Connecting 5V to a relay with coil voltage of 4.5VDC


Joined Mar 14, 2008
I would like to know how much voltage will be generated when we turn off the relay suddenly.
Inductive kickback voltage.
That depends not only on the coil inductance but the amount of stray capacitance in the circuit.
No matter what those two values are, the resulting kickback voltage will likely blow the transistor driving the coil, so you must add a suppression circuit, such as the diode across the coil shown in your diagram.
The diode reduces the kickback voltage to less than a volt.


Joined Aug 27, 2009
One of the reasons I use the 4.5v version of the 6GK relay is to direct drive it using a controller source/sink GPIO port. The sink capability (low) of the port eliminates the need for a external diode while the 4.5V version handles the voltage drop while sourcing (high) from a 5vdc VDD and the relay drive current requirement is only 23mA. That's in the spec for high drive output pin on most 8-bit controllers or if you have extra pins that can be paralleled ON THE SAME PORT (for synchronized port latch switching) for even more drive.