Home-made Variac

[Julian Kang]

I have found a video on YouTube showing how to make a home-made Variac which uses distance to moderate the current. I am thinking on using it for my demo fusor, any advice? http://www.youtube.com/watch?v=P9UjxG8s ... 098377BE18

Julian

[Nick Peskosky]

Julian,

If the name from this video alone wasn't a red flag, maybe you should reconsider your technical maturity for tackling a demo Fusor. Caustic lye and unshielded AC line currents can both lead to very nasty burns/death if not handled properly. A household resistive dimmer switch would be a much safer alternative (depending on your current loading) and would cost in the ball park range of $10 at a local hardware store. Furthermore, investing in a lab grade Variac would set you back <100$ and could be used in future power supply construction and experimentation.

I'm not trying to discourage you from pursuing alternative means of controlling your input power but rather emphasizing that compromising safety for cost is ethically frowned upon by every field of engineering.

Regards,

Nick Peskosky
NPeskosky@gmail.com

[Carl Willis]

This is a resistive ballast, and fundamentally functions differently than a transformer. It may or may not suit your needs. Consider one difference: if you power an unloaded 15kV transformer via this ballast, the output voltage will be close to 15kV throughout the entire control range, whereas if you power the same transformer via a Variac, you can vary output voltage smoothly from 0-15 kV regardless of load. The Variac controls voltage, whereas this contraption is a ballast resistor.

Is it cost-effective compared to a 1.4kVa Variac? Not in my opinion, especially considering it will be a poor substitute for a Variac in a lot of applications that use Variacs. Check Variac (or generic autotransformer) prices on eBay, and on the other hand, consider what it would cost to put this ballast together.

Safety and reliability are issues here. Being a big resistor, the liquid heats up as power is dissipated. 50-60 Hz AC is a low enough frequency that some electrolysis occurs on electrodes, making hydrogen and oxygen gases that in theory could accumulate and reach flammable concentrations. Lye is, of course, an aggressive and reactive chemical and not a particularly justified choice for an electrolyte in this project, in my opinion. It's unclear what the shelf life on this project would be, since lye scavenges water and CO2 from the air and is bound to get splattered around on parts that aren't intended to contact it. Liquid level will rise and fall over time, carbonates will precipitate and corrode the copper, lye spray will eat all the brass in the switches; condensation may well cause short circuits, etc. Sooner or later, you'll spill it too.

-Carl

[Richard Hull]

Yeah, a wet resitive "ballast" is no help here. Nothing on earth touches a wire wound "autoformer" (variac) for smooth, even sinusoidial control of mains AC.

I sold and still sell, on a limited basis, variacs. The issue is the shipping costs which rival the cost of the item itself.

In general...... 2amp variac 240 watts 2 pounds - variac $15.00 shipping $10.00
5 amp variac 600 watts 5-10 lbs - $35.00, shipping $15-$20.
10 amp variac 1kilowatt 10-20lbs - $65.00, shipping $30.00ups.
20 amp variac 2 kilowatts - $100.00, shipping $40-50.
30 amp 240 volt variac 7 kilowatts $350.00 shipping $100.00++ freight truck.

Regardless of price and shipping they are a deal. However, go to a hamfest and get 'em cheaper. I saw a naked, no knob, no casing, no dial variac at a hamfest last week with good brushes rated at 120-240 volts 15 amps (over 3 kilowatts of power) for $30.00! A steal! All you had to do was lug it to your car in 96 degree heat.

Richard Hull

[Chris Bradley]

I love it! Completely daft and dangerous! Just think of all that lovely electrolysed hydrogen wafting around, along with a bucket of caustic solution just waiting to disgorge its contents all over some HV electronics.

... that is, I love that someone else is bonkers enough to show me it working!

A household resistive dimmer switch would be a much safer alternative (depending on your current loading) and would cost in the ball park range of $10

A resistive dimmer?? Tell me more!

Or, do you mean a triac based dimmer switch?

No, no good for driving transformer primaries. Forget.

[Nick Peskosky]

'Resistive' was probably a poor choice of wording and most likely a Cardinal sin for me as an EE. Triac+Diac based dimmers are not the ideal option for controlling highly inductive/capacitive loads such as an NST but with the inclusion of a snubber circuit and a power factor correction capacitor they might be able to provide you passable power control. Response in the low power range will be rather poor due to the severe distortion of the input sine wave and medium power will likely cause the xformer to hum like a bumble bee, but overall as a cheap control option I believe it would be much safer than the aforementioned electrolyte based ballast. I've never attempted controlling a transformer in this way so please correct me if I'm completely out in left field. And to reiterate, I would personally employ an auto-transformer for this type of control scenario.

This site listed a functional circuit sans-snubber : http://www.neonshop.com/neonweb/nn/tdorr17.html

Nick Peskosky
NPeskosky@gmail.com

[Chris Bradley]

Triac+Diac based dimmers are not the ideal option for controlling highly inductive/capacitive loads such as an NST

'Not ideal'!? That's the statement!

You should try it. I've tried, because it'd be very useful if at all possible, so seemed worth 'a punt' despite the recommendations not to bother trying. I guess many have tried just to see, and many have failed. You get some very very weird effects with sharply cutting the current mid-cycle in a transformer of significant power. I've never heard any electrical nor mechanical components, let alone transformers, make the sort of tortured painful noises that transformers make if you feed them with a sufficiently beefy triac that can take the current switching and drive the transformer into regions of operation beyond any natural mode it is intended to go to.

You can tune it up to work providing you have a steady, predictable load, but that's not what you get with a fusor, nor with a multiplier stack feeding a fusor.

[Chris Bradley]

Has anyone tried running a DC current through a half of a centre tapped primary (of a regular transformer), as a means to control the saturation?

[kcdodd]

This is a resistive ballast, and fundamentally functions differently than a transformer. It may or may not suit your needs. Consider one difference: if you power an unloaded 15kV transformer via this ballast, the output voltage will be close to 15kV throughout the entire control range, whereas if you power the same transformer via a Variac, you can vary output voltage smoothly from 0-15 kV regardless of load. The Variac controls voltage, whereas this contraption is a ballast resistor.

Is it cost-effective compared to a 1.4kVa Variac? Not in my opinion, especially considering it will be a poor substitute for a Variac in a lot of applications that use Variacs. Check Variac (or generic autotransformer) prices on eBay, and on the other hand, consider what it would cost to put this ballast together.

Safety and reliability are issues here. Being a big resistor, the liquid heats up as power is dissipated. 50-60 Hz AC is a low enough frequency that some electrolysis occurs on electrodes, making hydrogen and oxygen gases that in theory could accumulate and reach flammable concentrations. Lye is, of course, an aggressive and reactive chemical and not a particularly justified choice for an electrolyte in this project, in my opinion. It's unclear what the shelf life on this project would be, since lye scavenges water and CO2 from the air and is bound to get splattered around on parts that aren't intended to contact it. Liquid level will rise and fall over time, carbonates will precipitate and corrode the copper, lye spray will eat all the brass in the switches; condensation may well cause short circuits, etc. Sooner or later, you'll spill it too.

-Carl

To be clear, I am not advocating anyone use a variable resistive load in place of a variable transformer. However, this type of thing was built in the beginnings of electrical grids as dummy loads. Although, they were smart enough to use salt water instead of lye-water. I currently plan on building a copper-sulfate water resistor to regulate the current to my coils from a battery bank. The idea is the copper sulfate won't eat the copper electrodes. I have no idea what lye would do. Also I'm needing to control around 100kW, not 1kW, and I won't be putting my hand right onto it either I can tell you.