[chbot] H-Bridge (was Re: Sensor Information)

[Charles Manning]

Herewith a reasonably good circuit for a simple H-bridge, roughly done but the 
idea is there.


It was designed by Tilden (the BEAM guy) and we've used them a bit.

How it works: 

The inputs go to the resistors at the left and right of the schematic. 

Let's consider an input on R1. When it does high this causes T6 to conduct 
which, in turn turns on T4 and T2. causing the motor to be powered in one 
direction.

When R3 is taken high, then T7, T5 and T1 conduct, turning the motor the other 
way.

If all the above were turned on simultaneously there would be a short circuit, 
so T3 is added as an "anti-smoke" transistor. If R1 input goes high then so 
does R4, causing T3 to turn on which forces T7 off, thus saving the day.

This circuit is common emitter so there is no voltage loss across the main 
driving transistors.

You need to experiment a bit with resistors. I would suggest starting with R1, 
R4, R3 all being 10k and R2 being 100 ohms.

R1 and (R3 + R2) control the maximum current so changing these to 1k would 
increase the current.

T1,2,4,5 are the main power transistors.
 T3,T6 and T7 can be low power devices.


Have fun.

-- CHarles


On Wednesday 25 March 2009 17:55:12 Hanno Sander wrote:
> Agree with Charles- from personal experience!
> Don't use the L293 for low voltage (<12V) systems!  You'll lose too much
> voltage to heat the device.  Best bet is probably to make your own h-bridge
> - it's not that hard...
> Hanno
>
> 2009/3/25 Charles Manning <cdhmanning at gmail.com>
>
> > I would not put the L298 high on the list because it is a common
> > collector device, just like the L293.
> >
> > The internal configuration of these devices means that the output voltage
> > does not swing all the way to the rail and you lose approx 1.5 to 2 volts
> > in the driver chip. This turns into heat, but also means that your motor
> > does not get driven as hard as it should meaning that it does not get
> > enough Weetbix to win that Sumo competition. That is perhaps not too much
> > of an issue when you're using 12V or more input, but obviously throwing
> > away 1.5V or so is bad news for low voltage systems.
> >
> > Many modern HBridge designs are "rail to rail" and do not suffer this
> > problem.
> >
> >
> >
> >
> > On Wed, Mar 25, 2009 at 3:54 PM, Andrew Errington <
> >
> > a.errington at lancaster.ac.uk> wrote:
> >> I actually like this design:
> >>
> >> http://www.mcmanis.com/chuck/Robotics/tutorial/h-bridge/bjt-circuit.html
> >>
> >> Because the opto circuit is very elegant (it combines optical isolation
> >> with 'smoke-free' logic).  I've never built one as I was not able to
> >> find a fast enough opto-isolator to do high-frequency PWM.   It may be
> >> that that is a non-issue however.
> >>
> >> Good luck with whatever design you choose.
> >>
> >> Andrew
> >>
> >>
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