The Beginnings of a Fusor Electrical System

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Re: The Beginnings of a Fusor Electrical System

Postby Rich Feldman » Sat Dec 31, 2016 4:25 pm

Looking good, Rex, but be sure to leave enough work for Duncan to figure out himself. :-)

If the resistor's location would permit it to be near cathode potential, then it might be a bias resistor.

In Coolidge tubes with three terminals at filament end, the isolated one doesn't go to a hot cathode (in my experience).
It goes to a metal part behind and around the filament, that goes by names like focusing cup, grid, or Wehnelt electrode.
That part can be optimized for zero bias (for simplicity), or for bias 100-200 volts more negative than filament-cathode.
In the second case, bias can be developed with a resistor, giving some HV current regulation as well as sharper focus.
wehnelt_bias.jpg
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My own mains-frequency XRT has that resistor and filament winding under the wrapping of one secondary winding. Took a while to figure it out from external resistance measurements.

Commonly, VF and VH are both 60 Hz AC, for "self-rectifying" Coolidge tube operation as Rex speculated. There's no HV current in half of each mains cycle. The average HV current is DC through the secondary windings, amounting to more than a few ampere-turns and shifting core flux toward saturation on one side. Sometimes (I don't know enough to say usually) compensated with a diode and power resistor in parallel, in series with the primary winding (and could be outside of HV tank).

Also, as a reminder, nominal voltage of X-ray transformer secondaries is generally peak, not RMS. The nameplate and meter on radiography systems say kVp.
Oh, and my old reverse-engineering post is still up, with image order reversed: viewtopic.php?f=11&t=4805&p=27643
Last edited by Rich Feldman on Sat Dec 31, 2016 7:38 pm, edited 1 time in total.
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Re: The Beginnings of a Fusor Electrical System

Postby Rich Feldman » Sat Dec 31, 2016 7:28 pm

On closer reading of Rex's post:
There is an odd thing that I haven't quite figured out. Looking at picture 2, terminal 4 is the white wire from the left-side coil. This is wired to a strap around the middle of the core (possibly ground?) and it also goes to a 10K fairly high wattage resistor that then connects to terminal 7, that is the connection to the white wire from the right-side coil. So far I can't guess what this resistor might be doing.

As you surmised, the two HV windings have their Start (inside) ends connected to the grounded core. Except one is connected through a resistor, for monitoring the actual current in the one and only HV circuit. In Duncan's transformer's original application, 10 mA would yield 100 volts.

Some designs bring that secondary wire out of the oil tank, as a terminal called Current Monitor, for use with an external milliammeter and/or resistor. If not connected externally, you would probably get a 10 mA arc from that terminal to ground.
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Re: The Beginnings of a Fusor Electrical System

Postby Richard Hull » Sat Dec 31, 2016 9:02 pm

Rex has given one of the best thoughts on this yet. The multiplicity of wires can be daunting. You can go to the bank with the assumption that anything on that terminal bar is low voltage only. A good, non-autoranging ohmeter and about 10 minutes with it in the hands of a skilled electronics buff, who knows HV transformers, would have the basics of what this is down pat. I despise auto-ranging anything. I can turn a knob to select a fixed range. Inductance will not fool a fixed range, old, crude DC ohmeter and a smart user. Too many generalized assumptions are made in auto-ranging systems and can leave the average user often wondering what is going on.

I still have, and occasionally use, one of a pair of old d'arsonval metered VOMs. They never lie and warrant an easily read, averaging of a fluctuating voltage that would have a digital meter going nuts.

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Re: The Beginnings of a Fusor Electrical System

Postby Bob Reite » Sun Jan 01, 2017 3:42 am

Ah yes! A good old Simpson 260! I personally still have the Heatkit MM-1 VOM that I built when I was in Jr. High.
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Re: The Beginnings of a Fusor Electrical System

Postby Rich Feldman » Sun Jan 01, 2017 10:21 pm

Happy new year, everyone.

Duncan, I know you haven't told us any ohms readings without also telling us the meter range settings. Autoranging issues are a red herring in this thread, and are probably my fault.

Back to your assignment, here's a first step that ought to be pretty educational. But before that, if you don't have a set of clip leads on hand after Christmas, get one. Maybe cashier will regard it as colorful Christmas lighting stuff, and give you the end-of-season discount. :-)
th.jpg
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At the transformer, let's begin with white and yellow wires that emerge around middle of paper wrapping that covers each secondary winding. Connect a reasonably fresh 9 volt battery between the white wire and one of the yellows. Now with your meter on a DC voltage range, measure the voltage of each battery terminal with respect to the grounded core (accessible on terminal strip).
If you get values around +4 V and -4 V, then measure and report V on all other terminals. I expect interesting values at the 10K resistor and at the red wire next to the two yellow wires at mid-secondary.

Most other accessible wires will probably read zero volts, but that doesn't mean they are connected to ground. If you leave the battery connections undisturbed, but move your voltmeter's common probe to the negative battery terminal, the HV circuit should have voltages between 0 and about +9 V. If wires that read 0 before are reading 0 again, with different meter reference voltage, then you know they are isolated from the HV circuit. That's easy to double-check with no battery, and meter in ohms mode.

There may be a small, harmless spark when you disconnect the battery. Have fun!
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Re: The Beginnings of a Fusor Electrical System

Postby Duncan Wilkie » Sun Jan 01, 2017 11:13 pm

I've measured 8.04 volts on the specified connections. Isn't that similar to what my multimeter does to measure resistance?
Some say the glass is half full. Others see it as half empty. I say it is twice as big as it needs to be.
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Re: The Beginnings of a Fusor Electrical System

Postby Duncan Wilkie » Sun Jan 01, 2017 11:28 pm

To help clarify, here's a picture of where the wires enter the transformer body from the connection board


20170101_172531.jpg
Some say the glass is half full. Others see it as half empty. I say it is twice as big as it needs to be.
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Re: The Beginnings of a Fusor Electrical System

Postby Rich Feldman » Tue Jan 03, 2017 10:51 pm

Duncan, I think you missed a key instruction, because of not knowing enough about electricity. There are online learning places much more appropriate than fusor.net. It's even better if you can engage a live teacher/mentor face to face. In fact it's essential, for safety among other things.

We get people here who should be sent away to learn basics. Here's one really good starter kit, with _no_ proprietary parts and no circuits to build without understanding. Power comes from a D cell (not included) with a rubber band around both ends to hold wires in contact. My wife found these kits at Marshalls before Christmas for $15.
Mr.Electricity.PNG

The next step is to introduce a multimeter, and series and parallel circuits more complicated than any in the kit. Circuit A might be child's play. What's the voltage on each lamp in circuit B? What's the voltage between points X and Y? It is sad to think of people seeking nuclear fusion before they can confidently answer those questions by inspection.
series.PNG
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You could check the answers by measurement, using your voltmeter and a few incandescent holiday lamps. The kind that come wired in series. Time to sacrifice your parents' least favorite light string. Cut and strip the green wires midway between sockets. When you grow tired of measuring voltage, measure and report the current in the wire between the first and second lamps.

If you want to get right back to your transformer, here's more detail (no substantive change) on the original assignment. Let's number the connections:
xfrmr-1a.jpg
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xfrmr-2a.jpg
xfrmr-2a.jpg (23.68 KiB) Viewed 651 times
Last time I asked you to connect points 1 and 4 to your 9 volt battery. Then tried to say: hold or clip the voltmeter's black probe onto point 8. Measure and write down the indicated voltage when the red probe is on points 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, and 12. Then move the black probe to point 1 and repeat the sweep of red probe through all 12 points. For extra credit, put black probe on point 4 and walk the red probe around yet again. You can put the 36 measurements in a 3 by 12 table with a story to tell.
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Re: The Beginnings of a Fusor Electrical System

Postby Rich Feldman » Thu Jan 05, 2017 7:07 pm

Duncan Wilkie wrote:I've measured 8.04 volts on the specified connections. Isn't that similar to what my multimeter does to measure resistance?

Yup, but there's more. I bet the full exercise will confirm one probable topology for the complete high voltage circuit, and give you the DC resistance of three windings and two resistors, regardless of how your multimeter's ohm modes behave with multiple-henry loads. You would also be helping with a lesson for inexperienced readers, about the customary meaning of "voltage" at single points.

I was serious about using miniature incandescent holiday lights for elementary experiments in voltage and current measurement on series and parallel circuits. An additional learning experience comes when you try to reconcile the V and I results with measured resistance of the lamps. Please be sure the string is unplugged, and stays unplugged, when you start cutting the wires. :-)
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Re: The Beginnings of a Fusor Electrical System

Postby Duncan Wilkie » Thu Jan 05, 2017 10:47 pm

Mr. Feldman
I do have some considerable experience with low voltage electrics (Arduino, Tesla coils, etc). I had that exact electricity kit as a kid. Good stuff.
I apologize for having led you to believe my lack of experience; I simply would like to exercise extreme caution in this process. Your original post said "If you get values..." and I wished to withhold from testing the other terminals because you do indeed know considerably more than I. I assumed the discrepancy in voltage confirms that there are two secondaries wired in series, however, I wished to confirm that my result didn't pose any risk to me.
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