Hayrettin Demir said:
All Relays have coil parallel diode...
So , this relays coils have not back EMF...
Yes, they do have back EMF.
That is why the diode is required.
The diode limits the back EMF to about 0.7 Volt and therefore protects the MOSFET and the switch contacts.
If the diode was not connected, the back EMF would be very large and it would destroy the MOSFET and cause arcing across the switch contacts as they open - thus eroding the contacts.
The 82 Ohm and capacitor are not shown for simplicity. But they are assumed to be connected across the relay coil as in previous diagrams.
If a resistor of 1000 Ohm was connected across the coil, the voltage across the coil at the instant the contacts open would be 20 mA * 1000 Ohm = 20 Volt.
I have simulated the cases with and witout a diode across the 82 Ohm resistor. See attachments
As you can see, the release delay is slightly longer without the diode.
This is because the time constant is 600 * 1000 uF = 600 mS with the diode and (600 + 82) * 1000 = 682 mS without the diode.
Since I don't know what relay you are using, I have assumed that the relay release current is 4 mA.