The supply voltage is from a car alternator and it is generating 14.4-14.5 volts. My first concern is the SonaLert maximum is 15 volts. That is very close to the upper limit. Placing a scope on the motor/sonalert terminals showed a 50 volt spike on turn-on. That is probably the cause of the destruction.
I am considering installing a series resistor of 12 ohms and a 12 volt zener (1N5349BG) to limit the spike voltage just to the sonalert. My concern is the power rating of the zener. I calculated the resistor to be 1 watt since the sonalert draws 200 ma. That would drop the supply voltage to 12 volts without any zener current. When the spike occurs I am hoping the 5 watt zener will be able to handle the surge. I measured the duration of the spike and it is about 20 ms. This should not damage the resistor even though it is grossly under rated during the spike.
Any thoughts about this solution or an alternative. I have looked at varistors but a 12 volt varistor has a max clamping voltage of 38 volts. That would not do the job.
Thanks,
I am considering installing a series resistor of 12 ohms and a 12 volt zener (1N5349BG) to limit the spike voltage just to the sonalert. My concern is the power rating of the zener. I calculated the resistor to be 1 watt since the sonalert draws 200 ma. That would drop the supply voltage to 12 volts without any zener current. When the spike occurs I am hoping the 5 watt zener will be able to handle the surge. I measured the duration of the spike and it is about 20 ms. This should not damage the resistor even though it is grossly under rated during the spike.
Any thoughts about this solution or an alternative. I have looked at varistors but a 12 volt varistor has a max clamping voltage of 38 volts. That would not do the job.
Thanks,