[sdiy] PWM control of Current Sink

[Tim Ressel]

Hi,

R5 may be too big at 1.6K, as it will drop 4.8 volts at 3mA. You can 
reduce R5 to say 470 ohms, and insert a voltage divider between pins 7 
and 3 of the op amp to compensate for the reduced feedback voltage 
across R5. I calculate 24K in series and 10K to ground for the divider.

--TimR

On 6/24/2015 11:52 AM, Tom Wiltshire wrote:
> 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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-- 
--Tim Ressel
Circuit Abbey
timr at circuitabbey.com