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Topic: Sharing Base Transistor fries H-Bridge! Why? (Read 870 times) previous topic - next topic


as newbie I am moving step by step thru different How-Tos and Tutorials. Until know I never managed to kill any component....
Now I am dazzled by the fact that I managed to fry my breadboard H-Bridge by connecting one NPN/PNP pairs base via a 1k resistor w/ ground while the second NPN/PNP pair was connected to 5V. This fried my 7805+ all transistors including my 9V bat. I was trying to make the circuit a little less "PIN" intensive to use and used the attached "***Kills NPN-PNP" circuit scheme.
When I connect all base pins separatly via a 1k resistor towards GND and VCC the motor behave as expected.

Can any one give my a helping hand to understand why this is happening and/or point to a circuit that works using only 2 instead of 4 Arduino pins without additional transistors e.g. BC337?

BTW: Thanks for the great help on Arduino/Electronics as well as for the Tutorials!

Best wishes, Robin


Congratulations, it is the classical how-not-to-make-an-H-bridge error.

Look for example at the left side of the bridge.
The base current for Q3 becomes the base current for Q1.
So Q1 and Q3 will turn fully on and cause a shortcut. The current through R3 can't help preventing it, because the base current is almost unlimited.

With the example that didn't fail, the base of Q1 is to ground, so Q1 is never active.
The base of Q4 is to 5V, so that is also never active.
The current flow through Q3 - M1 - Q2, so the motor is on.

If you understand how the current flows, you know how an H-bridge works.

You could connect the emitters to the motor, and make a 'safe' circuit.
Or you could use a driver ic. Like the old L293D, or a MOSFET driver.


Thanks Krodal!

Just to make sure I understood it. PNP/NPN pairs are across the load/motor not bypassing it causing a short circuit. The NPN on the same side is driven because between the base of the NPN and the base of the PNP are on the same side with no resistor to limit the current across the Emitter-Base diode of both causing a short circuit. IF this is true couldn´t I use a diode between the PNP - NPN to rectify the Base-Current of the PNP (Q3)?




Okay.... a diode wouldn´t make sense either. This would make the second half of the H-Bridge obsolete....

Cheers, Robin


The second circuit has 2 seperate resistors going to ground. The first has 2 transistors tied together going to ground.

in the second circuit the 2 transistors are completely independent. in the first circiut the bases are tied together and thus directly affect each other.


It can work, but you have to connect all four bases with a resistor to Arduino outputs.
Most of the time, it is used like this:
- One of the PNP-transistors is switched on.
- The opposite NPN-transistor (on the other side) is a PWM signal to regulate the speed of the motor.

If you want to turn the motor left and right, and control it with PWM, you have to use 4 Arduino pins, or use a special ic (that ic might also need 4 pins of the Arduino).
If you want to turn the motor left and right, without PWM, just two Arduino pins are needed, but the H-bridge needs 8 transistors, or 4 transistors with the emitter connected to the motor, to make it safer.

Here is such a 'safe' version with the emitter connected to the motor: http://amitraj.webs.com/roboticsfordummies.htm


Hi Krodal,

I implemented the example from the referenced website. Works fine  ;)

Thanks for the help

Cheers, Robin


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