[sdiy] PWM control of Current Sink

[rsdio at audiobanshee.com]

Your circuit cannot work across the full range. A current sink (or source) can only function when there is sufficient voltage potential for the current to flow in the desired direction.

Looking at the vc-echo page, the collector voltage is only 2.5 V, or less. The voltage can be as low as 2.35 V. That's labeled Ivco in your circuit. The problem is that your CV ranges to 5 V, which would only allow current to be sourced, yet the transistor can only sink.

Theoretically, your current sink might work in the lowest 20% of its range, give or take. Anything above 50% will surely fail, and it's likely to fail below 50% for a range.

The solution is to change your PWM-to-CV stage so that its output voltage is always below 2.35 V. You'll need to allow for any voltage drop across the Collector to Emitter of the transistor as well. Maybe 1 V max CV would work, but I haven't done the math. Then, adjust the resistor to produce the desired 3 mA when the CV is 1 V. That should work.

You might also need a base resistor, but I haven't looked too closely at that question.

Brian


On Jun 24, 2015, at 11:52 AM, Tom Wiltshire <tom at electricdruid.net> wrote:
> 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