From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Speed control for small d.c. motors
<3D8B6E81.6CC64148@worldonline.fr> <3D8BB559.9DDA2099@webaccess.net> <firstname.lastname@example.org> <email@example.com> <3D8F1C40.BA70EAA6@webaccess.net> <firstname.lastname@example.org> <3D8FAE95.15617BFA@webaccess.net> <email@example.com>
Date: Tue, 24 Sep 2002 11:48:32 GMT
NNTP-Posting-Date: Tue, 24 Sep 2002 04:48:32 PDT
Tony Williams wrote:
> In article <3D8FAE95.15617BFA@webaccess.net>,
> Chuck Simmons wrote:
> > What you say about Ra seems a little off. As it happens Ra/R3 should be
> > near R1/RV1. That is, I would expect Ra to be about 3.3 ohms.
> A 3.3 ohm Ra would make the tail current even worse
> than first estimated.
> RV1 is a 100-ohm pot, so 3.3 ohms is the minimum
> value for Ra. If RV1 has (say) a useable adjustment
> range of 10 to 100 ohms, that would allow for an Ra
> ranging from 3.3 to 33 ohms.
> The original comment about the 22-ohm tail resistor
> was based on measured Ra's [see below]. A guessed Ra
> of 6 ohms was assumed, which would give RV1= 55 ohms.
> (Which is nicely near the centre of a 100 ohm RV1.)
> At 5v max across the bridge that would be about 0.7V
> across the 22-ohm tail resistor... 32mA tail current,
> which does seem excessive.
> If (as originally suggested) the "22" was a typo, the
> first thought was 220 ohms, but I now see that 82 is
> another possibility. With Ra=6, this would give max
> tail currents of 3.2mA (220R) and 8.5mA (82R).
> I think that 8.5mA (82R) tail current is more in
> keeping with the base current needed to drive the npn
> output transistor.
> > I don't have any brush motors here.....
> I've measured the resistance (at stall) of three
> 6v to 12v, mini-drill type of brushed motors. They
> were roughly 5, 8 and 8 ohms.
> Tony Williams.
I don't understand your calculation at all. When the motor is running,
the BEMF dominates the drop across the motor. A motor with a 3.3 volt
winding might have a current of 100ma flowing and a voltage drop of 4
volts. The resistive part of the voltage would only be 0.33 volts while
the motor's generator voltage is the rest. In my 4 volt drop example,
there is about a 1 volt drop on RV1 and maybe 18ma in the 22 ohm
The idea of the circuit is that Ra and R3 are one leg of a bridge and R1
and RV1 are the other. The resistors are chosen so that the bridge is
very near balanced for the motor resistance. When this is done, RV2 sets
the desired BEMF of the motor and thus the speed. Though not shown so,
if RV1 is a variable resistor, the circuit will accomidate a range of
motor resistances with the minimum being 3.3 ohms. For a 6.6 ohm motor,
RV1 would be adjusted to about 50 ohms.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons firstname.lastname@example.org