Re: Charging a capacitor to 6kV

Robert Baer wrote:
birdburdy wrote:
On Apr 2, 7:51 am, Hattori Hanzo <OutintheS...@xxxxxxxxxxxxxxxxx>
wrote:

On Tue, 1 Apr 2008 22:22:53 -0700 (PDT), Tom Bruhns <k7...@xxxxxxx>
wrote:




On Apr 1, 4:56 pm, birdburdy <birdbu...@xxxxxxxxxxx> wrote:

I didn't trace out the whole circuit of my "fly swatter," but it
looked like a standard blocking oscillator to me.

You know the "Wheeee" sound the fly swatter makes when you turn it
on... what actually causes that? Stress in the coils? It sounds very
flyback-like.

I was amazed at how
high a voltage mine went to, since the output capacitor is rated at
630 volts. I'm sure it was either 3kV or 4kV; I don't remember for
sure (though a search of the group archive should tell you, since I
measured it in response to a similar question about a year ago). I
measured through a high-value precision resistor, probably 10^9 or
10^10 ohms. That puts much less load on the circuit than I'd expect a
multimeter to do. A 20kohm/volt meter would be only 40 megohms at 2kV
full scale.

I suppose the inductance of the 32 ohm side would be about 0.1H to
0.2H. How fast do you want to charge the capacitor? You need to put
(C*V^2)/2 joules into it, or 18 millijoules to get 6kV across 1nF.
Note: a typical audio transformer design isn't optimum for flyback
service.

Here are the transformers I have:http://www.maplin.co.uk/Module.aspx?TabID=1&ModuleNo=3709&doy=2m4

Wild guess: a peak current of 30mA in the secondary is OK.> Then on each cycle you could store about 0.1 millijoules in the

transformer, and the current will ramp up at about 10-20mA/millisecond
with about 2.5-3V applied, so the cycle time might be 2-3
milliseconds. Expect to take on the order of a second to charge the
cap--unless I missed a factor of 10^3 in my mental arithmetic, or am
way off-base with my assumptions about the transformer. I'd measure
some transformer parameters, and then SPICE a circuit

I need to learn how to use SPICE.

to see what
things looked like. Or--I'd just adapt that fly swatter and save some
effort.

Am I correct in saying that I can simply add a Villard cascade (1kV
diodes and 400V, 0.01uF capacitors) across the oscillating output from
the transformer?

I suppose it's the core of the transformer that whines. That's
usually the case. "Blocking" oscillator is another term for flyback.
A flyback can be driven by a separate oscillator, or it can be self-
oscillating.

The transformer data *** doesn't list the inductances of primary and
secondary, but since it's rated 300-3kHz, it may be a little lower
inductance than I guessed; my guess was based on about 100Hz low end.
Not a big deal. But to make it self-oscillating, it's convenient to
have a winding about half the number of turns as compared with the
winding driven by the collector. In this case, since you're looking
for a high flyback voltage, even lower turns ratio would be good: you
don't want to drive the base negative by more than about 5V. If the
primary (driven winding) flys back to 25V, say, you'd want a 5:1 ratio
to limit the secondary driving the base to 5V.

Yes, you should be able to use a standard multiplier cascade to get to
your 6kV. You have to worry about things like diode leakage and diode
capacitance when you get to high multiplications, but I don't think
6kV will be difficult. A secondary fly-back voltage of two or three
hundred volts shouldn't be too difficult with your little audio
transformer, and you may be able to get somewhat more than that. Note
that the cascade of 0.01uF caps will represent quite a bit of net
capacitance at the 6kV output node--maybe you don't need the separate
output cap at all.

SPICE: download LTSpice for free. There are quite a few example
circuits, and on Yahoo Groups, there's an LTSpice group that's very
helpful. Beyond drawing a schematic and making sure there are values
assigned to all the parts, there's not a whole lot you need to know to
use it.

Cheers,
Tom

I notice that not a soul in this thread bothered to tell this person
that whatever cap he wants to charge, it has to be able to handle the
voltage he wants to charge it up to.

So for 6kV charge point, you would want at least a 10kV capacitor,
minimum. 12kV or 15kV is better.

I'm using capacitors in series. I have three 2kV poly caps.



Also, you do not have to worry as to whether the source will do the job
or not. If you add voltage to these caps, they accumulate a charge. Time
is the only factor.

I've still got to give them a charge at a high enough voltage.
Capacitors in series might work to achieve a net high voltage rating, BUT...
...the voltage will divide according to the capacitance; the larger the capacitance, the lower the voltage - and that is starting from all being at zero volts and assumes sufficent time for full charging (of all).


Be careful. In rectifier circuits the voltage also divides up according to individual leakage and that can be grossly different. The imbalance can reach the point where one cap takes the lion's share and then ... *BANG*.

--
Regards, Joerg

http://www.analogconsultants.com/
.

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