[chbot] Cheap, low voltage, high current h-bridge

Richard Jones rjtp at ihug.co.nz
Sat Mar 15 11:04:11 GMT 2008


Here is a neat H bridge configuration that I think could be easily 
adapted to run with the Zetex

ZHB6718 : http://www.bobblick.com/techref/projects/hbridge/hbridge.html

The circuit as presented is a bit naff as the darlington drivers cannot saturate the output transistors, but using the Zetex chip with its amazing hfe and Vce sat in lieu of the darlington transistors gets around that. The additional driver transistors prevent smoke and provide the 3.3V interface. You would think there has to be a better way. Anyone else have a favourite low voltage H bridge circuit?

Richard


Andrew Errington wrote:
> On Fri, March 14, 2008 13:10, Hanno Sander wrote:
>   
>> Thanks Andrew for the ultra-fast and accurate reply.
>> The Zetex ZHB6718 is:
>> - cheap $1.30, single SO8 package for a complete 4 transistor NPN/PNP
>> h-bridge - low voltage: it loses < .1V at 1Amp current
>> - good on current: gain is >200 at <1Amp current, meaning drive current is
>>  <5mA.
>> A slight issue: on page 7, safe operating area, full copper, dc, they
>> imply that it's only capable of 5v, 400mV- maybe I'll have to double up to
>> reduce current on each one?  Had any experience with it?  Seems
>> straightforward to integrate- the Propeller outputs 3.3V up to 50mA, so
>> you could connect it directly, right? Hanno
>>     
>
> I think you are misinterpreting the datasheet.  That figure refers to the
> safe power dissipation.  What you see from that figure is *if* the
> H-bridge is conducting with a 100% duty cycle (i.e. DC) then the maximum
> load voltage is 5V and the maximum load current is 400mA.
>
> The Propeller outputs are irrelevant (here), since the Propeller turns the
> H-bridge transistors on and off.  They only need to source a small current
> (i.e. about 5mA as you calculated).
>
> I have used these for driving the Tamiya hobby motors.  The motors are
> rated at about 3V, but they can draw a lot of current, especially when
> loaded or stalled.  I have found that this little chip gets warm, but I
> haven't damaged one yet (but then I haven't been too demanding).  The
> particular thing I liked about this chip is that you can drive the motor
> with a lower voltage than the logic supply.  For example, if you drive a
> 3V motor with an L293D and the logic supply is 5V then the chip actually
> feeds the logic supply to the motor, which is not what I wanted.  With the
> Zetex chip you can drive the base of each transistor with 5V and the 3V
> supply voltage to the motor is unaffected.
>
> If you are driving your motor with a higher voltage than the logic supply
> then you will probably need to add two NPN transistors to drive the 'top'
> of the bridge, since the PNP transistors can only be turned off by pulling
> their bases high.  Connect the base of the two PNP transistors with a
> pull-up resistor to your motor supply, then use a pair of NPN transistors
> (controlled by the Propeller) to pull the bases to ground to turn the PNPs
> on (and invert the sense of your driving logic).
>
> Also, don't forget there is no inherent protection against short circuits.
>  You need to make sure your code does not turn on Q1 & Q2 (or Q3 & Q4) at
> the same time or you will let out the magic smoke.  You can put hard-wired
> logic next to the H-bridge to prevent this (which is the best way).  In
> fact, I found this really elegant circuit which does just that:
>
> http://www.mcmanis.com/chuck/Robotics/tutorial/h-bridge/bjt-circuit.html
>
> Oh, one last thing, these chips are robust enough that you don't need
> protection diodes across the transistors, at least, not for hobby stuff.
>
> Have fun,
>
> Andrew
>
>
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