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Re: [TCML] Primary short
Hi Dan,
I know it's painful at first. I have acquired/created 5 or 6 such "boat anchors" in the last 50 years. Cores and insulators can be salvaged for other projects and a sanded and repainted box can make an interesting doorstop. Also good for practicing depotting techniques (see archives). It's good to consider an NST or a Maxwell Cap as a uesd car with a broken odometer, "You pays your money, you takes your chances".
Matt D.
-----Original Message-----
From: Dan Casaregola <ag2z@xxxxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Thu, Jan 28, 2010 10:00 pm
Subject: Re: [TCML] Primary short
Hi Matt
Just thought I'd let you know the outcome of the "shorted primary", since you were kind enough to help me diagnose the problem. The primary was OK at .7 ohms, and the sec was OK at 8k ohms both sides. No voltage at the secondary withe primary voltage applied at 120 volts! Opened up unit, and began chipping out tar with a hammer and screwdriver, and finally got enough out to remove the transformer. 6+ pounds of tar chips. During the course of chipping, even tho I tried to be as careful as I could, I did strike the secondary windings, thereby opening the secondary. (DRAT)! I lifted the unit out, and upon careful examination of the secondary windings I found a charred section on the side of one of them which showed flakes of insulation. I carefully removed them, and wound up with a crater close to the core. Definitely a carbonized area. This was not a crack, Definitely a crater. This was definitely a total loss! Awful feeling after months of work. All I have left is the two insulators. The primary coils looked like they had been subjected to excessive heating (not caused by me) and showed some flaking of the insulation. #16 copper wire, only good for antennas now.
Before you ask, I used a Terry filter as prescribed, using the appropriate components. Also the safety gaps spaced 1/4 inch each side to ground. Again spacing determined by setting to fire with transformer alone at 120 volts. In conclusion, the only other piece of info you must know is this unit was old and used. It showed signs of paint flaking on the surface of case, and severe corrosion of the steel at bottom. However it did produce sparks initially.
Guess I start hunting for another. Thanks again.. Dan
----- Original Message ----- From: <mddeming@xxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Tuesday, January 19, 2010 11:44 AM
Subject: Re: [TCML] Primary short
Hi Dan,
It is highly unlikely that the problem is on the primary side of your transformer. It is very possible to have a high voltage short in the secondary of a transformer and still see normal resistance when it is looked at with the 1.5 to 3V supplied by a voltmeter. In a case like this, there may be a bare spot in the insulation on the windings in one or two different layers of the secondary, but the wires are not touching. However, when the secondary voltage gets high enough, an arc occurs between the two layers, or windings to core, or windings to case, which shorts the output (carbon tracking). A voltmeter will only tell you if a) one or more of the secondary windings have melted and left an open gap (infinite resistance), or b) if two layers have "spot-welded" together(lower resistance on one side).
The only way to verify this, (short of running the NST power up until you see the smoke) is to measure the output voltage from each output terminal to ground and you will see them start to diverge quickly when the arcing starts. For this you will need either a voltage divider or HV probe with built-in divider, or an old analog meter such as my Triplett 666-HH which has a built-in 0-5000 VAC capability
thus allowing me to check each side of a transformer with up to 60% power applied. (15 kV X 60% = 4500 volts each side) This is almost always enough to bring out any problems.
Hope this helps,
Matt D.
-----Original Message-----
From: ag2z@xxxxxxxxxxx
To: tesla@xxxxxxxxxx
Sent: Mon, Jan 18, 2010 10:42 pm
Subject: [TCML] Primary short
Has anyone out there ever had a 15kv/30ma NST suddenly develop a shorted
rimary during a short test? Less than one minute. The resistance of the
rimary became .7 ohms. Secondary is intact on both sides. This is an old
ardiner NST with spot welded bottom plate. Primary connections are along the
ower part of case making access very difficult. Thanks. Dan
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