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I am looking at driving an LED panel indicator from either 230VAC or 120VAC to inform an operator that a hazardous voltage is present. I want this indicator to operate with minimal additional electronics (i.e. not converting to a lower DC voltage through a SMPS) so that it is a direct representation of the mains voltage. Most indicators are designed to operate at a single voltage level and if you undervolt them (i.e. buy a 230VAC indicator and put 110VAC into it) they are sometimes too dim to be usable. They would also not look great on the panel as I would end up with various brightness levels depending on what mains voltage is applied (I will have some 24VDC indicators also on the panel too).

Would Zener diode clamping be a suitable? Using an online calculator http://www.petervis.com/electronics%20guides/calculators/zener/zener.html for a quick estimate (assuming 10mA panel indicator, 9.4mA Iz, 100VL and 230VS) my diode would have to dissipate 1W and the resistor 1.35W which would mean they could get quite hot. Is there is a more efficient way of doing this?

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Neon bulb (and series R) is simplest, it takes a "dangerous" voltage to strike. \$\endgroup\$ – Brian Drummond Aug 24 '17 at 11:48
  • \$\begingroup\$ Would you not need a different series resistor for either 230 or 110 operation? \$\endgroup\$ – D. Martin Aug 24 '17 at 13:01
  • \$\begingroup\$ Ideally yes, but practically no, because (once conducting) its voltage is fairly flat with current, like that of a LED. So if it takes say 55V, then V across R changes from 55V to 165V for a 3:1 variance in current - noticeable but acceptable. \$\endgroup\$ – Brian Drummond Aug 24 '17 at 13:08
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You can use a neon indicator as @Brian suggests or use a 240VAC LED indicator (since neons eventually fail 'off').

The apparent difference in brightness is not actually that great because of the logarithmic response of the human eye. The LED will be even less sensitive since it doesn't have much voltage drop compared to the neon. I would suggest buying a couple and actually trying it before rejecting the simplest solution.

If the difference really bugs you, you could use a capacitor dropper with series resistance and clamp the voltage with a zener (a resistive dropper will likely have excessive dissipation if you clamp it with 240VAC applied), but all that stuff is at mains voltage and it has to be dealt with in a safe manner- vs. just buying an approved LED or neon lamp assembly.

Edit: Here is an example LED panel indicator (similar to neon types in size- fits a ~8mm hole) that is rated 125 to 250VAC: (Bulgin P/N LE2950WL5A)

enter image description here

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  • \$\begingroup\$ I did try a 240VAC LED indicator from Allen-Bradley 800F-N7G (raise.rockwellautomation.com/RAConfig/…) and below about 180VAC the LED is no longer really visible, at 110VAC it is essentially off. \$\endgroup\$ – D. Martin Aug 24 '17 at 15:36
  • \$\begingroup\$ @D.Martin Interesting data point. I wonder what circuit they use inside...probably something fancier than a simple resistor.. I will add an example LED panel indicator that is rated 125V to 250V. \$\endgroup\$ – Spehro Pefhany Aug 24 '17 at 15:57

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