[sdiy] PWM control of Current Sink

Neil Johnson neil.johnson71 at gmail.com
Wed Jun 24 21:20:27 CEST 2015


Tom,

My guesses:

1/ Construction fault - in which case showing us a schematic won't help; or
2/ The 1k6 resistor in your V-to-I converter is too large because your
design is flawed.  I guess you didn't read all of Ryan's website...
especially the important bits... :)

Neil

On 24 June 2015 at 19:52, Tom Wiltshire <tom at electricdruid.net> 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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