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Re: [TCML] Transformer rating
Great questions. Neon sign transformers are typically rated by open-circuit (no-load) voltage and short-circuit current, hence: 15kv/30ma. Do the math and 15kvac x 30ma = 450VA, but that's not the whole story. NST's are also rated by "real" power, and it turns out that a typical 15kv/30ma NST is really only good for 250W when powering neon lamps, not 450W. The 450VA thing isn't "real" because 15kv at 0ma = 0VA (no-load) and 0v at 30ma also = 0VA (short-circuit). The data plate on my Transco outdoor type NST lists it both ways: 15kv/30ma and 250W. The cross-sectional area of the transformer core and the wire gage of the primary and secondary coils imposes the limit on output power.
NSTs are a special case. Unlike a "normal" transformer, you can safely short-circuit the output of an NST all day long, and it will provide no more than it's rated secondary current, nor will it draw excessive current from the wall outlet. This is due to a special shunted iron core which provides internal current limiting. I'm not saying NSTs don't have to obey V=IR, they most certainly do. For example, if you try your 1500 Ohm secondary load experiment, the NST will dutifully supply the full 30ma it's capable of. Applying V=IR, 30ma through 1500 ohms yields a potential across the resistor of only 45v. 30ma@45v works out to a puny 1.35W. Instead of causing the current to increase, the excessive load merely caused the secondary voltage to sink like a stone.
With NSTs, there's no free lunch. If you want more power, you'll either have to wire a bunch of NSTs in parallel or else unpot the thing and modify the core to allow more current.
Is this making any sense?
--- 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, 11:46 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
> Tesla mailing list
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