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Zapped 29th May 2011 07:38 AM

VR1 on the Placid (single) supply; aiming for 5.5v
First of all, my apologies for dragging this tired topic out for another go-round. I know the matter of Placid adjustment pots has been covered here, and on the Twisted Pear website forum, in several places with regard to the bipolar version of the Placid, but I remain uncertain about the values of pot VR1 that will work in the case of the the single voltage (non-bipolar) board. (I'm attempting to power a Buffalo board with 5.5 v; I'm using a Avel Lindberg 9 volt toroid for the digital supply; I pulled together my own set of parts using the BOM and my reading of forum advice on part values, which I may very well have misconstrued).

For the single voltage Placid board, can the pot VR1 be 20 ohms (rather than 200 ohms) as can be done in the case of the Placid bipolar supply? Right now on the Placid single voltage board I have soldered in place:

VR1 20ohm
VR2 10K ohm
R 6 2K (feedback R? Analogous to R10 on the bipolar board?)

If 20 ohms works for VR1 here, what is the initial resistance setting? The manual says begin at 100 ohms, which suggests (when using a 200 ohm device) a midpoint setting. Do I similarly set the 20 ohm pot at a midpoint of 10 ohms? Or at max resistance value of 20 ohms?

While we're at it. Since I have the same values for these pots on my bipolar board, I have the same question about initial resistance for VR1 there. The manual says set at 50 ohms (again, I assume for a 200 ohm device). Where do I set my 20 ohm VR1 devices for this bipolar board? One quarter of max resistance, or the whole nine yards? (I understand from forum advice that the VR2 setting should initially be max'd out, whether one has a 2K pot or a 10K pot in place).

Thanks for your help.

Russ White 30th May 2011 04:27 PM

VR1 should be 200R(or up to 1K) in this case not 20R. The original Placid and Placid BP are not the same circuit. Refer to the schematics.

Zapped 30th May 2011 06:53 PM

Thank you, Russ. I will change out the VR1 pot to a higher value as you suggest. I'm usually not one to fiddle much with circuit values on my own. I guess I just didn't read the various forum discussion threads closely enough, and failed to appreciate the differences between the Placid BP and the Placid circuit. To my layman's eye, the Placid topology looked pretty much like half of a BP placid, only aiming for a lower voltage output.

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