[sdiy] PWM control of Current Sink

[Tom Wiltshire]

Getting close to 0mA is important, since the PT2399 only gives long delays with small currents. I need to be able to get 50uA or less.

Roman suggested (off-list) using a current mirror instead of the op-amp current sink. The mirror would be fed with the 0-5V CV via a 1.6K resistor. Saves an op-amp.

This idea appeals to me since one way I thought to get around the need to go down to 0V in the PWM filter was to use a passive filter, and then I would need either op-amp. Mostly people don't use passive filters because they tend to be lossy, but since that's acceptable here, I could simply stick the PWM through two or three RC stages, adjust the resistor to account for losses, into a current mirror and bingo!

Ok, so it might not be hi-fi, but it'll probably be good enough. Breadboard experiments to confirm will probably be tomorrow. The simplest two-transistor current mirror I can find is this:

http://sound.westhost.com/ism-fig11.gif

The second transistor "loses" a bit of current compared to the first, since the first transistor supplies both base currents. That wouldn't matter here either, since it'd help push me closer to zero. The plan is to use the micro controller to do some calibration of the PT2399's delay time anyway, so absolute accuracy isn't that crucial.

Thanks,
Tom


On 26 Jun 2015, at 17:21, rsdio at audiobanshee.com wrote:

> R-R op-amps still do not fully reach ground. The MCP6L02 ranges from 0.035 V to 4.965 V, so that might pose a problem if you really need to reach 0 V for full range.
> 
> If Tom can find a way to redesign the circuit to extend down to 0 mA, then it's worth the effort.
> 
> Brian
> 
> 
> On Jun 26, 2015, at 5:22 AM, Roman Sowa <modular at go2.pl> wrote:
>> Just put the R-R opamp in there and you're done. I was sure the TL072 were supplied from +/- source, the schematics didn say anything about supply...
>> Anyway, as an example MCP6L02 from Microchip. There are hundreds if not thousands rail-to-rail opamp models in 5V supply range.
>> 
>> Roman
>> 
>> W dniu 2015-06-26 o 13:54, Tom Wiltshire pisze:
>>> 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:
>>>>> 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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