Re: speed controller help [TANKS]

[Frank Pittelli <frank-at-rctankcombat.com>]

Pete Curran wrote:
> Just did a quick search and came up with
> http://www.rc-airplane-advisor.com/electronic-speed-controller.html
> 
> Uses and old servo and power amp, 

Yep. Forgot about that approach.  Servo ICs use PWM to control the servo 
motor and that signal can be used to control power FETs to control a 
larger load.  But, keep one thing in mind:  Airplane motors only turn in 
one direction ... tank motors need to go forward and reverse.  I think 
the circuit referenced above only provides proportional speed in one 
direction (it only taps one of the servo motor wires).

The two wires between the servo IC and the motor carry a PWM signal in 
both directions, switching the polarity.  That is, in forward the signal 
is a square wave between 0 and +X volts, while in reverse the signal is 
a square wave between 0 and -X volts.  If you pass these signals through 
some discrete logic (good ole fashion NAND gates), they can be used to 
drive the motors in both directions using an H-Bridge arrangement of FETs.

You'll also want to isolate the power and control sides of the circuit, 
so that large power spikes from the motors don't blast into the ICs.

        Frank P.