[sdiy] PWM control of Current Sink

[Steve Lenham]

Hi Tom,

I think your problem is with the voltage compliance of the current sink.

IC1.1 forces your full CV (up to 5V) to appear across the emitter 
resistor R5, but for TR1 to act like a transistor, its collector must be 
at least some mV above its emitter. If it isn't, the "constant current" 
will be supplied from the opamp via the base rather than through the 
collector. Worse, IC1 will start pushing current out of the collector 
via the forward-biassed B-C junction.

I'm not that familiar with the PT2399, but the Williams schematic 
indicates that the voltage on pin 6 is only about 2.5V. That indicates 
you will start seeing problems as soon as the CV gets above 2V or so.

The Williams current sink gives the amplifier some gain, thereby 
requiring a smaller voltage across the current-sense resistor.

Hope this helps!

Cheers,

Steve L.
Benden Sound Technology


On 24/06/2015 19:52, 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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