[sdiy] PWM control of Current Sink

[Tom Wiltshire]

Thanks everyone for all the helpful responses.

Following on from what people said, I did some more investigations, and there are two problems:

1) The current sink's voltage compliance. The output voltage goes much too high for the 2.5V on Pin 6 of the PT2399. Lots of people suggested putting a potential divider in front of the op-amp to reduce the CV to a more reasonable level, and to reduce R5 to compensate.

2) The PWM filter is single-ended, and the op-amp can't get down to ground. This means there's an offset of about a volt, even with the PWM trying to set the CV to 0V.

I'm going to redesign this section to solve these problems and I'll report back when I get it going!

Thanks again,
Tom

On 25 Jun 2015, at 10:36, Roman Sowa <modular at go2.pl> wrote:

> I'd replace R5 with 100 ohms and add voltage divider 24k/1.5k from IC1.2 output to IC1.1 input. This will not go higher on delay chip input than 0.3V and should be comfortable enough for it.
> You may also add 10uF capacitor across the 1.5k reistor of the divider to filter out the PWM further.
> 
> Roman
> 
> W dniu 2015-06-24 o 20:52, Tom Wiltshire pisze:
>> 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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