To amplify a bit on Richard's comments on neon sign and other current limited leakage transformers (oil burner, etc.): one needs to understand the normal applications for these transformers. A neon transformer is designed to light a glow discharge tube (i.e. the sign tube). When the tube is unlit, it is non-conductive. It takes a fairly high voltage at very low current to establish the plasma (called a self sustaining glow discharge). Once the plasma is established, the voltage drop across the tube drops dramatically and the current increases (the resistance drops). The transformer is designed to operate according to this characteristic. When the plasma is self sustaining, the voltage across the tube can drop to just a few hundred volts.
Were the transformer not current limited, more and more current would pass through the tube (with a corresponding further decrease in voltage) until an arc is formed (high current/low voltage mode where metal is being evaporated from the electrodes). This would be a runaway condition that would destroy the tube and/or transformer.
Same with an oil burner transformer except that what is formed in the gap is an arc. The initial high voltage causes a spark and the current increases to a safe value as the arc forms. The sparking voltage may be a few kV, the arc voltage drops to a few volts.
The bottom line is for a 20 mA, 15 kV neon transformer, you will never get both. You'll get 15 kV open circuit but only a few volts at full current.
Steve
Created on Thursday, March 15, 2001 6:24 PM EDT by Steve Hansen