On Mon, 17 May 1999, Adam Bryant wrote:

> I recently picked up a number of 2N5192's surplus and would like to use
> them to construct an H Bridge motor controller (controlled from a PIC of
> course).  The motor I will be driving will draw around 1.5A.  My power
> supply is a 12v 5A battery.  The problem (if I read the datasheet
> correctly) is that the "Collector-Emitter Saturation Voltage" of the 5192
> is specified as Ic=1.5Adc, Ib=0.15Adc.  As I understand this, for my
> Collector-Emitter current to be the 1.5amps I need to drive the motor, I
> need to drive the Base with 150mA. Is this correct?  So my thought was to
> add another transistor between the PIC and the 2N5192's to amplify the
> signal from the 20mA the PIC can source to the 150mA needed for the
> 2N5192.  Seem reasonable?  Are there any other options for doing this
> using the 5192's?  I would hate to have to use 8 transisitors per motor
> H-bridge.

I'd strongly suggest ditching your 5192s and just buy a L298 or LMD18200
to handle your motor. The 298 will do 1A, 3A peak; the LMD is heftier.

This site has more info, theory, and a PCB:
        http://www.dprg.org/hbridge_theory.html

You can get a 298 surplus at BG Micro (http://www.bgmicro.com/) for $3.49
US.  Note: you'll need fast-blow diodes.  Or, a kit from HVW Technologies
is available for $25 CDN at
        http://www.hvwtech.com/robotics.htm#motor_drivers


But if you're desperate or really cheap: yes, as far as I know, you can
use a small transistor to switch the larger transistor: the pair forms a
relative of the Darlington amplifier, for which you can search for more
info.

I'm building a small 6-transistor H-bridge, and it's basically is a pain
in the ass.  But this is my first real-world project, so it's a Valuable
Learning Experience :)  I'm using a variation of this circuit:
        http://www.wizard.org/schem/motor.html



- j