H-Bridge With Momentary Switches Question

Thread Starter


Joined Mar 12, 2016
Hello all!

I have ran into a strange behavior with the H-Bridge after physically building it. Before I break down the issue, I just wanted to make a few corrections based on the schematic provided. Instead of IRF9630, I'm using IRF9640. I'm also using 2N3904 NPN BJT's instead of the 2N2222 NPN BJT. Instead of the 2.2kΩ resistor on the inputs, I'm using 1kΩ.

Everything else is exactly the same. The inputs are momentary switches, so instead of sending a pulse to the inputs, I'm just connecting them directly to a power source of 5V.

My issue is that I have built this circuit and for whatever reason, whenever I push the momentary switch, the motor will rotate just fine, but as soon as I let go of the switch, the motor is still rotating. When I look at the power supply, the current begins to spike.

The strange thing is that once I drop down the power supply to about 11.4 V, the motor behaves as expected. When I push down on the momentary switch, the motor rotates, when I let go, it stops. Doesn't matter which input switch I push. It behaves as expected.

Would anyone happen to know why the circuit behaves this way?




Joined Apr 7, 2016
Hmm well as the motor is an inductor i'll try to mantain the current very stable... V = Ldi/dt... that's why the current begins to spike... it doesn't allow drastic current changes... I highly recommend to put 1 diode in paralell with each transistor in the bridge :) paralell to CE in each transistor ... i'm not sure if i'm right but it sounds like i am