120V DC panel indicator question

Use a neon bulb, like the ol' reliable NE2, with about 120K in series, for the 120VAC. I believe they'll work on DC, but only one electrode will light. For 28VDC, use a plain ordinary LED with the appropriate series resistor, R = (Vsupply - Vled) / .01A.

Cheers! Rich

=46or lowest power LED wins:

Use LEDs for both. Remember use a 1W resistor for the 120V .... Calculate say 20mA at 120V =3D 6K OHM

Cheers

"High intensity" neon lamps such as the NE-2H (C2A) and the "mini-NE-2H" (A1C) only use 2-2.7 mA average current. I hope that you are as aware as I am of LEDs that get similarly bright at less current than this.

Most InGaN green LEDs (nominal wavelength 520 to 535 nm) in my experience do plenty good and well with average current of .5 mA or less.

Cheaper ones such as TT Electronics / Opto Technology OVLLG8C7 will do well in AC circuits with a halfwave rectifier and series resistor, along with a second diode antiparallel to the LED to protect the LED from reverse polarity resulting from leakage through the halfwave rectifier. I would say that one glows plenty bright in this application with .36-.4 mA average current, which means that a 150K resistor should be good here at 120 VAC. Although, I would like to caution that an LED with halfwave-rectified 60 Hz AC sometimes has its flicker being visible.

For a "more premium" super-low-current 120VAC indicator with cost likely getting to reducing ROI unless most-alarmist forecasts of future energy costs pan out, use a Nichia NSPG520AS or similarly super-efficient efficient-at-low-current low power green LED (maybe likely by Nichia). For less flicker, connect this LED to the DC leads of a suitable bridge rectifier (voltage rating can be low). Put the dropping resistor in series with one of the AC leads of this bridge rectifier - of value to pass maybe .1 mA, .15-.2 mA if you need brightness "on a high side". This means at 120 VAC, resistor value is 1 meg, 680 or 470 K if you need brightness on a higher side.

As for 28 volts - my experience suggests to me that this is often DC. If your 28 volt application is AC, use the above with resistor values roughly 22% of above. (33K for TT Electronics / Opto Technology OVLLG8C7 antiparalleled by a diode when used with a halfwave rectifier, 220K or maybe 150 or 100K with Nichia NSPG520AS with fullwave bridge rectifier).

If the 28 volt application is DC, I would advise .5-.55 mA with a cheaper InGaN green LED (47K resistor) or .11 mA, .25 mA high side with the best that Nichia offers in this area (resistor value 100K to 220K).

- Don Klipstein ( snipped-for-privacy@misty.com)

Ed