[sdiy] PWM control of Current Sink

Roman Sowa modular at go2.pl
Thu Jun 25 11:36:07 CEST 2015

I'd replace R5 with 100 ohms and add voltage divider 24k/1.5k from IC1.2 
output to IC1.1 input. This will not go higher on delay chip input than 
0.3V and should be comfortable enough for it.
You may also add 10uF capacitor across the 1.5k reistor of the divider 
to filter out the PWM further.


W dniu 2015-06-24 o 20:52, Tom Wiltshire pisze:
> Hi all,
> Can I get a sanity check please? I've got the following circuit:
> 	http://www.electricdruid.net/images/CurrentSink.png
> The idea of this is to control the PT2399 delay using PWM from a micro controller. The PWM filter provides the current sink with a 0-5V, and the current sink is supposed to sink 0-3mA from pin 6 of the PT2399. Ryan Williams used a similar design, but his current sink was more complicated because he was using an inverting sink as a CV mixer, whereas I have a known CV source with defined limits.
> 	http://sdiy.org/destrukto/img/pt2399_current_sink.gif
> (from http://sdiy.org/destrukto/vc-echo.html)
> Trouble is, my version doesn't work. The result I'm getting is a very short delay, as if a large current were always flowing. Changing the PWM duty cycle pushes the PT2399 out of range altogether and kills even the short delay. The PWM-to-CV part seems fine, and I get 0-5V output for my PWM input. The problem is the current sink.
> I'm not sure at this point whether I've got a fault somewhere (e.g. it ought to work, but doesn't)  or whether I'm just totally mistaken (e.g. it'd never work, even when well constructed).
> Any pointers appreciated.
> Thanks,
> Tom
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