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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Here is a short monograph by the Chair of the International Neon Association's Technical Commitee on the loading of NSTs, and important differences between European and American NST specifications:
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Uspring wrote ...
In the case of a purely resistive load max power is reached when the output voltage has dropped to sqrt(1/2) of the unloaded value.
In light of this revelation (which certainly looks familiar), may I revise my estimate above for an NST rated at 10kV, to 7kV under load, rather than the ~6kV I posted earlier?
(Ash Small wrote ...
There is a 'rule of thumb' method of working out the loaded voltage, but I forget it, For an NST plated at 10kV it's around 6kV, I think.)
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
dex wrote ...
anybody ,except me,wondering now what is going on with power transfers in some real practical situation where we have nonlinear resistive loads (such like gas filled tubes and neon signs)?
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Patrick wrote ...
dex wrote ...
anybody ,except me,wondering now what is going on with power transfers in some real practical situation where we have nonlinear resistive loads (such like gas filled tubes and neon signs)?
Yes! Me, me, me!
I ran into some problems in my first thread here, which concerned impedance matching of an RF supply to a plasma (using a Pi circuit if I remember correctly).
Because I couldn't predict the load, I couldn't use Smith charts. In some situations, the plasma has negative impedance. It became so complicated, I decided to use a microwave source instead, where tuning is accomplished using an adjustable waveguide.
I now have an industrial RF generator sat around doing nothing but taking up space in my workshop.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Andy wrote ...
my previous work here: might be useful, Andy.
I'm ordering some 5 watt resister, just got some microwave diodes and 1600pf 6kv caps in the post today, going trail out the cw doubler.
I think I might have wired the mains ground to the secondary side, would that have any effect....think might be safer to measure from the cw doubler.
What do people use to stop arcing between caps
This is the NST circuit I use for generating dual rail HV supplies for my electron-impact low energy photon source experiments needing just a few mA: And this is how I built it into the NST case: You will see that I have connected the mains Earth to the case and that this is also fastened to the transformer secondary centre tap - but remember that this is not a voltage doubler circuit. The centre tap on this kind of NST is earthed to the case by the manufacturer, and you should not attempt to disconnect this, as the transformer insulation is not designed to withstand 7.5 kV at the centre, and may fail in a dangerous way.
With the variac set to maximum voltage ouput - about 250V - the DC output voltage is about 7 kV per rail when 1 mA is drawn. If the voltage at the load is critical, then I connect the load first and then gradually increase the voltage by means of the variac until the desired voltage across the load is obtained, which is the easiest way of compensating for the NST current limiting voltage sag. A bit primitive and old fashioned, but it works very well with constant loads of a few mA.
With 1600 pF in a 50 Hz doubler you will find that the voltage starts to sag badly much above 500 μA.
Don't forget the bleeder resistors, or the energy asleep in the capacitors will wake up and bite you hard, even hours later, if you are careless!
Registered Member #4266
Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874
This is the NST circuit I use for generating dual rail HV supplies for my electron-impact low energy photon source experiments needing just a few mA: And this is how I built it into the NST case: You will see that I have connected the mains Earth to the case and that this is also fastened to the transformer secondary centre tap - but remember that this is not a voltage doubler circuit. The centre tap on this kind of NST is earthed to the case by the manufacturer, and you should not attempt to disconnect this, as the transformer insulation is not designed to withstand 7.5 kV at the centre, and may fail in a dangerous way.
Is this a bridge rectifier setup, can you wire the caps to the negative side instead of ground, without effecting the circuit.
Would this circuit work with sparkgaps, diodes are easy to get.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
OK, Andy, here is the simplest possible full-wave rectifier circuit using a transformer with an earthed centre tap like most of the older NSTs, such as the one I showed in the picture above.
To multiply the voltage from a transformer with an earthed (US: 'grounded') centre tap like an NST, you must use a circuit called a full-wave cockcroft and walton voltage multiplier, like this:
I think I can see what you are trying to do with your circuit, but it won't work at all as it is! A 20V battery will not cause sparks to jump across a gap at atmospheric pressure, except under very special circumstances, such as fantastically small micron-sized gaps, where different rules can sometimes apply.
Have a look at circuits for the Cockcroft-Walton generator - which use AC with diode rectifiers - and the Marx generator - which uses DC with spark gaps - and see if you can work out the differences between them.
PS: 10 pF is a very small capacitance, such as you might get by twisting two pieces of insulated wire tightly together for a few inches. It cannot store very much energy. In your circuit, the 10 pF capacitors might as well not be there at all, so small is the capacitance, and its effect on the circuit.
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My first spark
