Doepfer

Hi Dieter,

thanks for the advice. The PWM-based circuit that Derek mentioned  
should help overcome that "offset" voltage in the motor, right? (as in  
such a pulse-based circuit, you always apply the full 12V  
voltage.....just not all the time).

Basically the idea is to use a CV to modulate the PW of a VCO (the  
a-111 goes from 0% PW to 100% PW IIRC), then use this VCO to drive a  
power amp as shown here http://solorb.com/elect/pwm/ 
pwm1/    .....which basically is similar to controlling RPM with the CV.

(omitting the comparator circuit, and plugging the 111 into R9).

This is gonna be fun :-)


Cheers,
denis



On Dec 2, 2008, at 10:26 AM, <yahoo@doepfer.de> <yahoo@doepfer.de>  
wrote:

> > Hi all,
> >
> > i want to build myself a little VC electro motor, you know, one of
> > these little model-plane 12V motors but with VC RPM. It's not  
> supposed
> > to track V/oct cleanly, just be somewhat predictably controllable  
> via
> > a CV.
> >
> > I'm not much of an electronics genius, but i figured that i would  
> use
> > a non-inverting amp based on an OP-amp to buffer the CV, then use  
> the
> > output as the voltage to drive the motor (the second connector of  
> the
> > motor being connected to ground). Obviously, the Op-amp would be
> > powered by the a-100 +/-12 V, so the design should be able to output
> > -12...12V to drive the motor (with the negative voltage reversing
> > direction).
> >
> > Does that sound about right?
> >
> > cheers,
> > denis
>
> Denis,
>
> I don't think that a standard opamp can do the job unless you use a  
> motor
> that requires only a few milliamperes.
> The next step would be to add a power output to a standard opamp.  
> For motor
> control 2 power transistors added to the opamp output (one npn and  
> one pnp)
> should do the job. I can send you the schematics directly if you want.
> But this will not generate a RPM that's reasonably proportional to the
> applied voltage. Each motor has an "offset" voltage. Below this  
> voltage the
> motor will not work, i.e. you have to apply e.g. 6V before a 12V  
> motor will
> begin to run. Only in the working range (e.g. +6...+12V) you will  
> have a
> behaviour with RPM reasonably proportional to the applied voltage.
>
> Best wishes
> Dieter Doepfer
>
> .
>
>