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'[PIC]: pic controlled power supply'
2001\04\13@094435 by Gary Faria

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I want to control the out voltage of this power supply using a pic12c508.
The out put voltage needs to vary from 5-12V @ 7A.
I attached the schematic power.jpg
Can someone help me!

Thanks,
Gary

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2001\04\13@095513 by Gary Faria

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Sorry, The file was not attached.

I want to control the out voltage of this power supply using a pic12c508.
The out put voltage needs to vary from 5-12V @ 7A. I attached the schematic
power.jpg
Can someone help me!

Thanks,
Gary

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2001\04\13@105111 by Scott Dattalo

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On Fri, 13 Apr 2001, Gary Faria wrote:

> I want to control the out voltage of this power supply using a pic12c508.
> The out put voltage needs to vary from 5-12V @ 7A.
> I attached the schematic power.jpg
> Can someone help me!

Gary,

I didn't see the attachment, but let me tell you how I'd approach this.

First, I'm assuming you're DC/DC converter has a feedback pin that is used to
sample the output. Often times you'll had somekind of filter between the output
and the feedback pin to compensate various parameters (e.g. the effect of the
bulk capacitance, rapidly changing loads, etc.). But usually there's a point in
the feed back chain that you can introduce an analog signal to control the
output.

Now suppose the 508 is powered from a regulated voltage source. If you make an
I/O high, then this voltage will appear at the pin. If you make it low then
ground will be there. If you put a resistor between this pin and the DC/DC
converter's feedback pin (or wherever it's possible to introduce an analog
signal into the feedback path) and toggle it high and low you'll cause the
output of the DC/DC converter to switch to its extremes. If this resistor is
sized such that at one extreme you 5V and the other you get 12V, then you're
almost done!

Converting this digital output is straight forward. All you need to do is Pulse
Width Modulate and filter it. If you're fortunate, you may be able to use the
filter that is already present in the feedback loop. The PWM on a '508 is
challenging. However, I've got one that has 256 levels and single cycle
resolution. Grab a bottle of aspirin and look at this code:

http://www.dattalo.com/technical/software/pic/pwm256.txt

I promise it'll make your head hurt!

I'm not sure how you intend to adjust the output. But the PWM routine has over
half of its cycles available to do something like RS232, or pollling buttons.

Scott

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2001\04\13@113046 by Gary Faria

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Sorry, I posted the file.
http://gffaial.homestead.com/power.html

I want to control the out voltage of this power supply using a pic12c508.
The out put voltage needs to vary from 5-12V @ 7A.
I attached the schematic power.jpg
Can someone help me!

Thanks,
Gary

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2001\04\13@120911 by Scott Dattalo

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On Fri, 13 Apr 2001, Gary Faria wrote:

> Sorry, I posted the file.
> http://gffaial.homestead.com/power.html

okay.

You can drive the node between R18 and R19  with the PWM output:


     R18
      |
      +--- RA --+-- RB ---- PWM
      |         |
     R19       Cx
      |         |
     gnd       gnd


You'll need to resize R18 and R19 to get the whole range covered. (It looks like
the output is currently set for ~8 volts). Splitting the PWM resistor into two
resistors allows you to filter the PWM waveform into a smooth analog
control. When calculating the exremes, combine RA and RB together. I can't
really suggest resistor values without going through a detailed design - and I
don't have time for that...

hint: If you're really clever, you'd figure out a way to use the 431 as the
5.0 volt regulator for the PIC too.

Scott

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2001\04\13@122146 by Chris Eddy

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Gary, I have done this twice.  It takes a little thevenin analysis.  The
center pin on the 431 reference wants to see 2.50VDC.  So the supply
raises the output until this pin is 2.5.  If you inject a bit of current
into this node, you will satisfy the circuit sooner, and the output
voltage will be lower.  You should apply 0-2.5V to this node through a
resistor, the value of which is determined through circuit analysis.
When the command voltage is 0, the new resistor is in parallel with
another to ground.  You might get a notepad out and do some equations.

The easiest way to get 0-2.5VDC is to use a PWM output through an RC
pair.  Then add a resistor across the capacitor equal to the R.  You
will combine a filter and a divider in one.  Done right, you will have
0-2.5VDC.  You might choose to buffer it, or use a salen key to filter
it, it is up to the designer.

I also suggest that you only adjust over a specific range, say 25% to
100%.  Switchers are notorious for not working well outside of a
designed range of operation.  Test it well.

Have fun. Chris~

Gary Faria wrote:
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