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oucivileng

Fri Mar 06 2009, 05:40PM

Registered Member #1772 Joined: Tue Oct 21 2008, 05:23AM
Location: Athens, OH
Posts: 71

Awesome. How the heck did you find that page? I've been trying to find pics of one taken apart for a while and have had no luck at all.

hvguy

Sat Mar 07 2009, 04:13AM

Registered Member #289 Joined: Mon Mar 06 2006, 10:45AM
Location: Conroe, TX
Posts: 154

Anything rated 250VA but stating an output of 15KV/30mA is not right. Your right, VA ("apparent" power) is simply V x I, which should be 450 in your case. Watts, on the other hand, is the power factor corrected version of VA or "real" power. This could be anything and is not relevant to TCs. You can never have less watts than VAs.

Don't buy a 15/30 rated 250VA, somethings up.

Should you want more info....

oucivileng

Sat Mar 07 2009, 06:01AM

Registered Member #1772 Joined: Tue Oct 21 2008, 05:23AM
Location: Athens, OH
Posts: 71

Yeah, I have a basic understanding of power factors. I too thought something was up when I saw it rated at 250VA but then I went to the Allanson website and sure enough they exist. go to

enter 120 primary voltage, 15kv, and 30ma, then click search products. The second result is a 15kv/30ma transformer at 250VA. I think Dr. K is right about the pfc cap.

hvguy

Sat Mar 07 2009, 07:26AM

Registered Member #289 Joined: Mon Mar 06 2006, 10:45AM
Location: Conroe, TX
Posts: 154

Even with a PFC cap a device with an output of 15KV at 30mA can not be less than 450VA. This is because Power factor can not be less than zero and 15KV x 30mA = 450VA. Assuming a perfect power factor 450VA x 1 = 450W. SO this means that the transformer is not actually providing 15KV at the same time it is providing 30mA. Wired, huh?

This is the new trend in power supplies. Companies are now advertising power supplies that say something like this; "30V 30A output" but the power supply then stipulates a maximum power of 300W in the fine print. This means the power supply can provide any combination of voltage and current within the 30V/30A range that equals 300W (i.e. 15V 20A). BUT it CAN NOT provide 30V at the same time it is sourcing 30A because that would be 900W.

This is what these neon's are doing. Since it does not actually take 15KV to keep neon lit once its ionized the electronics inside the little black box on that transformer are cutting the voltage back to about 8.3KV once the tube has "started". This enables the company to advertise a 250VA rating and a better power factor while still saying the transformer is a 15/30. It would be more accurately stated at a 15KV MAXIMUM or 30mA MAXIMUM AT 250VA.

Does this make a bit more sense now?

Proud Mary

Sat Mar 07 2009, 10:32AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

hvguy wrote ...
Since it does not actually take 15KV to keep neon lit once its ionized the electronics inside the little black box on that transformer are cutting the voltage back to about 8.3KV once the tube has "started". This enables the company to advertise a 250VA rating and a better power factor while still saying the transformer is a 15/30. It would be more accurately stated at a 15KV MAXIMUM or 30mA MAXIMUM AT 250VA.

I saw an interesting graph of the I v V output of an NST, and the magnetic shunts had already pulled the voltage way way down by the time it reached the maximum current limit advertised on the box. There's some good writing on the subject over at Fusor Forum.

Steve Conner

Sat Mar 07 2009, 12:08PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I don't see anything wrong with the 250VA thing.

When a manufacturer says his NST is a 15/30, that means that it gives out 15kV open-circuit, and 30mA short-circuit. You can't have both conditions at the same time, unlike, say, a 30V, 2A benchtop power supply.

Running a neon tube in normal use, it'll give something less than 15kV, and something less than 30mA, and I don't see any reason why the product of the two shouldn't be, say, 250.

An EE egghead type would infer from this straight away that the maximum power available from the NST into a non-reactive load is one quarter of the product of kV and mA ratings: 112 watts for a 15/30. This certainly doesn't apply in Tesla coil duty, though, where the load is capacitive, so resonance effects allow you to get considerably more.

Herr Zapp

Sat Mar 07 2009, 04:06PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

oucivileng, Harry -

The "Unitran" feature has nothing to do with an internal GFI. It's just an NST integrated into a complete sheet-metal enclosure, so that an additional sheet-metal box for the NST and the low voltage AC wiring is not required.

The link Harry provided to the "modified" Unitran covered disconnection of the secondary center-tap only. The Allanson Unitran NST shown in the link was not a GFI unit.

None of the GFI-equipped NSTs I have opened up had a modular or easily-removable GFI module; all were potted into the unit.

Regards,
Herr Zapp

Proud Mary

Sat Mar 07 2009, 04:10PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Herr Zapp wrote ...

None of the GFI-equipped NSTs I have opened up had a modular or easily-removable GFI module; all were potted into the unit.

That's just what I was afraid of, and had warned about, if you'd care to read my earlier posts in this thread.

oucivileng

Sat Mar 07 2009, 04:49PM

Registered Member #1772 Joined: Tue Oct 21 2008, 05:23AM
Location: Athens, OH
Posts: 71

Ok, I read a little more from their GFI page and I agree with Herr Zapp. It just says that all Unitran transformers meet the UL requirements for secondary ground fault protection. However, it looks like the GFI module is in it's own section of the case and not potted.

If you look at the pictures of the modified unitran transformer, you can clearly see an open compartment where I assume the module had been before he removed it.

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