[sdiy] PWM control of Current Sink

Richie Burnett rburnett at richieburnett.co.uk
Thu Jun 25 00:15:39 CEST 2015

You can also get an additional pole of filtering in there with an RC between 
IC1.2 output and IC1.1 input if you want to further reduce ripple from the 


-----Original Message----- 
From: Tim Ressel
Sent: Wednesday, June 24, 2015 9:04 PM
To: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] PWM control of Current Sink


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.


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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> Synth-diy at dropmix.xs4all.nl
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--Tim Ressel
Circuit Abbey
timr at circuitabbey.com

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