[sdiy] PWM control of Current Sink

Tom Wiltshire tom at electricdruid.net
Wed Jun 24 20:52:47 CEST 2015


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




More information about the Synth-diy mailing list