[sdiy] PWM control of Current Sink

Richie Burnett rburnett at richieburnett.co.uk
Wed Jun 24 21:57:02 CEST 2015


What are the supply rails to the TL074?

You may need to use a rail-to-rail op-amp if you're running everything off a single rail supply,

-Richie, 


---- 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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