# linear PS's

Discussion in 'Electronic Basics' started by Abstract Dissonance, Jan 31, 2006.

1. ### Abstract DissonanceGuest

I'm building a PSU and I've got a 40VCT @.25A transformer. I want to have
two outputs of +V1 and -V2 volts... i.e., two outputs that are adjustable.
I've done it for the +V already using the LM317T and a circuit that they
have in there data sheet... I get from 1.25 to 30V...

I'm curious though as when I do it for the negative supply(V2) if I could
take the ground as V2 and the "hot" as V1 so I'd get V1-V2 = V1+|V2| to get
a new voltage(which would ~= 2*30)? or, say, if I needed two positive
"hots"(not sure what to call them) could I just reverse the ground the
V2(i.e., swaping ground and V2 "behind the scenes"(well, lets say with a
toggle switch)) or does this cause problems?

i.e., I'd like to get the most out of my psu but I'm not quite sure what I
can do... right now I just have some voltage regulators(LM317 and some fixed
ones).

One more thing... I was messing around monitoring the voltage and the
output(of the regulator) seemed to fluxuate with 1/100th of a volt or 2 and
at some points it was about 1/2 a volt or so. Is this normal? (I wasn't able
to monitor the input voltage at the same but it seems to fluxuate too).

Thanks,
Jon

2. ### John PopelishGuest

I suggest you use the center tap of the secondary as common, with a
bridge rectifier across the 40 volts. Connect 2 storage capacitors in
series, with the positive end of one to the + output of the bridge and
the negative of the other to the - output of the bridge. This should
regulator is fine for the positive output (with the reference divider
to the center tap). You need the negative version of the 317, and
LM337, for a symmetrical negative regulator.
Do you have a high frequency bypass capacitor (e.g. .1uF 50 V ceramic)
connected directly from the grounded end of the reference divider to
the regulator input, and a second one from the common to the regulator
output? If not, you may have an oscillating regulator.

3. ### MikeGuest

and the junction of the 2 caps to the center tap of the transformer
which will be your common or ground.
It cuts your max output voltage in half but gets you the + and - you
want.