Jump to content

VAN DE GRAFF HELP

TWATGOBBLER

Ok well I have a few questions here that I know you guys can probably answer for me no problem . I have made a Van Degraff generator and have used womens nylons as my belt and top roller is just stationary 3/4" pvc, 14" top load oh and im driving it with an old 1725 rpm ac motor . Im getting 1 1/2" - 2" arcs so  im figuring @ 150k-200k volts dc right? This thing isnt too bad but its just not enough...Sooooooooooooo I have now gone from the 1725 rpm motor and stepped it up a bit to 25,000 rpm variable speed motor now. Still running nylons and pvc top roller now I have seen people use a rotating glass top roller and a rubber flatbelt and have made signifigantly bigger arc's. So my problems are where would I get my hands on a rubber flat belt about 2"x 96" stretched out measurements or slightly less length ? And do any of you have any tips or any info to share about this generator for me like maybe if this thing can get dangerous at a certain point and possibly hurt someone or maybe idk whatever else anyone has to share. Thanks in advance everyone you guys are the best

Link to comment
Share on other sites

Link to post
Share on other sites

You're dealing with a lot of charge here, it's pretty damn dangerous as it is. I would find a lower voltage hobby if I were you. I Think your math on the voltage is right, given the dielectric strength of air at 3kV/mm at 50.8mm = 152.4kV. My intuition tells me it's less, but I haven't played around with voltages much higher than 240VDC. I just hope you know what you're doing. You've probably already seen it if you've gotten this far, but Electroboom has a couple videos on Van De Graff machines and Leiden jars, but again you're dealing with very high and dangerous E fields here.

 

edit:

Quote

if this thing can get dangerous at a certain point and possibly hurt someone

Google tells me you need about 100mA through your heart to kill you, so lets do some math. I've measured the resistance across my body, hand to hand, to be about 150k ohms. Ohm's law tells us that 100mA*150kohms = 15kV. Meaning if 15kV gets connected across my body, I'll die. Just like that, lights out. Assuming your arcs are starting at 2" away from the machine and the approximation of 152.4kV is correct, if one end of your body is grounded and the other makes contact with this thing, you have an extra order of magnitude on the requisite voltage to push that current. Please be careful.

ASU

Link to comment
Share on other sites

Link to post
Share on other sites

1 hour ago, Hackentosher said:

-snip-

Google tells me you need about 100mA through your heart to kill you, so lets do some math. I've measured the resistance across my body, hand to hand, to be about 150k ohms. Ohm's law tells us that 100mA*150kohms = 15kV. Meaning if 15kV gets connected across my body, I'll die. Just like that, lights out. Assuming your arcs are starting at 2" away from the machine and the approximation of 152.4kV is correct, if one end of your body is grounded and the other makes contact with this thing, you have an extra order of magnitude on the requisite voltage to push that current. Please be careful.

Not disagreeing with the sentiment at all since safety should be the utmost priority but I think he's still quite away off from dangerous. Hes definitely in the painful territory of electric shocks but there is a time (total energy component) involved in electrocution. A big part of electrocution is duration of the shock that actually interrupts the rhythm of the heart. I would liken it to momentarily starving an engine of gas. The revs will drop slightly but ultimately it will be right back to idle as if nothing happened. 

 

I doubt that the sphere of his VDG actually has enough capacitance to keep an arc going through his body for any longer than a few tens of nanoseconds which typically isn't enough to cause any real problems to your body. VDGs can be extremely high voltage but they're very low energy. This all depends on the size of the sphere though. 

 

With all that said though I am by a no means an expert when it comes to dying from electric shock. 

CPU: Intel i7 - 5820k @ 4.5GHz, Cooler: Corsair H80i, Motherboard: MSI X99S Gaming 7, RAM: Corsair Vengeance LPX 32GB DDR4 2666MHz CL16,

GPU: ASUS GTX 980 Strix, Case: Corsair 900D, PSU: Corsair AX860i 860W, Keyboard: Logitech G19, Mouse: Corsair M95, Storage: Intel 730 Series 480GB SSD, WD 1.5TB Black

Display: BenQ XL2730Z 2560x1440 144Hz

Link to comment
Share on other sites

Link to post
Share on other sites

1 hour ago, trag1c said:

With all that said though I am by a no means an expert when it comes to dying from electric shock. 

Lol me too

Quote

 A big part of electrocution is duration of the shock that actually interrupts the rhythm of the heart. I would liken it to momentarily starving an engine of gas. The revs will drop slightly but ultimately it will be right back to idle as if nothing happened. 

 

I doubt that the sphere of his VDG actually has enough capacitance to keep an arc going through his body for any longer than a few tens of nanoseconds 

I guess OP needs to evaluate their own risk. If I was playing around with this I don't think I'd take a "probably" as good enough to let my guard down or feel safe around it. You're probably right that the sphere can't hold enough charge to sustain a sufficiently energetic pulse to be fatal, but again I wouldn't exactly feel safe around it.

ASU

Link to comment
Share on other sites

Link to post
Share on other sites

Van De Graaff generators are generally relatively safe since they don't involve a lot of stored energy. That said, having taken 3" arcs of a Van De Graaff generator while doing demonstrations, I can say it's extremely unpleasant. As soon as you add a bunch of layden jars (capacitors), things get considerably scarier.

 

As a contrast to that, the 1.5 kV (100 times lower) voltage supply in a tube linear amplifier is very likely to be lethal if you come across it simply because of the energy available. Such a supply is likely capable of almost 1 A continuous, and it will have substantial stored energy in its filter capacitors. 

 

It's dangerous to assume that the body resistance is anywhere close to 150k. Maybe with dry skin against your meter probes, but pierce through the skin and you'll find it's a lot lower, on the order of a few hundred ohms (if I remember correctly). A high-voltage arc can easily pierce through the skin.

 

It's useful to consider the energy stored. If most of the energy is stored in a capacitor (as is the case in a Van De Graaff generator), then if you know the capacitance, it's just 0.5CV^2. The trick is knowing the voltage. The length of the sparks doesn't tell you much in that regard, since it varies with the shape of the electrodes and the humidity. It also depends heavily on air pressure (see Paschen's Law). 


You'd have a hell of a time hurting yourself with +48V phantom power from a mic preamp, but a +48V 1kA magnet power supply is a very real arc-flash hazard.

 

As far as I know, there's no hard-and-fast rule as to how much stored energy is dangerous, and a lot of it depends on the circumstances. At low voltages, the main concern is arc flash, at high voltages, the main concern is shock, up until the point when there is enough energy for it to be an arc flash hazard as well. In those cases, you want to be in a separate room. 


 

It should go without saying, but a Van De Graaff generator like this is easily capable of destroying most solid-state electronics, so take care. You can also do a number on yourself with decent-size motors- don't underestimate the stored energy in rotating systems.

 

Edited by H713
Link to comment
Share on other sites

Link to post
Share on other sites

  • 4 weeks later...

Awesome input everyone I do appreciate all the concern of me electrocuting the heck out of myself , I sometimes just look at some of the equipment and transformers ,super capacitors and just some of the crazy voltage multipliers and over powered fly backs I have collected over the years and wonder the same damn thing .....if some day one of these super dangerous contraptions will be the death of me? But until then I must build on lol and try to keep doing it as safe as possible so every bit of info ya 'll share I do try and retain as much as possible it does help. Now that I have 3 going on 4 VDG's I have been thinking about my next build and I think its time for some Tesla Coil fun. So if any one has built one or especially if anyone has built one with some big power I would love to pick your brain lol. Oh and yes VDG will screw up any solid state electronics devices a good ways away from the machine itself and especially if its left on and allowed to just build charge because when you get back to it the entire room feels charged and magically the tv and AVR receiver are all screwed up now lesson learned I guess. Again thank you all and have a great day ...
 

Link to comment
Share on other sites

Link to post
Share on other sites

  • 3 weeks later...
On 5/16/2021 at 10:55 AM, TWATGOBBLER said:

Awesome input everyone I do appreciate all the concern of me electrocuting the heck out of myself , I sometimes just look at some of the equipment and transformers ,super capacitors and just some of the crazy voltage multipliers and over powered fly backs I have collected over the years and wonder the same damn thing .....if some day one of these super dangerous contraptions will be the death of me? But until then I must build on lol and try to keep doing it as safe as possible so every bit of info ya 'll share I do try and retain as much as possible it does help. Now that I have 3 going on 4 VDG's I have been thinking about my next build and I think its time for some Tesla Coil fun. So if any one has built one or especially if anyone has built one with some big power I would love to pick your brain lol. Oh and yes VDG will screw up any solid state electronics devices a good ways away from the machine itself and especially if its left on and allowed to just build charge because when you get back to it the entire room feels charged and magically the tv and AVR receiver are all screwed up now lesson learned I guess. Again thank you all and have a great day ...
 

 

I'm not a Tesla coil expert (though I've worked on some), but any vacuum tube Tesla coil (which is probably the coolest type of Tesla coil, IMO) is an order of magnitude more dangerous than a Van De Graaff generator. All I'm going to say is start small in this regard... maybe something like a 6L6 (they're cheap). 

 

Also, google "cockroft-walton multiplier".  They're both fun and extremely useful. One of my friends built one that used an ordinary plate transformer (probably like 750 V) and a very long multplifier string to create something like a 60 kV supply. Then it arced over internally and took out his diode string. 

Link to comment
Share on other sites

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×