In the circuit shown, using a SRD-05VDC-SL relay module with 5V, 89.3mA on coil side and S8050 J3Y NPN transistor.
What are the right sizing of flyback and zener diode?
The diode must withstand V and the relay current.
The Zener diode is fully optional. Its voltage should be lower than V or the transistor will get negative CE voltage. Turn-on time also is important.
Only the diode is required. The zener is an unusual addition.
It can be added if a shorter release time of the relay is required.
Zener voltage must be lower than collector breakdown voltage minus supply voltage.
Leo..
For intermittent use only the pulse rating of the diode is really relevant, as it only carries current pulses. And 89mA is hardly large for a diode either! You'd struggle to find a diode able to be damaged in this circuit (!)
Normally you'd have decoupling on the supply to this circuit as well, between +V on the coil and the transistor emitter.
You seriously need to explain why you said that! 
Wawa got it right. 
Yeah. Reversed polarity 
The peak current thru the diode is the same as the current in the coil.
This topic was automatically closed 180 days after the last reply. New replies are no longer allowed.