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

[Richard Jones]

Zetex have Mosfet H bridges ZXMHC3A01T8 that could work off 3.3V drive 
to deliver about 1-2A, and more as Vgs goes higher. see: 
http://www.zetex.com/3.0/product_portfolio.asp?pno=ZXMHC3A01T8&h=76
Soanar have Zetex on their line card. I'll write to the list if I can 
get hold of either these or the ZHB6718 locally.

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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>
>   


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