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Subject: Re: Speed control for small d.c. motors
Date: Fri, 20 Sep 2002 20:52:49 +0200
Organization: Wanadoo, l'internet avec France Telecom
NNTP-Posting-Date: 20 Sep 2002 18:52:51 GMT
X-Mailer: Mozilla 4.79 [en] (X11; U; Linux 2.4.19 i686)
John Woodgate wrote:
> I have two vinyl disc players with Matsushita d.c. motors in which the
> AN6652 speed control chip has departed. They are known to be not very
> reliable and are now unobtainable AIUI.
> I want to replace with a discrete speed controller. I have an old
> Philips cassette recorder with a controller that looks sophisticated but
> originally didn't work because the AC127 germanium npn leaked like a
> sieve. But I can't see some of how it works anyway, which is annoying.
> The current sense resistor seems to be very low in value; only 17 mV
> across it. OK, all of and change in it gets to the emitter of the npn,
> through the diode level-shifters, but the gm of the Ge npn is not all
> that high at Ie = 4.5 mA. I don't see very much loop gain. This is what
> it looks like (use Courier font):
> +------1 kohm----------+
> | e BC328 |
> + 7.5 V (batt) o--------+----\_/---+--6.5 ohm--+---+------+
> 9,0 V (mains) | | | |
> +-||-+ 4.7 nF BA315 V |
> | BA315 V |
> +-100 ohm-+ | |
> | | | Motor
> 360 ohm | | |
> | |/ AC127 | |
> Speed adj. 100 ohm pot---|\e Ge npn | |
> | | | |
> 620 ohm +----------+ |
> | 560 ohm |
> 0 V o--------------+--------------------+------+
> The motor takes 26 mA at 3.7 V off load, which is almost the same as I
> want for the turntables, but I have a 14 V supply available. The BA 315
> diodes look like ordinary double-plug diodes, nothing special.
> With a 'good' AC127, it holds the motor current and voltage within close
> limits from 6 V to 12 V applied, but I can't get it to work with a
> silicon npn and a suitably adjusted 360/100/20 pot chain to allow for
> the larger Vbe; that is the major problem. I'm not surprised; Philips
> didn't put a Ge device in there for no reason. I suppose I'm missing
> something obvious.
> Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
> Interested in professional sound reinforcement and distribution? Then go to
> PLEASE do NOT copy news posts to me by E-MAIL!
I am not sure I can help with debugging your circuit, but I have another one,
very similar, and which uses only Si transistors. Beside the values of the Rs,
the most notable difference is that the upper leg of the pot chain is
connected to the Collector, not Base of the power bjt. I can post it on the
Web if you're interested.
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