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

[Andrew Errington]

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