Cockroft Walton Voltage Multiplier Problems


by Fronzbot
Tags: cockroft, multiplier, voltage, walton
Fronzbot
Fronzbot is offline
#1
Sep20-10, 02:38 PM
P: 63
Hey everyone.

So I like to have electronics projects going on to keep me on my toes during the academic year and my project I chose for fall ended up being a Cockroft Walton Generator. Basically, I was just looking to make a simple negative ion generator from it. I finally got the parts in today but am having SEVERE problems getting it to work properly or even figuring out what the hell is going on. This is my first HV project, so I'm pretty unfamiliar with this territory. I'm hoping someone out there can help me figure this stuff out.

First, the calculations. I used this site as a reference.

I wanted an output of -40kV at a current of 0.845mA. I chose 20nF as my capacitance and, as per the suggestion on the site, I made C1 & C2 = nC, C3 & C4 = (n-1)C etc... To save on cost (since I'm a poor college student) I chose 5 stages and was just going to use the input from my Function Generator which has a peak of about 5V and the frequency I settled on was 100Hz.

[tex]V_{out} = 2n * V_{peak} - (I_{load}/(6fC) * (4n^{3} + 3n^{2} - n))[/tex]
[tex]V_{out}= 2(5) * 5 - 0.000845/(6*100*20 \times 10^{-9}) * (4(5)^{3} + 3(5)^{2} - 5)[/tex]
[tex]V_{out} = 50 - 70.4166667 \times 570[/tex]
[tex]
V_{out} = -40087.5V[/tex]

[tex]V_{ripple} = I_{load}/fC = 0.000845/(100 \times 20 \times 10^{-9}) = 422.5V
[/tex]

So those values were perfectly fine for me. When I attached it up, though, it was only outputting -47VDC. Problem. I tried increasing the frequency and the max output I could get was -101VDC at an 8.3kHz Square Wave. I then added 5 more stages in (Stages 1 and 2 with 100nF, 3 and 4 with 80nF, 5 and 6 with 60nF, 7 and 8 with 40nF and 9 and 10 with 20nF) This time I got it up to -205VDC with an 83.2kHz Square wave.

I'm confused.

I've concluded that my output problem likely lies with the fact that I have such a small input voltage and if the circuit was attached to a more beefy supply, such as mains*, the output would be much greater. I've also concluded that these equations don't make any sense. I see no flaw with my arithmetic and so there are, as I see it, only two possibilities:
1) The equations are wrong
2) I have assumed something I shouldn't (such as Load Current, capacitance, etc)

Number 2 is the more likely case.

So anybody out there that can help me out?

*I should mention that that was just an example. I'm not going to connect this to mains for a few reasons, one of them being I don't have an isolation transformer on hand and another is I don't want to mess around with the mains, especially in my apartment.
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vk6kro
vk6kro is offline
#2
Sep20-10, 08:12 PM
Sci Advisor
P: 4,005
By coincidence, another post today has a circuit that might suit you.

See the excellent simulation by Bob S in this thread, :
http://www.physicsforums.com/showthread.php?t=429932

This appears to run at about 150 Hz, but this could be increased by decreasing the value of the 0.39uF capacitors.

You would attach your CW circuitry at the bottom of the diagram, on the secondary of the transformer.


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