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Quarter shrinker charging resistors

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Signification

Tue Jun 19 2018, 06:46PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

I think this will be really rough on the resistor: If I did the calculation correctly, then the resistor can dissipate 13.7kW max peak!! Assuming, (for worst case conditions), of 60mA, 102uF, and 7kJ. Although things won't be this bad, and only exist briefly, worst case, this large dissipation in the resistor bothers me. It depends on how long you intend to take to charge the cap to 7kJ--do you have a charge time in mind? As you probably know, 7000J stored in this capacitor is right at 12kV. However, this sounds light a fairly reasonable energy for first go at the coin...maybe a tad high. Most importantly, don't forget the high velocity sharp hot shrapnel!! Gort-suit maybe?

@ klughsmith: Yes, in my experience, a powered NST output will last and last shorted--BUT NOT OPEN-CIRCUITED.

klugesmith

Tue Jun 19 2018, 10:12PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Bowing in respect to Signification, for having gotten the quarter-shrinking thing down & living to tell us about it.

Sounds like 7 kJ is too much energy to make attractive shrunken quarters, unless the design is inefficient. (e.g. work coil turns count is poorly matched to the capacitance value, or the interconnection circuit has unnecessarily high inductance). With attention to those details, Mr.Mil might get good results with only 11 kV, which is within reach of a safely grounded NST with two-diode rectifier.

The presence of two HV terminals on the 102 uF capacitor opens up design options. In particular: use of full-wave bridge rectifier, for generous charging voltage margin. Now the NST case and capacitor case can both be grounded. That's safer for the devices as well as for humans. If left floating, the cases could be charged to hazardous voltage levels by unsymmetric leakage currents or corona currents. And that also puts un-necessary voltage stresses on insulation inside the NST and capacitor.

Using your 10K res (x2, with symmetric charger?) as a discharge resistor(s) might not be horribly abusive. Those big ceramic tube resistors are rated for 10x overload for 5 seconds. For a nominal 225 W unit, that's 2.25 kW x 5 seconds = more than 11 kJ. Sure, if the cap were at 11 kV then the initial current is 1.1 A, for 12.1 KW. But it decays with RC = 1 second. Total temperature rise of the resistor (inertially cooled) would be only 55% of the datasheet tolerable overload case.

p.s. my table includes 6 kV on the 52 uF cap, because IIRC that was enough to cut cans in half. Right before work was suspended in 2006. Might be a good warmup project for MrMilStar.
IMHO, magneforming projects like these would be appropriate topics for Electromagnetic Projectile Accelerators board.

MRMILSTAR

Wed Jun 20 2018, 03:32AM

Registered Member #62119 Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136

I don't have my calculator in front of me but I anticipate the maximum charge time to be no more than 30 seconds with a 10K ohm charging resistor. So, yes the power rating of the resistor looks way overloaded even with a 225 W resistor. But, as was stated, you have to look at the duty cycle which is less than 30 seconds. If you went strictly by the power rating of the charging resistor needed, you would end up with a resistance you just can't reasonably build at the power level needed. The same arguments apply to the discharge resistors. I plan on using ten 5K ohm resistors rated at 225 watts each in series to get a 50K ohm discharge resistance.

Even though the power level of these resistors looks excessive, I have studied what other have built with their quarter shrinkers. Over-loaded resistors seem to be the norm because they rely on low duty cycle operation for cooling.

I don't want to connect any part of this device to mains ground. I don't want to take the chance of an accidental high voltage discharge getting back into my house wiring. The NST should be fine with the case ungrounded because the core is still internally tied to the center tap of the secondary. All circuits will be disconnected from main power before firing to prevent any possibility of a discharge getting into the house wiring. No ground appears to be necessary because no one will be touching anything on the device during operation. All circuit activation will be done either by air or tension strings with springs. There will be no wires of any kind between the device and the operator. After discharging the capacitor terminals will shorted.

klugesmith

Wed Jun 20 2018, 05:35AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

When charging the capacitor through a series resistor, the power & energy lost in the resistor is commonly MUCH less than the power and cumulative energy going into the capacitor.

For example, if you switch the NST on full blast, charging current starts at 60 mA and gets smaller after that. 600V drop in 10k res. 36 watts, max.

Or even with a non current-limiting transformer, if you limit current by dialing the voltage up gradually with a variac, as Signification suggests -- energy loss in the R is a tiny fraction of CV^2/2.

The resistor power gets large (and wasteful) only if you are depending on that resistance to limit the current. For example, switching on a 10 kV, 1 ampere DC power supply. Where are you going to plug that in?

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