Christmas Comes Early!

This forum is for specialized infomation important to the construction and safe operation of the high voltage electrical supplies and related circuitry needed for fusor operation.
Andrew Seltzman
Posts: 692
Joined: Mon Feb 02, 2004 1:02 am
Real name: Andrew Seltzman
Contact:

Re: Christmas Comes Early!

Post by Andrew Seltzman » Fri Jun 09, 2017 9:38 pm

The spellman power supplies do parallel nicely and will not sustain damage. I have paralleled 2 of the 40kV version on my fusor, see:
viewtopic.php?f=6&t=10294&start=100

One thing to remember is that these supplies are not stiff voltage sources, they are cockroft-walton multipliers; current only flows in one direction. If you parallel two of them and set them to different voltages, the output voltage will be the one of the supply set to greater voltage with that one supplying all the current. In practice, running them in constant current mode will definitely work in all conditions, but I have also found that by connecting the current program line of the "slave" supply to the current monitor line of the "master" supply and running both voltage program lines at the same voltage set point will cause both supplies to current share equally. You may need to adjust the slave supply to a slightly higher voltage to make this work, but in my case I did not need to do so.

The control loops of these supplies could be modified easily, but there is no need to do so.
Andrew Seltzman
www.rtftechnologies.org

ian_krase
Posts: 451
Joined: Mon Nov 28, 2016 7:48 am
Real name: Ian Krase

Re: Christmas Comes Early!

Post by ian_krase » Sat Jun 10, 2017 5:15 am

Interesting. Though still not something I would try with my own supplies.

There's another result of them not being "stiff": they take time to change between voltage and current mode, and can output a much higher pulse current then the control is set to. This means they can electrocute you even if the current is set to a "safe" level, and it also means that, unlike, say, an NST, they aren't good for arcing because the pulses of very high current heat stuff up.

Andrew Seltzman
Posts: 692
Joined: Mon Feb 02, 2004 1:02 am
Real name: Andrew Seltzman
Contact:

Re: Christmas Comes Early!

Post by Andrew Seltzman » Sat Jun 10, 2017 8:21 am

They do have filter capacitors on the output and thus can output a higher current pulse during an arc, but the energy storage is small. That being said since they can output 5mA or more continuously they can electrocute you, pulse or not. It's never good to intentionally arc switching power supplies continuously at a high rate, but this is not the normal operation in a fusor unless you are doing it intentionally. The PTV series of supply are an ideal candidate for a fusor, and I would recommend this model, you could probably get near 1e6n/s once it is modified for 350W.
Andrew Seltzman
www.rtftechnologies.org

ian_krase
Posts: 451
Joined: Mon Nov 28, 2016 7:48 am
Real name: Ian Krase

Re: Christmas Comes Early!

Post by ian_krase » Sat Jun 10, 2017 6:38 pm

Do you still need a ballast resistance?

Andrew Seltzman
Posts: 692
Joined: Mon Feb 02, 2004 1:02 am
Real name: Andrew Seltzman
Contact:

Re: Christmas Comes Early!

Post by Andrew Seltzman » Sat Jun 10, 2017 7:20 pm

I've never used one with my switching power supplies.
Andrew Seltzman
www.rtftechnologies.org

John Futter
Posts: 1294
Joined: Thu Apr 22, 2004 2:29 am
Real name:
Contact:

Re: Christmas Comes Early!

Post by John Futter » Sat Jun 10, 2017 7:30 pm

Ian
Unless you can guarentee that you will not have plasma breakdowns then I would use a ballast to limit dI/dt excursions in the psu
I will put up a photo of what happens to a multiplier stack if you let the supply continuously collapse the output voltage.
the first components to complain are the multiplier capacitors usually somewhere near the centre of the multiplier

Andrew Seltzman
Posts: 692
Joined: Mon Feb 02, 2004 1:02 am
Real name: Andrew Seltzman
Contact:

Re: Christmas Comes Early!

Post by Andrew Seltzman » Sat Jun 10, 2017 8:03 pm

Ian,

As John said, the word is "continuous"; an occasional spark won't harm the supply, but if you are continuously arcing(like surface tracking on your ceramic grid insulator) the supply may be damaged. More advanced supplies designed for x-ray tubes, like the DXM series, have an arc fault counter that will shut down the supply after a given number of arcs in a time window. The PTV does not have this feature (it does have current limiting), so don't leave it arcing.

I tend to favor not using a ballast resistor since it complicates the HV component train by adding another series component between the supply and grid which then needs to be insulated as well resulting in a larger more complex system. Also consider cooling issues, lets say you want a 100k ballast resistor and have a 10mA system at 40-50kV. At 50kv, an arc/short will draw a peak current of 500mA from the capacitor. During normal operation, the resistor will drop 1kV at 10mA dissipating 10W. It's not a lot of power, but if in an enclosed/insulated system it may overheat. Also consider an RL snubber (parallel resistor and inductor), that way the resistor will snub the di/dt spike, but the inductor will provide very low DC resistance for normal operation.

It's not that hard to include either one, but then again my fusor runs very stably, arcing isn't a major concern for me, and I haven't had any problems.
Andrew Seltzman
www.rtftechnologies.org

ian_krase
Posts: 451
Joined: Mon Nov 28, 2016 7:48 am
Real name: Ian Krase

Re: Christmas Comes Early!

Post by ian_krase » Sat Jun 10, 2017 8:15 pm

How would you make the inductor? Coil of HV wire?

Andrew Seltzman
Posts: 692
Joined: Mon Feb 02, 2004 1:02 am
Real name: Andrew Seltzman
Contact:

Re: Christmas Comes Early!

Post by Andrew Seltzman » Sat Jun 10, 2017 8:45 pm

I've had good success wrapping copper magnet wire directly around the resistor from terminal to terminal. This was a snubber for a 100kV, 50A system though and the resistor was about 2ft long 1.5" dia, for a smaller resistor for a fusor you might need better insulation to prevent flashover between the windings.
IMG_20150901_134527848.jpg
IMG_20150901_134536374.jpg
Andrew Seltzman
www.rtftechnologies.org

ian_krase
Posts: 451
Joined: Mon Nov 28, 2016 7:48 am
Real name: Ian Krase

Re: Christmas Comes Early!

Post by ian_krase » Sat Jun 10, 2017 9:12 pm

Cool. Makes sense, I guess. Does the assembly get potted or oil-onsulated, or just stay in air?


(And also what values would you use for a current controlled switched supply, contrasting with the faq which is for transformers​ mostly)?
Last edited by ian_krase on Sat Jun 10, 2017 9:20 pm, edited 1 time in total.

Post Reply