a voltage-controlled current sink / voltage-controlled resistor

Posted on Feb 6, 2014

Control a current through several current-mirror device (specifically the IREF pin on the TLC5940 ) using a single potentiometer. I`ve tried using a modified Howland current souce as in the following schematic: While this works OK with resistor values as shown for dimming a LED, it doesn`t work as a current sink given a 1. 24V voltage source at the node marked sink. I am using an LM324

a voltage-controlled current sink / voltage-controlled resistor
Click here to download the full size of the above Circuit.

quad op-amp. If someone can suggest some avenues to explore for a voltage controlled I-sink, or voltage-controlled resistor (VCR), or an R-controlled resistor, etc. I would appreciate it. I`d like a minimalist circuit, if possible, as I`ll need to control at least six current mirrors (TLC5940s) for my application, with a single pot/trimmer, and accuracy is not really critical in this application. People seem to have built VCRs out of photocells + LEDs, however, this seems a bit cludgey. I suppose an optocoupler is an option, however, I think linearity will be an (admitedly not huge) issue. A MOSFET can be used as a voltage controlled resistor, as long as it doesn`t dissipate too much power - although, this would be a fairly small resistance. Kurt E. Clothier May 19 `13 at 2:15 People seem to complain about MOSFET/JFET circuits because of biasing issues, I guess. It seems that non-linearity can be overcome by feeding the drain to the gate through a resistor, right Do you want to make your comment into an answer angelatlarge May 19 `13 at 2:28 Something like that. I`ve seen it done, but I don`t understand it quite enough to make a full answer. Maybe someone else can come up with something more definitive. Kurt E. Clothier May 19 `13 at 2:41 @angelatlarge From a prior experiment, the variation in manufacturing within a single batch of MOSFETs may give wide differences in the linear part of the curve, and in gate capacitance...

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