Let's Make Robots!

The darn H-bridge

I just want to be able to reverse a current. How hard can it be?

I just want to be able to decide direction of the motor to my pic, why does it have to be so complicated?

Well - perhjaps it doesnt! Jip / Jimmy, the most clever guy in here just drew me this and send it to me.P2290333.jpg

The smallest and most-likely-to-burn-something H-bridge in the world!

If it works, I will make a full walkthrough about it.

If it burns my robot.. well.. he did warn me ;) 




Transistors are seen from top (usually you see them drawn from below) 


Motor will turn one way when


A = low, B = low, C = high, D = high

Other way when A = high, B = high, C = low, D = low


Motor will halt if A = high, B = high, C = high, D = high


... shortcut and smoke if A = low og B = high or C = low and D = high




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The voltage at the base pin must be higher than the voltage at the emitter to turn an NPN on. If you put in an NPN where there are PNPs you would need the base-pin to go beyond the voltage supply for the motor. Of course this is possible either using voltage doublers or just keeping the motor voltage lower than the supply voltage.

Actually the h-bridge shown above was never meant to be in this forum. Frits asked me to show him the simplest possible (no protection or anything) h-bridge using the transistors he had (BC547 and BC557) and so I did :-). If I were to build an H-bridge myself I would start off by using my HEXFETs (rated for around 20 A) instead of the BJTs in this design and I would add protection schottky diodes like you describe (although the HEXFETs have built-in body diodes for protection) and I would add filtering capacitors to absorb voltage spikes from the motor. I think I would also use some bridge driver chip to drive the HEXFETs instead of interfacing them directly from the microcontroller.

If you connect A&D and B&C you limit you modes of operation. With a full bridge you get CW rotation, CCW rotation, brake and coast (and of course melt-down :-)). You will have to limit yourself to three of these if you do the connection. As previosly said this is a simple bridge not really meant for others than dare-devils like Frits :-).

- Jimmy

"The voltage at the base pin must be higher than the voltage at the emitter to turn an NPN on."

Erh... of course I meant that the voltage at the base pin must be higher than the voltage at the COLLECTOR to turn an NPN on :-)

- Jimmy

You are right, of course. I just thought i remembered a design with only NPN transistors. I'll see if I can dig it up somwhere and see how it works (if it works at all...)
There is an error in the pin description. To turn the motor one way, open C+B. The other way is A+D. Think of the transistors as a switch and follow the power from Vmotor, through the open transistor (A or C), through the motor and through the other open transistor (B or D)

I think the pin description is right. Remember that PNP transistors turn on when input is low and not high. NPN transistors are the other way around. Did I miss something else?

- Jimmy

Well, I missed the PNP/NPN part. :) But why use NPN and PNP? Why not NPN all the way? And connect A&D and B&C in your drawing to get 2 control lines instead of 4. 

 Also, remember to put protection diodes on the transistors. Put them across collector/emitter against the current direction.

Dang I keep forgetting to click "reply" on the right comment... look below somewhere to find a comment to your comment :-)

- Jimmy

There are two problems with this design. 1 is the transistors. the 547 and 557 are designed for low current. I doubt you can drive a motor with it. 2 is that if you turn on A+B and/or C+D then yes, the motor will halt, but you will most likely see the whole thing go up in a puff of smoke. If you open both transistors in either side, you create a short between Vmotor and GND...

Yes it's true that the transistors are only rated for about 100 mA of current which I also pointed out to Frits in the mail I sent him. I guess he left that bit of information out when putting the thing online :-).

Aren't motor drivers supposed to go up in a puff of smoke? :-D... I'm going for drama and not movement ;-).

- Jimmy


I just saw that I turned the NPN transistors in the wrong direction on the picture! They should be rotated 180 degrees... The collector pins from PNP and NPN are supposed to be connected.

- Jimmy