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Russ, I'm starting to build the Placid, but at the same time I want to learn something about the circuit. I started with the reference by picking up two JFETs with 10 mA Idss when Vgs=0, so I shorted the gate to source and I get a constant 10 mA through the LEDs and get 4.1V. Then with the low pass 2K/100uF, I used 2.15K resistor. I believe there are no issues here. Also, VR2 is there to turn on the power PNP transistor QP1 and adjust its current, correct? In addition, c12 and c7 are bypass for the power to the opamps, so .1uF is OK. Here is a picture  And for 14V I'm using fixed resistors according to the opamp gain formula: Vout=Vref(1+R10/R4)=4.1V(1+5230/2150)= 14.07V  Edited by user Monday, February 8, 2010 11:38:29 PM(UTC)
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hi glt,
Look fine so far. :)
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Hi Russ, been reading other posts regarding oscillation with Placid.
is the compensation network still 1nf/221 ohm and 22pf? Or do you have new values?
Thanks.
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Both are optional (the 1nf and the 221R I mean), and I would not use either by default. I actually have been rewriting the manual to show that the 1nf/221 combo is almost always not desirable. I actually added it in the beginning to test various things that could impact output impedance. The new version of the cct does not include the 1nf/221 network any more. BTW, I have not been able to reproduce any oscillation myself, regardless the compensation scheme. So I am very curious what was going on that thread. If anyone has such an issue though the first thing I would do is remove the 1nf/221 network. (simply removing either part will accomplish this, it does not matter if its the R or the C as they are in series). EDIT: I was able finally to get it to oscillate, but using 150pf instead of the 22pf totally cured it. It took trying 4 different opamps (same type opa227) but one did oscillate with only 22pf, but not at all with 150pf... Cheers! Russ Edited by user Saturday, February 13, 2010 6:32:00 PM(UTC)
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glt, see my edit above. I would use 150pf (up to say 220pf) instead of the 22pf if you see any signs of oscillation. Brian and I will likely change the kits.
Cheers!
Russ
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New compensation network: C10/C3 increased to 220pf and C8/R13 (and C4/R7) not needed. I have devised some low profile heatsinks from L-shaped aluminum (from Home Depot) for stacking two supplies. Device on heatsink is not the actual device for the supply. Also, install the 1 ohm resistor on the back side of the board and insulate the connections under the heat sink.  Edited by user Monday, February 15, 2010 8:18:43 PM(UTC)
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Good job man. :) You are a quick study.
I really like the heat sink idea. I have used something similar myself. :) They should actually be more than adequate.
Cheers!
Russ
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Russ, thanks for the feedback. I'm planning on using pre-regulation, to keep voltage drop in the power transistors at 3-4V, so power dissipation should be small.
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glt wrote:Russ, thanks for the feedback. I'm planning on using pre-regulation, to keep voltage drop in the power transistors at 3-4V, so power dissipation should be small. That will rock. You will get the benefit of less heat and incredibly awesome PSRR.
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