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'[PIC]: Voltage multiplier'
2001\09\15@015327 by Jinx

face picon face
part 1 756 bytes content-type:text/plain; (decoded 7bit)

Hi all, could I have a little help please. I've got an application
that amongst other things requires a variable voltage from
10V to 20V. It's to charge a large cap for a solenoid pulser.
I've got this voltager quadrupler circuit that I could drive with
5V PWM from a PIC pin. The multiplier may not get to 20V,
but 15V or more should hopefully be enough

Questions -

How do I select the capacitors for best efficiency at a
particular frequency. Ripple is not especially important,
neither is frequency ?

Will varying the M:S ratio of the signal produce a variable
DC at the output of the multiplier ?

Do I need a load R apart from the solenoid reservoir cap ?

What possible down sides are there for the PIC ?

TIA



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2001\09\15@033336 by Dave Dilatush

picon face
Jinx wrote...

>Hi all, could I have a little help please. I've got an application
>that amongst other things requires a variable voltage from
>10V to 20V. It's to charge a large cap for a solenoid pulser.
>I've got this voltager quadrupler circuit that I could drive with
>5V PWM from a PIC pin. The multiplier may not get to 20V,
>but 15V or more should hopefully be enough

>Questions -

>How do I select the capacitors for best efficiency at a
>particular frequency. Ripple is not especially important,
>neither is frequency ?

If you have a charge-pumping type of circuit like this, and the
voltages on the input capacitor, flying capacitor, and output
capacitor are all the same and are all nearly constant (e.g., as in an
ICL7660-type voltage inverter) you can get efficiencies approaching
100%.

But that's not the case here: you're discharging the solenoid
reservoir cap, then recharging it through this multiplier gadget
consisting of [I assume] capacitors and switching diodes.  The
voltages on all the caps are different, and are changing in time.  The
net effect is that as the multiplier string charges the reservoir cap,
it does so in a process that pretty much resembles an exponential RC
charging curve.  The PWM drive frequency, along with the size of the
multiplier capacitors, determines the effective "resistance" involved.

Bottom line is, the efficiency won't be anything to write home about.
But how fast you can replenish the reservoir cap will depend on the
drive frequency- the higher the frequency, the faster the charge
pumping- and the multiplier capacitor sizes.

>Will varying the M:S ratio of the signal produce a variable
>DC at the output of the multiplier ?

Nope, not unless you go so close to 0% or 100% duty cycle that the
pumping capacitors just don't have enough time to complete charge
transfer before the opposite half of the cycle starts.  50% duty cycle
is best, something close to 50% is usually adequate.

>Do I need a load R apart from the solenoid reservoir cap ?

Nope.  The voltage on the reservoir cap will asymptotically approach
the multiplier factor (4X?) times the peak-to-peak drive voltage,
minus diode drops.  It won't just keep rising, if that was your
concern.

>What possible down sides are there for the PIC ?

All I can see might be output pin loading.  This could be tricky, as
your circuit is going to tend to draw output pin current in short,
sharp spikes.  Offhand, without a data sheet in front of me, I can't
hazard a guess as to a safe limit.  Could you perchance use some kind
of buffer?  I've used a 74HC244 octal bus driver with all sections
paralleled to do this kind of thing (in a voltage doubler, at least)
and it did the job.

Hope this helps a bit...

DD

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2001\09\15@035307 by Jinx

face picon face
When you say efficiency approaches 100%, if I limit the
PIC pin to say 15ma at 5V (75mVA), I could expect
around 3.5mVA at 20V ?

>Do I need a load R apart from the solenoid reservoir cap ?

> Nope.  The voltage on the reservoir cap will asymptotically
> approach the multiplier factor (4X?) times the peak-to-peak
> drive voltage, minus diode drops.  It won't just keep rising,
> if that was your concern

The PSU book I lifted the circuit from showed no component
values and a "load resistor" in all of the various 2x, 3x and 4x
circuits. They must have used "load resistor" to represent
the actual circuitry that used the voltage, not a fixed component

> Hope this helps a bit...

> DD

It helped immensely. Thanks

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2001\09\15@052940 by Jinx

face picon face
> ICL7660-type voltage inverter) you can get efficiencies
> approaching 100%.

Sorry, I skipped ahead of myself in the previous reply and
read that for this circuit I won't get 100% efficiency

>Will varying the M:S ratio of the signal produce a variable
>DC at the output of the multiplier ?

This variable voltage has another application apart from
charging the solenoid cap btw

> Nope

What I'm thinking then for the variable voltage part of it is to
drive the multiplier with a 50% square wave and get what I
can in the way of voltage (V++). Take V++ through a series-
pass PNP into an RC filter. The base of the PNP will have a
pull-up to V++ and an NPN to ground, controlled by a 2nd
PWM from the PIC. Sound feasible ?

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2001\09\15@091022 by Russell McMahon

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part 1 3450 bytes content-type:text/plain; (decoded 7bit)

Jinx,

The circuit as you have shown it will not work in your application.
There's nothing wrong with the circuit per se - just that they have implied
something in it which is not necessarily obvious.
ie the  sine wave they show is a true bipolar signal where first it has the
polarity +/- and then -/+
. A PIC PWM output etc is single sided going +/0  and then 0/0. Trace
through your diagram using a square wave and you will see that it doesn't do
what you want.
Fortunately a variation on the theme (attached) allows as many stages as you
need. There are other and probably better versions but try this for
starters.

NB:    Note that in the attached circuit you can return the BOTTOM of C3,
C5, C6 etc to the bottom of C1 = input square wave.
This has some advantages at low voltage. (For higher voltages where this
circuit is often used it is more traditional to do it as shown here).

BASIC OPERATION:

Voltages in brackets are the theoretical values for a lossless input switch
and 0.6 volt diode drops and perfect transfer of "voltage" between caps.
Each stage adds 3.8 volts (5v - 2 diode drops).
Numbering is because I drew two ccts and this is the 2nd one.


- Vin low
- Vcc (5 volts) charges C7 to 4.4 volts (Vcc 5 volts - 0.6 v diode drop).
- Vin square wave rises to 5 volts.
- C7 bottom rises to 5v so,
- C7 top rises to 9.4 volts.
- D7 charges C11 to 9.4-0.6 = 8.8 volts.
- Vin low
- C8 charges to 8.8-0.6 = 8.2v
- Vin high
- C8 top to 13.2 volts
C12 charges to - 12.6v
and so on.

Stages can be stacked as required.

In practice voltages divide between caps in proportion to charge holding
capacity, diodes lose energy etc.
If cap swings are large energy efficiency per stage tends to be 3.8/5 = 76%.
Cascade a few stages and this adds up. Small swings per cycle improves this.
Using small Schottky diodes helps.

Larger caps will be needed at the bottom and small er further up the
cascade.
These CAN be designed but common sense and a little playing will get you
started.
Start with eg 0.1uF monolithic ceramics for pump caps and small
electrolytics for reservoirs.

This is a "half wave" system.
If two antiphase square waves are available the initial circuit or something
similar to it may be OK.

More frequency = more throughput provided energy is largely transferred in
time available.

More comment avail;able if needed.


       Russell





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2001\09\15@100514 by Dave Dilatush

picon face
Jinx,

When I replied to your original post I hadn't even seen the attached
diagram.  Russell is correct in saying that this particular circuit
isn't suitable: it appears designed for a transformer input, wherein
the input voltage goes both below and above ground.  It won't work off
of a logic output from a PIC port pin.  The diagram he posted, I
suspect, is the correct one.

>What I'm thinking then for the variable voltage part of it is to
>drive the multiplier with a 50% square wave and get what I
>can in the way of voltage (V++). Take V++ through a series-
>pass PNP into an RC filter. The base of the PNP will have a
>pull-up to V++ and an NPN to ground, controlled by a 2nd
>PWM from the PIC. Sound feasible ?

Perhaps.

One extremely important thing to settle here, before going any
further, is how much current do you need out of this thing?  These
stacked cap/diode/cap/diode/cap/diode... multipliers can generate
arbitrarily high voltages from a finite input voltage (given enough
diode/cap stages, of course), but once you get beyond a doubler or
tripler, these things aren't really good for more than a couple of
milliamps output unless driven from something a **LOT** more powerful
than a PIC pin.  The times I've used multipliers like this, it was to
supply only a few hundred microamps.

First, from the variable voltage output you cited above; what's the
required output current?

And second, how much current is required by the solenoid when it is
on, how long is it on when it is on, and how often do you intend to
fire it?

If the total of these two currents, averaged over time, is more than a
couple of milliamperes, I think you need a switching regulator.  One
of the chips from National Semiconductor or Linear Technology might
do.

Dave

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2001\09\15@103322 by Jinx

face picon face
> First, from the variable voltage output you cited above; what's
> the required output current?

2 or 3 mA is enough

> And second, how much current is required by the solenoid
> when it is on, how long is it on when it is on, and how often
> do you intend to fire it?

An instantaneous pulse from the cap will do. It's part of a clock
mechanism that uses a pair of ratchet drivers. They operate
on alternate minutes, so there are 2 minutes to charge the
1000uF cap for each solenoid. The PIC presently does back-
up timing, power management, push buttons, LEDs etc. If I
could use it to generate the voltage for the solenoids then
the 12V gel cell can be replaced by a 6V. Although nominally
12V coils, the solenoids will operate from 8V (albeit weakly)
to 25V (pulses only, not constant DC). I know from other
clocks I've made that connecting the cap to the solenoid for
1/4 second is enough to drive it reliably

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2001\09\15@104332 by Dave Dilatush

picon face
Jinx wrote...

>> First, from the variable voltage output you cited above; what's
>> the required output current?
>
>2 or 3 mA is enough

OK...

{Quote hidden}

Hmmm.  I think you might be in luck.  I'd use the diagram Russell
posted, though perhaps with one less cap/diode stage.  And I wouldn't
drive it directly from a PIC pin, rather from a high-current buffer of
some kind, as in my first reply.

This can work.  It's worth a try.

Dave

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2001\09\15@104758 by Jinx

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part 1 502 bytes content-type:text/plain; (decoded 7bit)

> There are other and probably better versions but try this for
> starters.

They offer this as a floating output variant. I see that yours
includes Vcc, whereas mine do not. Noted your and DD's
comments about the lack of a -ve swing from the PIC. The
first picture I posted sounded OK as it was described as a
single-ended design. The only previous experience I've
had with multipliers was a Cockcroft-Walton for a negative
ion generator based on mains input


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2001\09\15@105419 by Jinx

face picon face
> This can work.  It's worth a try

Now I've got some pointers I can potter about. Signal diodes
like 1N914 would be alright here wouldn't they ? Could I use
Ge or Shottky diodes to minimise loses if things get tight ?

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2001\09\15@125755 by Dave Dilatush

picon face
Jinx wrote...

>Now I've got some pointers I can potter about. Signal diodes
>like 1N914 would be alright here wouldn't they ? Could I use
>Ge or Shottky diodes to minimise loses if things get tight ?

Hmmm.  1N914's would certainly be fine for a really low-current supply
of this type; I used them in a circuit which delivered, IIRC, about
ten volts at a hundred microamps.

I'm concerned about possible problems with peak current: when you
drive this kind of circuit with a square wave, as from a logic output,
the current through all of the circuit elements comes in the form of
very short, sharp spikes.  These are troublesome since, although the
average currents can be calculated accurately- and might be quite
modest- the instantaneous currents are indeterminate.  They can be
quite large, and can have damaging effects which are hard to predict.

Germanium, I think, is out; I'm not aware of any germanium devices
that would be suitable, and germanium tends to leak like a sieve,
anyway.

If it were my choice to make, I think I would opt for Schottky devices
in this application, and my customary choice would be something from
the 1N5817 family- whichever device has a sufficient breakdown
voltage.  They're 1 amp devices, I think, which might seem overkill;
and they certainly aren't as cheap as a "jellybean" silicon signal
diode.  But they'd do the trick, have a low voltage drop, and not
invite failure from overstress.

Devices like the 1N4001 series or other garden-variety power rectifier
would not be suitable, I'd think: too much potential for problems
related to reverse recovery time.

Hope this helps a bit...

DD

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2001\09\15@181423 by Brent Brown

picon face
Hey Jinx,

I think that if you put a series resistor between the PIC pin and the
first two caps, then the circuit would have more of a response to
pulse width. This might simpler than trying to regulate it after the
multiplier stage. Use feedback from the output to an ADC or
comparator input to control output voltage

Another idea is that if your circuit already has a MAX232 chip or
similar you could get ~20VDC between the +10V and the -10V
outputs.

Have fun with it.

Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile/text: 025 334 069
eMail:  spam_OUTbrent.brownTakeThisOuTspamclear.net.nz

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2001\09\15@182910 by Jinx

face picon face
> 1N914's would certainly be fine for a really low-current supply
> spikes
> can have damaging effects which are hard to predict.

It's a good idea of yours to buffer the PIC with gates. I've got
a few HC244s and 4049/4050s. For the sake of a few cents
it's worth doing

> invite failure from overstress.

Not interested in repairs - this unit is installed up a 30ft
concrete column. It goes in, it stays in

> Devices like the 1N4001 series would not be suitable,
> I'd think: too much potential for problems
> related to reverse recovery time.

I've a few UF4004s around. I can try those until I lay my hands
on some Schottky (btw, thanks for spelling Schottky correctly -
after all those years learning to spell Schmitt I said Shottky)

Hope this helps a bit...

Thanks again. I'll post circuit and results when done, promise.
It may come in handy for someone else. I can already see a
use as the PSU for an H-bridge reciprocal driver for another
clock. That presently uses 2 x 12V cells (27V) and is very
bulky

And thanks of course to RM for the circuit, I'll be into it today

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2001\09\15@191749 by Dave Dilatush

picon face
Jinx wrote...

>It's a good idea of yours to buffer the PIC with gates. I've got
>a few HC244s and 4049/4050s. For the sake of a few cents
>it's worth doing

4049/4050 running at 5V aren't very beefy.  Run 'em at 15V and they
can whump out some juice, but at 5V, HC244 would be better I think.
As a disclaimer, I'm doing some guessing here not having the
datasheets before me; check it out.  Another possibility for this
might be some kind of "clock line driver" chip.

>Not interested in repairs - this unit is installed up a 30ft
>concrete column. It goes in, it stays in

Whoa!  Concrete?  Hmmm.  Yeah, use big diodes.  :)

>(btw, thanks for spelling Schottky correctly -

Glad I succeeded in that.  Had to type it a couple different ways to
see which one looked right...

>Thanks again. I'll post circuit and results when done, promise.

Sounds like you're good to go.  Let me know if I can do anything more.

Dave

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2001\09\15@202049 by Jinx

face picon face
>4049/4050 running at 5V aren't very beefy.  Run 'em at 15V
>and they can whump out some juice, but at 5V, HC244 would
>be better I think.

Yes you're right. However, could you feed back the multiplied
voltage to power the 4050s ? Or is that getting a bit close to
"something for nothing" ?

> As a disclaimer, I'm doing some guessing here not having
> the datasheets before me; check it out.  Another possibility
> for this might be some kind of "clock line driver" chip

Or just a plain npn switcher transistor driven by the PIC. It
doesn't sound like the shape of the wave into the multiplier
is super-critical for this application, it's only ending up as DC

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2001\09\15@210102 by Dave Dilatush

picon face
Jinx wrote...

>Yes you're right. However, could you feed back the multiplied
>voltage to power the 4050s ? Or is that getting a bit close to
>"something for nothing" ?

Hmmm.  Nope, I think that falls into the "something for nothing"
category.  If you could magically reach inside the 4050's and connect
the MOSFET drains to your +5V and drive the gates from the higher
voltage you're generating, then you'd get somewhere; but no go.

>Or just a plain npn switcher transistor driven by the PIC. It
>doesn't sound like the shape of the wave into the multiplier
>is super-critical for this application, it's only ending up as DC

Yeah, but you need to have push-pull so you'd need a PNP as well; then
you'd have to be careful to prevent any overlap of the transistor ON
times to keep from having current wasted straight through the pair.
No, not worth the aggravation (IMO).

<BRAIN FART ALERT>

Here's an idea: forget all this stuff, and use a MAX232 running off
+5V to generate +/- 10V to charge your solenoid reservoir capacitor
through a suitable current-limiting resistor (maybe 1K), so you don't
overload the MAX232.  If you only need one pulse per minute, the MAX
should handle that easily.  I = C * dV/dT = .001 * 20 / 60 = 333 uA
average current.  You get 20 volts total out of the thing, and even
though it's in the form of +/- 10V, that can be dealt with easily.

MAX232 are great switched-capacitor voltage converter chips; just
ignore the unnecessary transceiver stuff it's also got inside it (do
tie off unused inputs to GND or VCC, though).

</BRAIN FART ALERT>

Dave

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2001\09\15@212145 by Jinx

face picon face
> >Yes you're right. However, could you feed back the multiplied
> >voltage to power the 4050s ? Or is that getting a bit close to
> >"something for nothing" ?

>Hmmm.  Nope, I think that falls into the "something for nothing"
>category.  If you could magically reach inside the 4050's and
> connect the MOSFET drains to your +5V and drive the gates
> from the higher voltage you're generating, then you'd get
> somewhere; but no go.

Agree. 7406 then ? Open collector, 5V logic input

> > transistor

> No, not worth the aggravation (IMO)

I'm not at home to Mr Aggravation today

> Here's an idea: forget all this stuff, and use a MAX232

Good idea. I've was thinking about what Brent Brown said
and had some thoughts along those lines afterwards. The
circuit doesn't have a transceiver in now, but no problem at
all to include. PIC-compatible, low-fuss, low-cost

Cheers

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2001\09\15@225703 by Dave Dilatush

picon face
Jinx wrote...

>Good idea. I've was thinking about what Brent Brown said
>and had some thoughts along those lines afterwards. The
>circuit doesn't have a transceiver in now, but no problem at
>all to include. PIC-compatible, low-fuss, low-cost

Yeah, the more I think about it the more I think the MAX232 is a
winner.  No fuss, no muss, and a nice 20V output.

Dave

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2001\09\16@180826 by Olin Lathrop

face picon face
> The circuit as you have shown it will not work in your application.
> There's nothing wrong with the circuit per se - just that they have
implied
> something in it which is not necessarily obvious.
> ie the  sine wave they show is a true bipolar signal where first it has
the
> polarity +/- and then -/+
> . A PIC PWM output etc is single sided going +/0  and then 0/0.

Note that the AC input voltage in Jinx's circuit is completely AC coupled to
everything else.  Any DC componenet on it is irrelevant except for the
capacitor voltage requirements.

The problem with driving it from a square wave instead of a sine is
effeciency.  If the input voltage goes thru a sudden step, then something
will suddenly have a voltage drop accross it.  It's not going to be a
capacitor, so that leaves the diode or the switching element in the square
wave voltage source.  This will waste power until the capacitors catch up to
the new input voltage.

One fix is a small inductor in series with the square wave voltage source.
The inductor takes the sudden voltage jump, which decreases as the current
builds up and the caps charge up.  You will need both positive and negative
flyback diodes from the square wave voltage source to its rails.  Make the
inductor as small as you can to limit the peak currents to tolerable levels.
You then have to make sure the switching frequency is low enough to make
sure the inductor is fully discharged before the next square wave edge.  I
would do it like this:

1  -  Decide the maximum tolerable instantaneous current from the voltage
source and thru the diodes.

2  -  Pick the smallest inductance to guarantee the maximum current from #1
is not exceeded.

3  -  Find the longest possible inductor charge then discharge time per
square wave edge.

4  -  Set the total square wave period somewhat longer than twice the time
from #3.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, .....olinKILLspamspam@spam@embedinc.com, http://www.embedinc.com

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2001\09\17@032143 by Vasile Surducan

flavicon
face
Yes, but not with only one doubler !
I had working with a family member of 232, max 3232
and have unpleasant surprise to have only -5V and +8V at outputs.
This means, a doubler is not enough for +20V

Vasile


On Sun, 16 Sep 2001, Dave Dilatush wrote:

{Quote hidden}

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2001\09\17@054531 by Dave Dilatush

picon face
Vasile Surducan wrote...

>Yes, but not with only one doubler !
>I had working with a family member of 232, max 3232
>and have unpleasant surprise to have only -5V and +8V at outputs.
>This means, a doubler is not enough for +20V

The device data sheet shows output voltage under various conditions.
With no load on the transceiver outputs, and only a few milliamps
drawn from the V+/V- terminals, you should get very close to +/- 10V
out of a MAX232A; otherwise, something is wrong.

DD

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2001\09\17@064514 by Brent Brown

picon face
Vasile Surducan wrote...
>Yes, but not with only one doubler !
>I had working with a family member of 232, max 3232
>and have unpleasant surprise to have only -5V and +8V at outputs.
>This means, a doubler is not enough for +20V

Interesting. I decided it might be worth doing some quick
measurements.

I have an Analog Devices ADM202 in a PIC circuit right here beside
me. With a 5.00V supply, 0.1uF caps and absolutely no load it is
producing +9.83V and -9.62V (19.45V between outputs).

When I connect it to the RS232 port of a PC it drops to +9.00V and
-7.90V (total 16.90V) with a 4.90V supply. No comms taking place.

The supply dropped to 4.90V because on this board I have a 22R
resistor and 100uF cap decoupling the 5V rail to (hopefully) prevent
noise from the charge pump getting into a radio receiver on the
same supply rail.

Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile/text: 025 334 069
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2001\09\17@103208 by Alan B. Pearce

face picon face
>The only previous experience I've
>had with multipliers was a Cockcroft-Walton for a negative
>ion generator based on mains input

The circuit you posted as vx4b.gif is a classic voltage doubler circuit,
often used in circuits where there is only a single non-centre tapped
winding, and both positive and negative voltages are required.

The circuit posted by Russell is a Cockroft-walton circuit, as used in the
EHT circuit of TV sets. Do take note that this circuit will change its
multiplier value depending on the input waveform type. As an example a
typical TV EHT rectifier will run as a tripler when used with the very
narrow pulse from a flyback transformer, but will run as a 5x multiplier
when driven with a 50:50 ratio waveform. I do not think this means you can
get reliable regulation by varying the mark-space ratio of the drive
waveform, but it may be worth a try for someone using a similar circuit for
LCD bias or similar non-critical applications.

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2001\09\17@200303 by Jinx

face picon face
I've uploaded a few bits and pieces re multipliers

http://home.clear.net.nz/pages/joecolquitt/multipliers.html

Russel's CW is redrawn (correctly I hope) for comparison
to a negative ion generator

Olin, is that 3rd diagram what you meant ?

Also there is a very old product of mine, a piezo whistle. I
used a TBA820M audio amp running at 40kHz to get 29V
from two 7.2V rechargeables. This is the PSU for a 555
which drives one BC337 for each piezo. The little screw
shown is to adjust the frequency of one to make it beat
against the other. I intended to make that a page of its
own - it's based on a 68HC705K1 but the code is basically
just timer/LCD and would be easily portable to a PIC. But
the whistle is buried in the St Catherine files (patron saint
of carpenters and lost causes ;-)) ) and I'm still "running up
that hill" as far as time goes, but one day..........

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2001\09\18@071930 by Olin Lathrop

face picon face
> Olin, is that 3rd diagram what you meant ?

Yes, except that I'm a bit worried about driving it directly from a PIC
output pin.  Maybe it would be OK if the two flyback diodes were Schottky.
Another possibility is to use a buffer gate or a double emitter follower.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, RemoveMEolinTakeThisOuTspamembedinc.com, http://www.embedinc.com

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2001\09\18@075421 by Roman Black

flavicon
face
Hi Jinx! :o)
Sorry I was riding around the countryside this weekend,
or I would have replied sooner. :o)

Here's an interesting suggestion; why not use a boost
converter with one diode and one inductor, and obviously
a filter cap, and drive it from the PIC output?? The
PIC can control frequency and duty cycle and give good
control of the output voltage generated.

You may need a couple of other parts, but you should
be able to get 55% efficiency very easily, possibly up
to 75% if you fiddle with it. So for your 3mA output
@ 20v you may require about 20 to 25mA, but this should
be do-able. There are some minor issues with keeping
voltages in spec, but again I think its very do-able.

The advantages are HUGE. You could tie the output voltage
through a resistor divider into the PIC analogue input,
giving proper negative feedback, so the PIC can control
ramping and regulation. :o)

Even on a simpler PIC you could use an RC network as a
default "analogue" input, using a discharge and time
delay system which I know you have used in your other
projects. Not as quick, but still gives voltage sensing
and hence proper negative feedback.

Maybe the addition of a few cents for a NPN transistor
(BC327??) after the PIC output would give higher power
20v conversion, possibly up to 20v @ 50mA with the small
RF inductor, and save the PIC from a lot of possible
out of spec problems. :o)

As far as complexity and board space goes, I think this
idea beats any voltage multiplier system that people
have suggested in terms of size and performance and
definitely efficiency. I found from my fiddling in the
last SMPS challenge that a tiny cheap RF type inductor
will give good performance in buck circuits at 75% to
85% efficiency, up to 100mA output.

Multiplier circuits??
The multiplier circuits will expose the PIC output pin
to some serious out of spec voltages. Efficiency will
suck as all the current must go through a number of
diodes, the PIC can only generate a power squarewave
of about 3.5v to 0.7v, about 2.8v total squarewave(!!)
(max possible efficiency 56% with perfect multiplier,
then allow for all the diodes...) and the main charging
and dischaging (energy transfer) occurs at peak currents,
absolutely the worst efficiency system for getting power
from a PIC output which have poor saturation performance.

I just think that the PIC-controlled simple boost
converter will be very cheap, very sophisticated, very
simple, very small, and offer a lot of advantages.
I also think that there will be a lot of piclisters
who would benefit from a circuit like this, to generate
1v to 30v voltages at a few milliamps controlled from
a PIC with just a 5v supply. Very handy circuit. :o)
-Roman



Jinx wrote:
{Quote hidden}

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2001\09\18@081933 by Jinx

face picon face
> Hi Jinx! :o)
> Sorry I was riding around the countryside this weekend,
> or I would have replied sooner. :o)

Hiya, no hurry. Hope you had a good time - bike back
together I take it

> I just think that the PIC-controlled simple boost
> converter will be very cheap, very sophisticated, very
> simple, very small, and offer a lot of advantages.
> I also think that there will be a lot of piclisters
> who would benefit from a circuit like this, to generate
> 1v to 30v voltages at a few milliamps controlled from
> a PIC with just a 5v supply. Very handy circuit. :o)

As far as I can tell this hasn't been addressed before
and I expect it will be useful. You mention the possibility
of putting some strain on a single o/p pin so why not
use two or more in parallel. Don't think I've seen that
implemented, but instinctlively I'd use a protective buffer
instead

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2001\09\18@084351 by Jinx

face picon face
> Yes, except that I'm a bit worried about driving it directly
> from a PIC output pin.

I'd use Dave Dilatush's suggestion of buffering with gates

> Maybe it would be OK if the two flyback diodes were Schottky.
> Another possibility is to use a buffer gate or a double emitter
> follower.

Getting a little spoiled for choice with all these alternatives. It
looks as those I'll be a busy boy this week trying them out

And then you lot can tell me where I went wrong ;-)))

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2001\09\18@085158 by Roman Black

flavicon
face
Jinx wrote:
>
> > Hi Jinx! :o)
> > Sorry I was riding around the countryside this weekend,
> > or I would have replied sooner. :o)
>
> Hiya, no hurry. Hope you had a good time - bike back
> together I take it

<cheeky> Ummmm, yep I prefer to ride it when it has
BOTH wheels on it...</cheeky> ;o)


{Quote hidden}

Yep, very good suggestion, also keep in mind
that for a buck or boost SMPS converter there
only needs to be ONE switching element, normally
a transistor or fet. So this leads to efficiency
gains, like using the PIC output pin(s) only
for sink with a diode, like an NPN, gives a few
advantages. Maybe also the buck/boost converter
could be designed to use the bilateral PIC output
to both sink and source current to the inductor
giving possibly greater performance with a small
inductor, either in terms of efficiency or total
current out, etc.

I haven't really seen much on the list about this
but i'm sure it would be handy, especially with
the PIC's ability to power 50mA per port could
give some nice outputs from a very simple circuit.

What did you think about the PIC controlling
voltage or current directly via controlling freq
or pwm? Something well suited to a buck/boost SMPS
converter, but not really practical with the
multipliers mentioned?
-Roman

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2001\09\18@102031 by Jinx

face picon face
> What did you think about the PIC controlling
> voltage or current directly via controlling freq
> or pwm? Something well suited to a buck/boost SMPS
> converter, but not really practical with the
> multipliers mentioned?

True. I did ask if PWM could be used to alter the voltage
but it was soon pointed out that this couldn't be achieved
with the multiple diode-cap system. The Cockroft-Walton
would have its place though, say for an LCD. They need
funny voltages sometimes. Get 10V or 15V out of a PIC
and invert it with 7660s

Remember Harold Hallikainen's DC feedback control
of lighting voltage ? Similar to what you've suggested,
IIRC measure the output voltage to adjust for variations
by changing the phase angle switching point

(off to bed. All these post-2am finishes are doing me in)

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