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Registered Member #477
Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
Neither of you capitalized the first letters of your sentences, either Hehehhh...
But getting to the question: Can you post a couple of pictures? Wouldn't want to recommend 'arc self-start' tests unless it's obviously a NST. My personal favorite method for measuring NST (and other HV transformer) output voltages is to use a known-good 15kV NST in reverse (15kV because that's about tops for NSTs, so you don't generally have to worry about overvolting your good unit). High-voltage probes for DMMs are nice, too. Some people might recommend just feeding line voltage into the HV winding and measuring the output on the low voltage side to establish the ratio, however I've found that to be extremely inaccurate in most cases (because of the high impedance of the HV windings of a NST, line voltage across these windings often will not result in enough current to magnetize the core, leading to a lower-than-expected voltage on the LV side and an exaggerated ratio--"Woa! A 50kV NST!! No WAYYY!!!").
For a guesstimate of the voltage, provided you're sure it's a NST, you can draw some arcs. Arc self-start distance will vary according to atmospheric conditions, electrode geometry, etc., however you can come pretty close. Using paper clips or other small-gauge wire for electrodes, 7.5kV should easily self-start at 1/4", 9kV 3/8", 12kV 1/2", and 15kV 5/8".
Registered Member #661
Joined: Sat Apr 21 2007, 03:17AM
Location: Ventura, CA
Posts: 41
What I do to check transformers is just feed like 1 Volt AC into them and see what output I get. 9-12 Volt AC wall worts are pretty common, you can use one of those too. Works great for me but be careful, feeding 12 Volts AC into a 12 kv NST will give you 1200 Volts on the secondary side which would give you a nasty shock if your not paying attention. I prefeer the 1 Volt AC way.
Registered Member #477
Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
If you have a variac, try it! But many/most DMMs won't go over 600V, and many don't even go that high. So you'll be forced to stick with only a couple of volts (tops!) in. Then you may be subject to the same not-enough-magnetizing-current problems as when you feed 120V into the high side. The scary part about the 1V trick is that, if you don't get enough magnetizing current, the output of the HV winding will be low, leading you to underestimate the ratio. Going the other direction, you'll overestimate. I suppose I like the idea of overestimating more than underestimating when it comes to something like this. Anyway, I may be all wet; I've never tried feeding just 1V into a NST. It seems sure to produce a "high" voltage, but the accuracy seems likely to be pretty bad.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
I could see typing nts instead of nst once, but after noticing it was both in the title and the post I thought you had discovered a new type of hv transformer...
In any case, as long as you are sure that it isn't solid state, have a GFI breaker on it, dimmer, etc, the easiest way is probably to feed it a few volts in and measure the output voltage with a cheep voltmeter. Better yet would be to make a 1000:1 resistive divider (like 10meg:10K or something) and give it full voltage.
Registered Member #530
Joined: Sat Feb 17 2007, 07:56AM
Location: Victoria BC, Canada
Posts: 178
The absolute easiest way to measure the voltage output of an NST is to simply put 120VAC into the secondary of the NST. Then with a multimeter measure the primary voltage. This is a very very safe way to find the working Voltage rating of a transformer!! Some examples below:
9600 VAC NST = 120V into secondary - 1.5 V off primary (80:1 ratio) 12,000 VAC NST = 120V into sec - 1.2 V off primary (100:1 ratio) 15,000 VAC NST = 120V into sec - 0.96 V off primary (125:1 ratio)
---you will have to float NSTs to get an accurate measurement... this works on MOTs, PTs, Pole Distribution Transformers. The trick is get the accurate reading of two sets of windings. You may want to experiment and use 240V then divide the voltage out by 2 to achieve a decent voltage input/output ratio.
Registered Member #661
Joined: Sat Apr 21 2007, 03:17AM
Location: Ventura, CA
Posts: 41
Let me know if it works for you Aaron. I've always used this way of testing NSTs, funny I never thought of just running them in rerverse. Anways I have a 26 volt AC wall wart and a small variac. Running 1 volt on the low side gives me exactially 100 Volts on the high side and metering both sides gives me a ratio of exactially 1:100 all the way up to 26 volts. So I haven't had a probelm with under magnetizing the core, but I could be feeding it a lot of amps, haven't really check that yet. My cheap DMM says it reads up to 1000 volts but it reads up to 2600 volts just fine. eheheheh, only do that to my cheap one.
Registered Member #546
Joined: Fri Feb 23 2007, 11:43PM
Location: Albuquerque, NM
Posts: 239
I use a 1000:1 resistive divider like ... suggested. it's actually like 1001:1 since it's a 10M:10K resistor (R1+R2/R2) but whatever. I read like 13V on my 12/30 STN (Sign transformer of neon- beat that)
Make sure your resistors are rated for 20KV or higher.
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nts voltage
Hehehhh...
