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

[Michael Pearce]

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

Not a true statement they are still BJT's so there is still a
significant voltage drop (0.2 to  0.7V per transistor).

Modern MOSFET's with low RDS-ON  (<0.02ohm) will be much less lossy.
Can get MOSFETS with 20 Amp peak and 4 Amp continuous in a SOT-23 or
smaller package.
But Bridges are a little more complex to make using FET's (May Need
dead time gaps etc)


Mike



2009/3/25 Charles Manning <manningc2 at actrix.gen.nz>:
> 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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