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Over-current capacity of NST secondary winding

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MRMILSTAR

Mon Aug 06 2018, 10:08PM

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

I am designing a quarter shrinker with a NST as a power supply. The NST has a maximum output current of 60 ma. I am currently picking a protection resistor to connect between the NST output and the full wave bridge (FWB). My thinking is that if the FWB fails as a short circuit during charging, I want to limit the back current from the capacitor to the NST secondary to prevent damaging the NST.

Does anyone know the over-current capability of a NST secondary rated at 60 ma maximum output? I have found a 100K ohm wire-wound resistor rated for 225 watts. This would limit the current to about 150 ma for the NST secondary for a 15KV capacitor charge in the event of a FWB failure. Does this seem adequate? My intuition tells me that a NST secondary rated for 60 ma should be able to handle 150 ma for short periods (less than 1 minute).

Patrick

Mon Aug 06 2018, 11:38PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

The problem with answering that question is that the heat capacity of fine wire is overcome quickly, and your left with toast.

I used primary side rectification on an NST when in high school, not realizing i wrecked the impedance and cuased all sorts of secondary garbage in voltage and current. i used a variac and would slowly turn up the voltage from 0-120 vac, and watch the current needle go erratic at 7 amps when it should have been like 1-2 amps steady. By some great miracle i still have that NST.

So it is possible to expect overload survival for a short time, but NST's get sensitive when you start changing things. Some people re-wind them, some press out a few shunt plates and so on. Theyre engineered to operate as intended, so deviate from that at its risk.

Not sure what you mean by a 100k resistor at 225 W !? is it rated for HV ? if so thats impressive. If that doesnt answer your question ill need a schematic.

Hazmatt_(The Underdog)

Tue Aug 07 2018, 02:38AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

There are many here that will tell you that the NST will run in CC mode shorted into its nameplate rated current limit... in your case 60mA is all it will deliver.

However, I am of the other camp, if you overload a NST past 150% of its nameplate load, 90mA, it will die.
If you shunt the secondary winding, the IIR losses will cause the wire to burn-out. It's only 36 AWG and it will not sustain a fully energized transformer in short circuit.

It will current limit to 60mA in a Jachob's ladder, but that is not the leads connected together 'shorted'

But I can do an experiment sometime and report what I find, experiments are great to verify.

NST's are designed to operate continuous duty at the RMS voltage of the nameplate (because Neon tubes act as diodes) at their rated current, mainly because they are not insulated to run open circuit, or at their open circuit voltage (on the nameplate). You can talk to Franceformer as well if you like.

MRMILSTAR

Tue Aug 07 2018, 03:27AM

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

In answer to the resistor question, the 100K ohm resistor is a wire-wound resistor not rated for high voltage. That being said, people have successfully used wire-wound resistors in quarter shrinkers as high voltage current limiting resistors in the charging circuit between the NST secondary and the FWB. They do have the benefit of being quite long (12 inches) which helps prevent flashover. They are also successfully used as charge dissipation loads, with several in series, for the capacitor to discharge in case of an unsuccessful firing and to bleed off excess charge after a successful firing. . The 225 watt rating means that the resistor is rated to dissipate 225 watts continuously. However, since the discharge time is one minute or less, this rating can be exceeded for this short amount of time without destroying the resistor.

Let me phrase my question another way then. For those that have built a quarter shrinker, what method of protecting the NST secondary have you used in the event of FWB failure to short-circuit? Or do you just assume that the FWB will not fail as a short circuit?

MRMILSTAR

Wed Aug 08 2018, 12:24AM

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

Instead of a current-limiting resistor, here is another idea for protecting the NST secondary. On E-Bay I found microwave oven fuses rated for 5 KV and 0.65 amps. These are fast-blow fuses. I propose connecting 4 of these in series, possibly under oil or wax, to yield a fuse rated for 20 KV at 0.65 amps. This fuse would be connected between the NST secondary and the FWB. I am hoping the fast-blow feature of these fuses will protect the NST secondary in the event of a FWB failure to short circuit.

After thinking about it, I don't think this will work. Lets assume there are four 5 KV fast-blow fuses connected in series in an attempt to achieve a composite fuse with a rating of 20 KV. If an over-current condition occurs then one or more fuses may blow. It is certainly not guaranteed that all 4 fuses will blow simultaneously. Lets assume only one fuse blows at first. Since a single fuse is only rated at 5 KV, a plasma arc will form between the ends of the internal blown fuse link because 20 KV is being applied across it. As the other 3 fuses blow afterwards, the arc will be spread successively between the blown fuse link ends in each individual fuse.

Thoughts?

klugesmith

Thu Aug 09 2018, 09:51PM

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

Valid concern there, about HV fuses in series. Not a whole lot different from oft-discussed issues when HV capacitors or diodes are connected in series. Have you searched the Interet, particularly OEM literature from fuse-makers, about your question? There are many fuses with 5-digit voltage ratings high on telephone poles.

Regarding limited-duration overloads of power resistors, NST's, etc, I agree that there's value in considering the total heating energy (in joules) and the thermal mass of the device. The initial temperature of your NST secondary winding will be lower than it would be in normal luminous-tube service.

How about using a full wave bridge rectifer with very conservative ratings, and accepting the risk that a very improbable combination of diode failures might destroy a NST? Like choosing screws to attach a big TV screen to a wall.

IMHO, one magneforming risk stands out. That's if your capacitor ever discharges into an inductive work coil while the charger is still connected. If the capacitor voltage reverses, then the circuit current suddenly has a path through forward-biased diodes in your rectifier. Could be thousands of amps, unless there's a charging resistor to limit it. Fortunately, the initial stored energy should be mostly dissipated by normal losses in the HV pulse circuit, before the voltage reverses.

[edit] Regarding the charging resistors: As we discussed in another thread, their heating during a normal charge cycle is vastly less than the potential heating from "backwash" if rectifier fails while capacitor is highly charged. Might be a good place to use a DIY water resistor. Doesn't take that much water to suck up even 10 kJ in a single event, without any damage.

MRMILSTAR

Fri Aug 10 2018, 12:48AM

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

I found a 15.5 KV, 0.5 amp used fuse on E-Bay for a reasonable price ($60). I may use that. I have thought about just taking the small risk of blowing the NST. I have one of the rarer 15 KV, 60 ma units though and I would rather minimize the risk to it. I am going to make the FWB from 12 KV, 350 ma MOT diodes. 3 in series on each leg. That should give me 36 KV for the FWB. Since the peak output of the NST is 21 KV, that should be plenty of overhead.

Sulaiman

Sat Aug 11 2018, 10:32AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Have you considered neon tubing

or a 6ft fluorescent lamp ?
etc.

They should easily ignite and allow charging
but would break the circuit if the FWB fails ... KABOOM !

,,, and the NST is safe.

Blast shielding and PPE recommended

------------------------------------------------ ---------------
More seriously, unless you have the correct type of fuse,
the result can be dumping all of the cap bank energy into the fuse&FWB ... BOOM !

The resistor is not 'cheap' but it should be safer than most diy options.
Note that 15.5 kV x 0.5 A = 7750 Watts ... it will heat up really quickly !
------------------------------------------------ ---------------
On the assumption that you will want to safely discharge your cap bank quite often,
the resistor used to limit damage to the FWB can be used for safe discharge by removing input power then (remotely or very carefully) shorting the + and - terminals of the FWB.
-----------------------------------------
Add some kind of cap bank voltage indicator(s) to the cap bank to reduce risk.
(a string of cheap resistors and a neon lamp as a minimum)
If you are extremely lucky you get to make up to TWO mistakes.

MRMILSTAR

Tue Aug 14 2018, 03:48AM

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

Doh! I should have written "15.5 KV, 0.5 amp fuse"; not resistor. I have received the fuse. It is hefty at about 12" in length and about 1.5" in diameter. The discharge resistors are separate. They will probably consist of ten 5K ohm resistors in series for a total of 50K ohms. Each resistor is rated at 225 ohms. I plan on using a a charging voltmeter and a discharging voltmeter so that I know the charge on the capacitor at any time.

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