240v pilot light wiring

[gtomax]

Pretty rookie question here but I want to take a vintage 240v pilot neon and wire it into a multi primary PT. Because of the primary winding taps I'm assuming it'll function normally if I connect it to the 240v taps even though the mains are running off the 120v taps. Should function normally no matter if the voltage selector is set to 120 or 220 or 240 etc. as long as the neon is wired directly to the 240 primary taps?

[Roe]

yes

[martin manning]

I think maybe. Do you have a wiring diagram for the transformer?

[R.G.]

Roe is correct. >IF< the transformer is a multitap primary with both 120 and 240 inputs, >AND< you wire the 240V neon across the 240Vac taps, it will act as an autotransformer and make it get 240Vac no matter if the primary is set for 120 or 240. Or other taps, if they are there for 100, 110, 220, 250, etc. Transformers make every turn have the same volts across them, so if the number of volts per turn is correct on any one section, it's correct on all of them.

But neon don't really have a voltage, other than a breakover voltage. Depending on the exact gas mix and whether there is thorium in the electrodes or not to lower the break over voltage, they usually snap over at 70V to 100V - period. Once a gas tube breaks over, it suddenly conducts a lot more easily and sucks a lot more current if it's not limited by a resistor or some such outside the bulb.

The current in the tube and the resulting brightness are set by an external resistor. So 120V neon bulbs differ from 240V neon bulbs only in the resistance in series with them. But a neon for 240 will be dim on 120. A 120V neon will be very bright and might burn up the electrodes and die on 240 from the high current. Maybe. I haven't tried it.

Note that you could also replace the 240V neon with a 120V neon and wire that across the 120 taps on the primary. Same theory.

[martin manning]

I don’t know that the primary of this transformer is a single winding with multiple taps. If that is the case, fine, and I understand that is the reason for the >IF<. The other possibility is that the transformer has the very common arrangement where two separate windings are connected either in series or in parallel to get the different voltages. Then there is no connection that will always be at 220V.

[R.G.]

You're right - if there are two primaries that get disconnected and reconnected series/parallel, then a 240 neon will be a problem.

In that case, I'd connect the neon across one of the 120V primaries and either adjust the series resistor if it's available, or replace the bulb with a same-size-but-120v rated one that fits. A single 120V section will always be 120V.