# How does a 120VAC neon indicator lamp respond to a leading-edge triac dimmer?

I bought a dirt cheap toaster oven to convert to a solder reflow station, and I noticed in the teardown that it apparently has a neon power indicator. It goes out instantly when it loses power instead of fading like an incandescent, and it measures open-circuit when off.

I'm not opposed to cutting it out of circuit, but I also thought it might be nice to have a rough indicator of what it's actually doing, if it actually works that way.

Here's the circuit:

simulate this circuit – Schematic created using CircuitLab

So:

• Does it "just work" and produce an incandescent-like end result?
• Can it confuse the triac? (probably not because it's in parallel with the resistive heater and my software is going to hold the opto on anyway)
• Is there something else I ought to know about using neons and triac dimmers together?

## 1 Answer

You can leave the neon in place, it will neither confuse the triac, nor alter your calculations in any way. When the oven heater is on the neon will glow.

The neon is not like an incandescent in any way, it is a gas plasma (near enough) and is essentially open circuit when below it's trigger point. Your circuit is slightly wrong in that the neon will have a series resistor to limit it's current after striking. The resistor is likely in the 100k ohm plus range.

• Even though it's a heater with relatively slow response, I was planning on actually dimming it instead of just on/off. (The software will operate as "phase-locked PWM" at 120Hz.) Given a ~100k series resistor in the neon, I agree in that I can't see how it would affect anything except itself, but just out of curiosity, what would the neon do with that? Would it have more of a threshold behavior? Just off if it doesn't receive the strike voltage, and just on if it does, with some flickering around the threshold? – AaronD Oct 13 '17 at 15:04
• At lower power to the heater the neon may flash (intermittent strikes) or not light, at high power it will operate normally. Most neon's strike in the 70-100 V region. You will not damage the neon. – Jack Creasey Oct 13 '17 at 15:32
• Okay, so it sounds like my prediction is correct then: on like usual above ~40% power (just a guess, but definitely <50%), off below ~30% (another guess), and flickering in between. Thanks! – AaronD Oct 13 '17 at 15:54