When you want both supplies to live independant, you have to feed them independant. Try this: connect the middle of the transformer windings to ground, use 2 buffer elco's in series, the middle to ground. Your supply will work perfect, up to 15V+- Leo van Loon leo.van.loon@tip.nl -----Oorspronkelijk bericht----- Van: John Midgley Aan: PICLIST@MITVMA.MIT.EDU Datum: woensdag 18 februari 1998 13:56 Onderwerp: (OT-ish) PIC Power Supply Well, quite OT actually I'm filling in a few days before my PIC programmer arrives by constructing a power supply. Simple job. Couldn't be easier. Doesn't work. I thought it would be handy to have +/- 1.2V to 20~30ishV to hand, with the two rails independently adjustable. I'm using a 317 and 337, as you'd expect. Individually, connected to an unregulated source of +20V or -20V (respectively) they do what they're supposed to. The 'adjust' pin looks at a voltage divider formed from a 330ohm resistor and a 5kohm pot. Twiddle the knob and Vout = 1.25(1+Variable/330). Lovely. Then I connect them together; they share the series-connected dual windings of a 15V transformer (to give 30V), a bridge rectifier, and earth. Now each 'tracks' the other, so that if I've got +20V and -20V 'dialled in' on the two pots, when I reduce to, say +10V, the negative rail slowly approaches -10V. Now if I dial in -5V, the positive rail reduces to about +5V. The way the 'uncommanded' voltage changes looks like a curve of a cap, charging (or discharging) through a resistor. There is a 10uF cap from the 'adj' pin to ground 'to improve ripple rejection'. In the words of Marvin Gaye, What's Going On? Sorry to be so long-winded, but I've stared at this for several evenings, and I don't know where else to turn (sob). Regards John Midgley