Fusor Forums

Just a note, in some experiments here I've gotten a heck of a lot more than usual out of an NST by hanging a cap across the secondary to resonate out the series inductance of the built in "ballast".
I used a whole bunch of nice polystyrene caps in series to get the voltage rating and value right.
In my case that was roughly 45 .47u/1kv caps in series in a plexiglass box.

Edit, if you do this, you have to be careful running under no load conditions, as the voltage can rise above what the NST can insulate.

At that point, it seems like you can get 15kv and 60 ma BOTH AT THE SAME time out of a transformer rated at 15kv/60 ma, unlike what usually happens with all that series L they built in. This is reflected in a bit more draw from the line and more heating in the transformer, of course.

I doubt you need all that lead till you get to higher voltages, other than at the view port. These just don't put out all that much till you get to around 40kv or so input in standard SS chambers. You can, however, pick up both X rays and a pretty nasty sunburn out of the view port - there's a lot more short UV than you'd think coming out there. Found out the hard way.

Here we use some leaded glass from a hospital radiology room - BillF scrounged it from a remodeling. It really cuts down the radiation of both sorts. I have also used the faceplate of an old color CRT, but it doesn't shield as well - it's not clear (gray), and you have to polish out one side to get optical quality (a ton of work with cerium oxide). But it's cheap if you see one in a dumpster.

Jake -

From what I see, you have the bottom two stages of a voltage multiplier, which should produce an UNLOADED voltage of roughly 2x the peak to peak voltage across the transformer terminals.

The NST's secondary may or may not be referenced (or connected) to the case ground. If so, your voltage measurements should be more or less as you indicated....(they could be either peak voltage if your 0.2 volt meter is a digital LCD type meter, or they could be averaged if you use a D'Arsonval type (moving coil) analog meter.)

However, you should for safety reasons and general principles, verify if your transformer actually has a center-tap connection to ground. Without the ground point, the secondary can experience voltages greater than it is intended to see. ( Doug already mentioned this)

You can just connect an ohmmeter between either end of the isolated, (and of course unpowered!!!) transformer and the case ground point. If you see a definite resistance value, a few hundred to a few thousand ohms, then the transformer DOES have a mid point connection to ground.

If you get a very high value ( megohms or more) then it is not mid point grounded.

Be careful....

Dave Cooper

While I've yet to see a floating xfrmr, I suppose such things exist. To add to what Dave said - remember that a many henry inductor is going to take some time to get an ohmmeter reading - longer than you might think. I got myself faked out on that on an old 60hz X ray transformer - didn't wait long enough.

It should be noted here that besides turning in a working Fusor at home, Jake is an ardent participant at the North West Nuclear Consortium and has his hands in our 50 KV Fusor as well, and he also is an intern for an aerospace company at which he is far from a token contributor, and he is deeply involved in robotics, rockets... The list goes on, but I think you get the point. Jake is an exceptional young man who will never flip a burger at McDonalds. Go Jake!

Dave Cooper wrote:
> The NST's secondary may or may not be referenced (or connected) to the case ground.

Sorry, I don't follow. Clearly the secondaries are referenced to ground. (Therefore the output will bob up and down with the voltage cycles of that lower output.)

Chris -

Simply a matter of "due diligence"....being careful. One learns in science, not to assume anything unconsciously. Explicit assumptions are often necessary, but when it come to high voltage safety, one should never assume connections are one way or another, without at least attempting to verify those assumptions.

Some transformers have secondaries that do not have a fixed ground connection, both ends exit through HV terminals. I have one that was made this way, at home. To use it for my purposes, it was necessary to open the case and create the ground connection at the secondary mid-point.

Dave Cooper

Dave Cooper wrote:
> One learns in science, not to assume anything unconsciously

True enough, though I think it is more often the case in science that the obvious gets overlooked ... in this case, the outputs have to be referenced to ground, else no circuit would exist and they would not drive a current into Jake's fusor grid....

So, no assumption made - the secondary is definitely earthed....

I just posted a FAQ on neon grounding.

viewtopic.php?f=11&t=4883#p27721

Richard Hull