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[DPRG] Big Trak Half Bridge

Subject: [DPRG] Big Trak Half Bridge
From: Balter Wiggin manny132 at hotmail.com
Date: Mon Jul 3 02:42:58 CDT 2006

I recently saw an article on www.robotroom.com about a seventies toy, the 
"Big Trak," and theres a half bridge motor driver in there that uses a split 
power supply to obtain bidirectionality. For a diagram, 
http://www.robotroom.com/BigTrak.html scroll down to the second diagram.

Also, i took a sort of poetic license in my description below. I removed the 
pullup resistors to the left on the diagram, and removed the coupling cap on 
the motor. But, my questions remain the same with or without the resistors 
and cap.

One lead of a motor is connected to a supply of three volts, and the other 
to two transistors. One transistor is an NPN transistor that leads to GND. 
The second transistor is a PNP transistor that leads to a supply of 6 volts 
(this is where it gets weird). When the PNP transistor is active, the motor 
spins the opposite way of when the NPN transistor is active. Is this because 
of the split power supply engineering or because electron holes (V+) flow 
from high to low?
Here is my ASCII representation of how the power supply is (i think).

6V+--------(+   3V cell   -)------3V+-------(+     3V cell     
-)---------V-/gnd

Does this "downhill" current of holes act as actual current would? I whipped 
  up a prototype circuit to a line follower based on the same concept. 
Tomorrow I intend to build it on a solderless board, so warn me before i set 
anything on fire :P

By the way, does anyone know of a relatively fast rechargeable battery 
trickle charger for a 3.6V/280mAh battery and a 3.6V/300mAh battery?

Gosh, those Big Traks look like fun...



Thanks for y'all's time:
Walter

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