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'[EE]:Capacitor Charge'
2001\02\19@112608 by Andy Faulkner

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Here's an interesting challenge.

How can you charge a 450V 160uf Electolytic capacitor
from a 12V 1A source, quickly as possible to a
pre-defind charge upto 300V.

Once charged I want to disconnect the charge circuit
and connect another circuit that will quickly
discharge the capacitor.

A bit like manually connecting the capcitor to a
charge circuit that is connected to a DC meter, when
it reaches 300V manually connecting it to a discharge
circuit.

Charge circuit connected via a relays so once charged
I can fully disconnect it from the charger.

Also just to make it a bit more complex, I want to
test that the capacitor has reached it's charge and
get the result into a micro controller.

Think of an automated machine that tests low power
high voltage ignition coils .

Known variables

Charge must reach 300V or higher

Once 300V is present on the capacitor it must be
verified and then injected into the discharge without
any effects from the charge circuit being possibe.

While the capacitor is being charged it must not be
connected to the disscharge circuit.

Supply cannot be over 12V 1A

Charge speed is important.

Andy Faulkner





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2001\02\19@113020 by Bond Peter S-petbond1

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> Once charged I want to disconnect the charge circuit
> and connect another circuit that will quickly
> discharge the capacitor.

Bridge wire?

Peter

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2001\02\19@114113 by Roman Black

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Andy, its not hard to do. I've pulled apart a low
cost "personal protection" device, the ones that
zap the attacker with a high voltage.

It was very simple, 2x C cell batteries for high A
low V supply. A simple one-transistor inverter
(self excited) and simple low-turns transformer
with a single rect diode. That charged a 2kv polyprop
cap to a few hundred volts, and very quickly,
about 1/20th of a second.

Then there was a simple sparkgap, so when the cap
volts got high enough it jumped the gap and this
energy dump was fed straight into a low turns
primary of a simple autotransformer. The secondary
of the autotransformer was about 120kV, at about
20 "zaps" per second. This is similar to a tesla
setup.

Interesting that the crude mass-produced thing
just keep charging the first cap, the inverter
was not disconnected at all for the secondary
discharge/sparking.

One transistor, one diode, and a couple of discretes.
And no PIC chips in sight!
-Roman



Andy Faulkner wrote:
{Quote hidden}

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2001\02\19@123527 by Spehro Pefhany

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At 08:26 AM 2/19/01 -0800, you wrote:
>Here's an interesting challenge.
>
>How can you charge a 450V 160uf Electolytic capacitor
>from a 12V 1A source, quickly as possible to a
>pre-defind charge upto 300V.

You need an inverter, obviously. At 1A/12VDC, you get
12J/s energy. The energy in the capacitor at full charge
is  0.5*CV^2 = 7.2 J. It's thus possible to charge the
capacitor in 0.6 second if the inverter had zero
losses. So, maybe 1 second in practice. See a book
on switchmode power supply design for details
(eg. Pressman).

>Once charged I want to disconnect the charge circuit
>and connect another circuit that will quickly
>discharge the capacitor.

You can use something like MOSFETs or IGBTs.

Best regards,
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2001\02\19@124813 by Michael Rigby-Jones

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{Quote hidden}

The charging speed is limited by your specified power source.

300volts and 160uF give a total charge of 0.048 Coulombs
For the sake of argument say the PSU can provide an maximum current of
12/300 = 0.04 Amps.
t=0.048/0.04=1.2 seconds.  In practice the PSU would probably be able to
supply higher currents when the cap has a lower voltage accross it but this
gives some kind of idea.

The traditional manner in which this is done for igntion coils, is to charge
the capacitor via the discharge load (i.e. the coil), and a SCR is connected
directly across the charging supply.  Firing the SCR discharges the cap into
the load, but also shorts the output of the supply which has to be designed
for this kind of operation.  You also have the disadvantage of having the
load in series which will increase charge time.


      Discharge
300v   load
------UUUUU-----
  |            |
  |            |
  --          ---
  /\ SCR      --- Cap
  |            |
  |            |
___|____________|
0v

I'm no expert on switching converters, but all the Capactive Discharge
igntions I have seen use a ringing choke type converter.

Detecting when the voltage on the cap has reached 300volts should be
trivial, a high impedance potentiometer, dropping the voltage to a level
suitable for intput into a comparator and a suitable reference should do the
trick.  As long the the total impedance of the potentiometer is large
enough, impact on charge time should be negligible.

Mike

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2001\02\19@130723 by James Paul

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Andy,

If you charge at a constant current, the voltage will rise linearly
as a function of time.  So use your 1A source, and charge until the
terminal voltage gets to your cutoff point.  To get the cutoff, you
could through trial and error measure the terminal voltage of the
cap as it is charging, and see how long it takes to get there.
Do this several times to get a good feel for the time interval, and
then just charge that long before switching.

                                           Regards,

                                             Jim








On Mon, 19 February 2001, Andy Faulkner wrote:

{Quote hidden}

EraseMEjimspamjpes.com

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2001\02\19@133249 by David VanHorn

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At 10:07 AM 2/19/01 -0800, James Paul wrote:
>  Andy,
>
>  If you charge at a constant current, the voltage will rise linearly
>  as a function of time.  So use your 1A source, and charge until the
>  terminal voltage gets to your cutoff point.  To get the cutoff, you
>  could through trial and error measure the terminal voltage of the
>  cap as it is charging, and see how long it takes to get there.
>  Do this several times to get a good feel for the time interval, and
>  then just charge that long before switching.


I did this in a different scale for a thermal printer.
A current-limited boost SMPS charged 1000uF of output caps as fast as it
could, to 32V, from whatever it could get on the input. The PS then signals
the micro when it's ok to print the next dot-line. That requires as much as
19A from the caps.

This let me run the printer WAY down on line voltage, and still get good
printing. Final operating range was 70-140VAC into the wall-wart, using a
20W transformer.

Even at very low line, the current limiting kept me from overheating the
transformer.
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2001\02\19@135327 by Chris Eddy

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Andy;

The most traditional way of meeting your goal is with a flyback
converter.  It can be self oscillating or driven, but it is almost
immaterial, since feedback does not exist in a traditional sense.  You
will want to pursue the discontinuous mode.  The continuous mode lends
itself to feedback in a much easier manner, and you have no such
feedback.  You just reach 300VDC and stop.  A comparator will sufice.  The
goal is to design the flyback with the ratio that you need at a reasonable
duty cycle.  The duty cycle can be chosen for a target value, and then let
the current sense in the current mode controller cut it off on every
cycle.  This will charge up 'packets' of energy in the transformer, which
are then transfered to the secondary.  If you have guestimated a duty
cycle, you can then select a winding ratio to meet this goal.  Say you
choose the ratio for a max possible voltage of 400VDC.  The unit will than
have the capacity to meet 300VDC while production tolerances wander all
over the place.  Be careful that you do not try to apply the equations for
continuous mode, they are all different.  I suggest you buy the Pressman
book on switching supplies if you are going to go any further. It is like
opening a treasure chest of stuff you cannot find anywhere else (or at
least in one place).

A forward converter might do the job too.  Refer to Pressman, again.

Also, refer to automotive CDI ignition and xenon flash power supply
designs, they are both very similar.  If this is a one of, you could steal
the gust of a flash!

Rub your rabbit foot, this one could be a pretty big job (because of the
small details).
Chris~

Andy Faulkner wrote:

> Here's an interesting challenge.
>
> How can you charge a 450V 160uf Electolytic capacitor
> from a 12V 1A source, quickly as possible to a
> pre-defind charge upto 300V.
>

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2001\02\19@141905 by David VanHorn

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At 04:30 PM 2/19/01 +0000, Bond Peter S-petbond1 wrote:
> > Once charged I want to disconnect the charge circuit
> > and connect another circuit that will quickly
> > discharge the capacitor.
>
>Bridge wire?


Vat of mercury. "Plunk-ZZZAP!" :)
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2001\02\20@013837 by mmucker

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> Here's an interesting challenge.
>
> How can you charge a 450V 160uf Electolytic capacitor
> from a 12V 1A source, quickly as possible to a
> pre-defind charge upto 300V.
>
> Once charged I want to disconnect the charge circuit
> and connect another circuit that will quickly
> discharge the capacitor.

I remember a demo of this circuit in my ninth-grade physical sciences
course.  The discharge circuit was a Craftsman flat-bladed screwdriver
shorted across the two cap terminals.  :)

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2001\02\20@051855 by P.J. McCauley

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I do this sort of thing on a grander scale. (12 milli Farads up to 6KV) My
charging circuit is of little use to you. I discharge in an emergency into a
50 Ohm load using a HT relay, or into my main load using a stack of SCRs. If
you only want current to flow in one direction into your load, this might be
the way to go.

Joe

{Original Message removed}

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