y is the flash tube needed? im not criticizing, i just dont understand. wouldnt the spark gap create an ac current to pass through the capacitor instead of charging since its fluctuating?
***It's not a ringing AC spark like in an LC circuit. As the VDG's potential goes above that on the capacitor, plus the value set by the gap (and/or the xenon tube, or neon) the gap sparks and the charge transfers onto the cap. Voltage continues to build and the cap gets charged up, no diodes needed, because the spark is DC.
But these machines definitely will charge a capacitance without an intervening gap, as I keep finding out when I've been fiddling with one and then touch a ground. Yipes!
If you just put the cap terminal directly up to the VDG machine, it usually won't charge up quite so well, because of 2 things: first, the metal and other stuff near the terminal tends to make high filed strength zones that bleed off corona, and second, some of the VDG charging effect depends on electrostatic induction and that needs a bit of voltage to get started. So I find that using a small gap lets the voltage build and the charge transfers in bunches rather than in a continuous current.***
ha ha thanx 4 replying man, both of those replies will probly help me w/my own vdg(once it actually starts working) so if i understand correctly, the charge isnt continuous but its still dc because the current never reverses direction?
Let's see--during charging, or normal operation of the VDG, charge is pumped onto the top sphere and accumulates (thus raising the voltage) until the air breaks down and corona losses offset the charge rate. If you are charging a cap thru a small gap, the voltage on the VDG builds until the gap breaks down at at few hundred volts over the existing cap charge. So the cap gets boosted by that increment of voltage, and the cycle continues until the corona limit is reached. So yes, those are DC.
But once the cap is charged up, if you discharge the cap thru a short or some external circuitry, then of course the spark is AC and rings down according to the inductance in the circuit, which can be adjusted to vary the "damping" or the number of cycles in the ringdown.
Yes, that's a downward correction estimate for small spheres or points and non-grounded electrodes and perhaps perturbed fields from closeness to objects, walls, etc. Also most HV that experimenters encounter (Tesla coils, flyback transformers, etc.) is AC and RF. This will spark at lower (rms) voltages than DC, usually.
Try googling "spark gap tables" for precise gap-voltage relationships.
For "casual" experimentation with DC EHV, I usually figure 10-20 kV / cm, and ES voltmeters usually agree.
It is ridiculously easy. Way easier than a Bonetti or Wimshurst machine, and the Dirod.
The VDG is the lo-tech way to HIGH VOLTAGE!!
Sure, nanoamps don't do much budging, but it's fun to look at.
Made another last night--switched positions of roller materials and a longer column, otherwise identical. Reversed polarity confirmed--and the positive one is WAY stronger voltage and much more current, for some reason. Faster belt speed? A purple rubber band instead of blue?
The bottom roller is pressed directly onto the motor shaft.
The top roller is a Teflon bit, crowned slightly, rolling without bearings on an aluminum tube axle, which in turn is slipped thru two small grommets, which are just resting on top of the tube inside the top sphere, held in place only by the rubber band tension. It's as simple (and lazy) as I could imagine. The top brush is a bit of brass screen material cut and soldered to a bit of paperclip and hotglued into the appropriate position.
y is the flash tube needed? im not criticizing, i just dont understand. wouldnt the spark gap create an ac current to pass through the capacitor instead of charging since its fluctuating?
igotapochahontas 3 years ago
***It's not a ringing AC spark like in an LC circuit. As the VDG's potential goes above that on the capacitor, plus the value set by the gap (and/or the xenon tube, or neon) the gap sparks and the charge transfers onto the cap. Voltage continues to build and the cap gets charged up, no diodes needed, because the spark is DC.
But these machines definitely will charge a capacitance without an intervening gap, as I keep finding out when I've been fiddling with one and then touch a ground. Yipes!
TinselKoala 3 years ago
If you just put the cap terminal directly up to the VDG machine, it usually won't charge up quite so well, because of 2 things: first, the metal and other stuff near the terminal tends to make high filed strength zones that bleed off corona, and second, some of the VDG charging effect depends on electrostatic induction and that needs a bit of voltage to get started. So I find that using a small gap lets the voltage build and the charge transfers in bunches rather than in a continuous current.***
TinselKoala 3 years ago
ha ha thanx 4 replying man, both of those replies will probly help me w/my own vdg(once it actually starts working) so if i understand correctly, the charge isnt continuous but its still dc because the current never reverses direction?
igotapochahontas 3 years ago
Let's see--during charging, or normal operation of the VDG, charge is pumped onto the top sphere and accumulates (thus raising the voltage) until the air breaks down and corona losses offset the charge rate. If you are charging a cap thru a small gap, the voltage on the VDG builds until the gap breaks down at at few hundred volts over the existing cap charge. So the cap gets boosted by that increment of voltage, and the cycle continues until the corona limit is reached. So yes, those are DC.
TinselKoala 3 years ago
But once the cap is charged up, if you discharge the cap thru a short or some external circuitry, then of course the spark is AC and rings down according to the inductance in the circuit, which can be adjusted to vary the "damping" or the number of cycles in the ringdown.
TinselKoala 3 years ago
ive read that a 1 inch spark means ~30,000 volts or ~10,000 per cm
danielhattie2000 3 years ago
Yes, that's a downward correction estimate for small spheres or points and non-grounded electrodes and perhaps perturbed fields from closeness to objects, walls, etc. Also most HV that experimenters encounter (Tesla coils, flyback transformers, etc.) is AC and RF. This will spark at lower (rms) voltages than DC, usually.
Try googling "spark gap tables" for precise gap-voltage relationships.
For "casual" experimentation with DC EHV, I usually figure 10-20 kV / cm, and ES voltmeters usually agree.
TinselKoala 3 years ago
It is ridiculously easy. Way easier than a Bonetti or Wimshurst machine, and the Dirod.
The VDG is the lo-tech way to HIGH VOLTAGE!!
Sure, nanoamps don't do much budging, but it's fun to look at.
Made another last night--switched positions of roller materials and a longer column, otherwise identical. Reversed polarity confirmed--and the positive one is WAY stronger voltage and much more current, for some reason. Faster belt speed? A purple rubber band instead of blue?
Experimentation continues.
TinselKoala 3 years ago
You make it look easy :D
AdminOnDuty 3 years ago
The bottom roller is pressed directly onto the motor shaft.
The top roller is a Teflon bit, crowned slightly, rolling without bearings on an aluminum tube axle, which in turn is slipped thru two small grommets, which are just resting on top of the tube inside the top sphere, held in place only by the rubber band tension. It's as simple (and lazy) as I could imagine. The top brush is a bit of brass screen material cut and soldered to a bit of paperclip and hotglued into the appropriate position.
TinselKoala 3 years ago
Nice. What did you use for roller mounts?
AdminOnDuty 3 years ago