120v pilot light w/240 Mains

[RockinRocket]

Need to determine what resistor to use to allow a 120v pilot light to work with 240 mains. Sure I could buy another light but that would be to easy. To make the situation seem even more silly to you the amp will probably all ways only use 120v mains but has a selector for 120/220/240 so want the amps light to work just in case.

Can anyone help with the answer and teach me the math on how to determine this ?

[PaulD]

You need to know how much current the light uses then use ohms law to calculate the resistance (you should also calculate the power dissipation to ensure you use a suitable wattage resistor)

[sluckey]

Is the pilot lamp a 120v neon type? If so, just put a 100K in series.

[wpaulvogel]

If you connect the 120 volt bulb to the 120 volt tap on the transformer it will always work with whatever voltage is selected. The transformer will output the voltage to the tap.

[RockinRocket]

Well I don't know how much current the light uses so I guess a simple 100k resistor should be fine?

Didn't think the lower volt mains taps would step down the volts. Learned something new. Sadly I only have wiresand no lugs, would probably be easier to just use a resistor.

[SoulFetish]

I think that connecting it to the 120V tap is the best solution.
You should really do that for a couple of reasons, first for most types of neon bulbs, the design current rating is around 0.6-1.6mA.
In enclosed lamp assemblies I usually see around 150k current limiting resistor values, limiting the current to 0.8mA. Limiting the current can extend the life of the bulb, which is good. Plus the relationship of current to brightness is such that they are not linear, and you can limit the current pretty significantly without noticing a decrease in apparent brightness. This will give you longer life in the bulb. Problem is that too low a current and the neon becomes unstable, and can cause flickering and other weirdness. So, you might want to tack in a 100k resistor and make sure the neon is operating correctly.

Plus, it's not simply doubling the resistance if you want to operate it at, say, 240V.
Lets take a run of the mill NE2 neon lamp; Manufacturers specify a nominal resistor value of 100k for use at 120V, and a nominal value of 540k for use at 240V for the an expected life cycle of 25,000 hours.

[R.G.]

You're getting good advice. You really can't get to a solution without knowing what the indicator type is and what current it needs.

You might do some tracing of the power wires. If it's a 120-240 option amp, it is likely that it was designed for this problem, and the indicator may already be attached to the 120V tap. That would be a reasonable thing to do in a good design.

Neon brightness is set by the current through the bulb, and that's set by the resistance. If you put the resistor for a 240V connection on the resistor and only feed it 120, it may get pretty dim.

Note that the same advice applies to the 240V transformer output. You can put the resistor for 240V on the neon and always run it from the 240V tap. The transformer goes both ways.