Since we use CMOS technology and silicon transistors today, it’s very important that our circuits must be protected.
Let’s take our relay module.
On the white cover we see the coil of the relay bypassed by a diode. This diode is a so called freewheel diode. When the relay is deactivated, the collapsing magnetic field causes an induction in this coil. The direction of this current is opposite to the power connected. If this diode would not be present, this spike, which can reach 40 Volts, can destruct the relay driver and other components.
The diode is so mounted that this spike is completely suppressed.
Unfortunately, despite the depiction of this diode, it might not present in the module (like in my case).
So, I gently removed the white cover and placed a BAT85 Schottky diode across the relay coil as seen in the picture.
Pay attention to mount it as shown. I used a little white sticker as insulation.
Now, we need to be careful when assembling a circuit.Reversing the relay may damage the relay driver transistor !
Therefore it’s advised to use a protection resistor of 120 Ohms in series with the relay.
Since the relay is a 5V type, adding this resistor will prolongate it’s life and will save our battery life as well.
So it might be wise to add this resistor int the circuits of the manual whenever a relay is used.