Re: [TCML] Transformer rating

[Quarkster <quarkster@xxxxxxx>]

Nicholas -

The power handling capacity of a conventional transformer can be rated in several ways. Small "instrument" transformers may be rated with a secondary output voltage and current. This indicates the maximum continuous output current the transformer can supply at the rated output voltage, given a maximum permissible temperature rise in the windings.

Larger power distribution transformers may be rated in KVA (kilo volt amps), the product of secondary output voltage and current, usually specified with a maximum temperature rise and a maximum ambient temperature.

Neon sign transformers have a very special core design incorporating "magnetic shunts" that limit the coupling  between primary and secondary windings. This effectively limits the output current regardless of the load impedance.  A 15/30 NST can supply  15kV into a very high impedance load,  or 30ma into a low-impedance load, but never 15kV and 30ma  simultaneously (at least not without a parallel resonant capacitor across the secondary).

NSTs are designed with this current limiting feature to match the impedance characteristics of a gas-filled luminous sign tube. A typical "neon sign" tube will have very high impedance initially, when the full 15kV is required to ionize the gas and vaporize the tiny amount of mercury that is usually present. As soon as the gas is ionized and begins to conduct, the impedance drops, and the magnetic shunts limit the maximum current to around 30ma. Once the tube is conducting, the actual NST output voltage may only be 800-2000 volts, depending on the length and diameter of the tube.

Regards,
Herr Zapp

--- On Tue, 5/19/09, Nicholas Goble <ngoble@xxxxxxx> wrote:
From: Nicholas Goble <ngoble@xxxxxxx>
Subject: [TCML] Transformer rating
To: tesla@xxxxxxxxxx
Date: Tuesday, May 19, 2009, 9:46 AM

I don't understand how what it means when a transformer is rated with
voltage AND amps.  For example, a 15kV/36mA NST or something smaller like a
14V/1Amp wall adapter.  Is the transformer a voltage or current source?

I'm just thinking back to basic physics V=IR.  If the transformer supplies
15kV and I have a load resistance of 1500ohms, why can't I draw 10Amps?

Nicholas Goble


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