I have looked up some existing questions and did my own calculations for my own problem.
I am switching a relay with a BJT transistor and would like to protect the transistor from voltage spike on relay coil, after the transistor stops conducting.
I decided to connect two diodes in parallel with my relay. A schottky and a zener diode.
First I determined/checked up the relay datasheet. My relay has nominal coil voltage of 5V (this is what I will be using for switching my relay minus the Vce voltage drop) and 35mA coil current (I used 40mA in my calculations).
I then searched for a BJT. I found one with Vce_max equal to 40V and Vcb_max equal to 60V. Maximum power dissipation is 200mW.
Next I picked a schottky diode with Vf_max equal to 0.3V, repetitive reverse peak voltage 30V and Iforward 1A and a zener diode with 2V Vz @ 5mA Iz. I looked at the graph in the datasheet which shows current/voltage curve and at the coil current which is 40mA voltage drop across zener diode is 2.7V.
In my opinion these chosen elements are okay, can anyone confirm this?
The circuit is not meant to switch relay with high frequency. It's meant for opening/closing garage doors.
From the diodes I have chosen is my assumption correct that voltage drop across coil will be around 3V (sum of both voltages of zener and schottky) after the transistor stops conducting and coil starts to discharge through diodes?
Thanks in advance.