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PICList Thread
'(OT-ish) PIC Power Supply'
1998\02\18@075234 by n Midgley

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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

1998\02\18@093306 by Leo van Loon

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face
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
spam_OUTleo.van.loonTakeThisOuTspamtip.nl


-----Oorspronkelijk bericht-----
Van: John Midgley <.....John.MidgleyKILLspamspam@spam@ENORFOLK-HA.ANGLOX.NHS.UK>> Aan: PICLISTspamKILLspamMITVMA.MIT.EDU <.....PICLISTKILLspamspam.....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

1998\02\18@093405 by Miller, Steve

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face
   John,

I believe your problem is the bridge recitifiers.  A bridge rectfier circuit
produces  floating outputs.  What you need to use is either a half or full wave
rectifier circuit with the ground attached directly to the transformer winding.
This will keep the ground from floating and keep your power supplies from
tracking each other.   If you can arrange your transformer secondaries into a
center tap configuration, then use this center tap as the ground.  Once this
grounded center tap is in place, use full wave rectifiers to produce your
positive and negative supplies.

----- Steve

1998\02\18@100350 by Brian Schousek

picon face
John-
My SWAG: check the secondary voltages. Are they holding firm at 15V AC or
are they tracking the output voltage of your dialed in voltage?

Brian

{Original Message removed}

1998\02\18@120510 by n Midgley

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face
>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+-

Sorry to be ignorant, but what exactly *is* a 'buffer elco'?

>I believe your problem is the bridge recitifiers.  A bridge rectfier circuit
>produces  floating outputs.  What you need to use is either a half or full wave
>rectifier circuit with the ground attached directly to the transformer winding.
>This will keep the ground from floating and keep your power supplies from
>tracking each other.   If you can arrange your transformer secondaries into a
>center tap configuration, then use this center tap as the ground.  Once this
>grounded center tap is in place, use full wave rectifiers to produce your
>positive and negative supplies.

I think both these replies are saying similar things, but before I devastate
North Norfolk with a huge explosion - can I treat a transformer with dual
secondaries wired in series as a centre tapped transformer, and earth the
'middle' of the coil. If that makes any sense.

Many thanks

John Midgley

1998\02\18@171802 by Leo van Loon

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face
'buffer elco' is common in Dutch. In English: filter capacitor? reservoir
capacitor?
Your conclusion is right: use center tap as common ground.

Leo
-----Oorspronkelijk bericht-----
Van: John Midgley <EraseMEJohn.Midgleyspam_OUTspamTakeThisOuTENORFOLK-HA.ANGLOX.NHS.UK>> Aan: PICLISTspamspam_OUTMITVMA.MIT.EDU <@spam@PICLISTKILLspamspamMITVMA.MIT.EDU>
Datum: woensdag 18 februari 1998 22:19
Onderwerp: Re: (OT-ish) PIC Power Supply


>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+-

Sorry to be ignorant, but what exactly *is* a 'buffer elco'?

>I believe your problem is the bridge recitifiers.  A bridge rectfier
circuit
>produces  floating outputs.  What you need to use is either a half or full
wave
>rectifier circuit with the ground attached directly to the transformer
winding.
>This will keep the ground from floating and keep your power supplies from
>tracking each other.   If you can arrange your transformer secondaries into
a
>center tap configuration, then use this center tap as the ground.  Once
this
>grounded center tap is in place, use full wave rectifiers to produce your
>positive and negative supplies.

I think both these replies are saying similar things, but before I devastate
North Norfolk with a huge explosion - can I treat a transformer with dual
secondaries wired in series as a centre tapped transformer, and earth the
'middle' of the coil. If that makes any sense.

Many thanks

John Midgley

1998\02\18@181420 by H.P.d.Vries

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face
John Midgley wrote:
>
> >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+-
>
> Sorry to be ignorant, but what exactly *is* a 'buffer elco'?
>
> >I believe your problem is the bridge recitifiers.  A bridge rectfier circuit
> >produces  floating outputs.  What you need to use is either a half or full
wave
> >rectifier circuit with the ground attached directly to the transformer
winding.
{Quote hidden}

That's the trick.

Something like this:

     * | *               +----------+
 ----* | *----(1)--------+ ~        |
     * | *               |         +|--------->  +15V
~    * | *----(2)--> GND |Rectifier |
110 V * | *               |         -|--------->  -15V
 ----* | *----(3)--------+ ~        |
     * | *               +----------+




The center tap(2) is ground, Hook your favourite rectifier (coulnd't
draw one any better in ascii ;) ) up with the
two other wires on the secondary side.

To make a nice DC - voltage (to get rid of the big ripple)
connect two buffer-elco's in series between +15 and-15V (watch polarity)
The bigger the better.. If you take 35V types youll surely be safe.

Hans.

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