Wouter van Ooijen wrote:

>I want to drive a modest (1A 24V) motor. The circuit must be potted, so
>heat must be minimal, so I want to use FETs. For simplicity I will use
>N/P fets.
>
>  
>
N-Fets have lower resistance, and therefor create less heat.
There are special drivers to use them on the "highside".
(I'm trying to build a 24V/90A motor driver right now)

>Question: if I want to have the motor turn forward at half speed, should
>I 
>- turn on left-high and right-low for 50%, and none for the other 50%,
>or
>- something like left-high/right-low for 75% and left-low/right-high for
>25%?
>
>In other words: to control the power in one direction, do you change the
>on/off ratio, or the forward/backward ratio?
>
>  
>
A DC motor is running or not. And if it's running it runs at "full speed",
it slows down depending on the torque it has to deliver, so you probably 
need some kind of feedback.
(I intend to use angle position and speed as feedback signals, and 
control the system with a PID controller,
written in JAL ;-)
Both forward/backward and on/off should work for you, but you also have 
to consider the anti-parallel diodes over the FETS. As I think you've no 
ar simple feedback (maybe you could use generated EMK) on/off is good 
enough and consumes the least power.
I think forward backward control has sense if you need high torques at 
low speed, and you have an adequat feedback.
Think of a servo: it has almost full torque at zero speed, because it 
uses 50% forward / 50% backward.

Stef

>Wouter van Ooijen
>
>-- -------------------------------------------
>Van Ooijen Technische Informatica: www.voti.nl
>consultancy, development, PICmicro products
>docent Hogeschool van Utrecht: www.voti.nl/hvu
> 
>
>  
>
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