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Van de Graaff generator vs. Greinacher Cascade for lifting hair

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Finn Hammer

Sat Sept 27 2014, 03:44PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

All,

I have built a couple of Van de Graaff generators for the local Electricity Museum, where they are used to demonstrate "Lifting Hair" alongside with the Tesla coil demonstrations. These VDGÂ´s face a lot of run time, and therefore they also demand service. Another point is the limited current delivered from these machines, 80ÂµA is not a lot.
I have been thinking about replacing them with Greinacher Cascades, and am wondering if there are any obvious reasons to not do so. Since it would be easy to create one that could deliver houndreds of ÂµA, and do so without any obvious need for regular service, I am in favour of the Upgrade from Conveyor belt to solid state.

What says you?

Cheers, Finn Hammer

Antonio

Sat Sept 27 2014, 05:46PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

80 uA is a huge current for a VDG of the size used for this experiment. 8 uA would be more typical. A cascade with some safe method of current limiting of course works too. You don't need a lot of voltage for a good effect. 50 kV is probably enough. 10 stages with microwave oven diodes and suitable capacitors, enclosed in a tube for safety and protection from dust, with a flyback transformer for excitation?

Uspring

Sat Sept 27 2014, 06:59PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

One obvious problem is shock. The VDG has a few pF. Dunno what you need for a cascade but it shouldn't add up to much more.

Sulaiman

Sat Sept 27 2014, 10:09PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I think that to replace the seemingly safe vdg with anything, it should be at least as 'safe'
which may not be easy esp. stored energy and insulation.

Steve Conner

Sun Sept 28 2014, 08:53AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I think if the stored energy issues could be sorted out, it would be a fine idea. The trouble is knowing how much stored energy is "safe". Also, the diodes in a CW stack are prone to blowing if the stack arcs to ground. A resistor in series with each diode can help.

Derek Woodroffe has a nice CW stack powered by a NST that he has used for electrostatic demos at teslathons. I don't know if anyone has dared to touch it.

High frequency drive might be an option to ease the tradeoff between output current and stored energy. If you're looking for a HF driver circuit, Anders and I can probably help.

Wastrel

Sun Sept 28 2014, 12:07PM

Registered Member #4095 Joined: Thu Sept 15 2011, 03:19PM
Location: England.
Posts: 122

VDGs are essentially constant current generators and the only capacitance is that of the dome and person. There are no failure modes that result in dangerous current to the dome, the energy is provided through mechanical work. They are as about as safe as it's possible to get for such high DC voltages.

CW ladder current increases with decreasing load when it's working properly and output can be all but shorted to the electrical input, which provides all of the power for the output, if it fails. I don't think It's a good plan to try to turn an inherently high performing unsafe design into a high performing safe one.

Antonio

Mon Sept 29 2014, 09:16PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

I have used a large disk machine for this experiment with success. It is insensitive to humidity and produces 100 uA if turned fast (what is not necessary). It is possible to experiment with both polarities, and the silent operation is a great difference.

Newton Brawn

Tue Oct 07 2014, 03:53AM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

What is the total capacitance of the electrostatic machine? and discharge voltage?

Is it possible find out how much energy (joules) can be discharged through human body without pain ?

I wave experienced discharge of 10kV in my hands with no feeling sensation. The estimated energy is less than 10milijoules.

Antonio

Wed Oct 08 2014, 01:24PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

With the Leyden jars disconnected the capacitance is of a few pF. Shocks in this experiment are due to the charge stored in the person when charged. This amounts to a capacitance of a good fraction of 100 pF. 100 pF at 100 kV corresponds to 0.5 joule, but the actual capacitance and voltage are lower.

Newton Brawn

Wed Oct 08 2014, 09:53PM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

Hi !

I am thinking in a Greinacher multiplier (C-W)
with four stages- 4 capacitors of 11pF and 4 diodes
The source could be a 10700V, 6800Hz "sinusoidal"
generator.

Generator = 15kVpk, 6.800Hz,
C1 = C2 = C3 = C4 = 11pF
Ctotal = 11pF
HV terminal max voltage = 60kV
Discharge energy = 0.02J

C= 11pF, F= 6.8kHz, >> Xc= 2.12megohm
The generator current to the HV terminal current
(grounded) = 5mA
Can this current produce some adverse effect in a person ?
Is it safe ?

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