On Mon, Mar 21, 2011 at 1:00 PM, IVP wrote: > Hi all, > > could I have suggestions for controlling an LM317 please > > In the attached diagram, the 10k pot is used to set the voltage > of the HV PWM outputs > > What I need to add is a modulation control, which simulates > the pot being rotated left and right. The pot will stay, modulation > is just another mode > > The frequency will be 10Hz at most, sinusoidal. That's not a > problem. Reading the LM317 datasheet, I see an example of > a Power Follower, using an LM195, but I don't think it's going > to be suitable > > IF I'm right assuming that ADJ can't be directly voltage-controlled > (correct me if I'm wrong) then adding modulation will require either > grounding the pot through a FET, so that it can be switched out, or > paralleling it with some resistive element > > Such as - > > Digital pot > PWM-ed 4066 switch (effective R =3D Ton/Toff) > Linearly-driven FET > > The first two imply some additional logic, perhaps a 12F. The > third seems simpler, if the FET could be is used as a resistor > with R (simply) controlled between Ron and 10k ohms. If the > pot is left in circuit, then R would be from Ron to Roff > > Any other ideas ? Another thought that might work (not tested, but from a logical point of vi= ew) The ADJ pin of the LM317 is one end of the ref. current source and using a pot to vary the current sink, you vary the voltage; that's the general application note. What I think is possible: Use the PWM output of the PIC, use it to drive a transistor: this should result in a variable voltage accross the transistor. Low pass filter this to get a clean voltage reference which is someway related to the PWM signal. Using this voltage reference, convert it to a current source that can be scaled in some way proportional to you require, use this current source to sink appropriately from the ADJ pin. I guess, this might work. --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .