From: Tony Williams
Subject: Re: Speed control for small d.c. motors
Date: Tue, 24 Sep 2002 09:03:13 +0100
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NNTP-Posting-Date: Tue, 24 Sep 2002 08:03:41 +0000 (UTC)
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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
> 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.