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Neon indicators in lighting circuits

Postby paddy1 » Sat Nov 27, 2010 6:00 pm

How does a neon indicator work in a light switch that has no neutral connections ? Supposedly, the neon indicator is 'alight' / on when the luminaire is off and vice versa. With a 1 way switch, I presume the indicator neon connections would be between common/line/live and L1 (is there any other option) but in the case of 2 way switching this would become more tricky as you would have common + L1 / L2 to choose from. If the indicator were to work as above, eg on when luminaire off and vice versa, how would you wire the actual neon connections ?
PS does a neon indicator bulb / wiring set itself have polarity ?
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Postby ericmark » Sun Nov 28, 2010 7:14 pm

I have seen these switches for sale where the edge of the switch lights up but how not sure. If they connect between line and switch line that would mean when switched off there would still be a current flow although only a few milliamp. With electronic discharge lamps this would build up and cause the lamp to flash every so often as the lamp tried to start. So clearly should not be permitted. The only other option would be to use the earth as a return again not really the way to do things.

There are clues in the adverts "# Works with many energy saving light bulbs
# List of compatible Energy Saving light bulbs" and "Not compatible with 240v LED light bulbs" The "Litswitch" does not say how much current can flow when switched off but likely just a couple of milliamp. It is noted the method shown in their PDF for connecting two way switches is not the method used by most electricians. In fact it as shown would cause EMC problems as they show different route for feed and return cables.

Nearly every manufacture publishes how there products comply with a BS or EN BS number and I see no such claims on this site.

Wikipedia says:- Illuminated switch
An illuminated switch in the off position

An illuminated light switch has an internal light source (either a neon lamp or an LED) which allows the user to locate the switch in the dark. Most European illuminated switches are two pole requiring the live and neutral wires to pass into the switch which enables the neon to be powered directly from the mains via a resistor. The internal light source in a single pole illuminated switch derives its power when the switch is OFF from current passing through the external light bulb. Single pole illuminated switches work well with incandescent bulbs, non-electronic fluorescent light fixture and halogen lighting, but they can cause a few compact fluorescent lamps to behave erratically. The current through the neon lamp in the switch can slowly charge the internal input capacitor in these lamps, until they begin to operate and produce a brief discharge. This cycle may repeat indefinitely in some cases, resulting in brief repetitive flashing while the switch is in the "off" position.

The point is that although the current is low when connected to a switch mode device it can build up to a dangerous level and I would only fit one in exceptional circumstances.
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Postby sparx » Sun Nov 28, 2010 8:19 pm

Hi Paddy1,
What a great question!!
OK as you say with one way switch since the neon is in series with the lamp and passes enough current to light up without affecting the main light.
As has been said on here before some low energy mini fluorescent lamps can still flicker/glow once they have been on as the neon leakage current is almost enough to relight them.
presume when you say switch you mean the finger plate fitted around it as I don't know of any illuminated standard light switches.
As an aside the neons do not have polarity but do operate around 90Volts so need a series resistor built in.
As to 2way switches ...I don't know if they can work, drawing the circuit says not if wired in single cores in conduit in conventional way but if done as 2way 'conversion' method where one switch has a 3core and the other has the 3core and the 2 core switch cable then if wired across L1 & L2 it should work .
Thanks for a thought provoking post,
wonder if others will answer also ,
regards Sparx
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Postby paddy1 » Mon Nov 29, 2010 9:51 pm

Have just done a bench test on this one, and it does work across L1 and L2 with the live feed (Common ) to the 2 way switch with the neon and the common of the 2nd 2way switch to the luminaire. I used an incandescent bulb fot this test, so not sure what would happen with low energy flourescents for example. I presume the polarity of the indicator neon wires doesn't matter since this is an AC circuit. There are no neutral wires involved here and so I also presume the neon works via a very small 'trickle' current through the bulb in the luminaire and hence to neutral.(taking the bulb out extinguishes the neon) The type of switch I am talking about is similar to an MK mastershield MK56401 (outdoor switch) rather than an illuminated bezel for an indoor switch, but presume the same applies. Thanks to Sparx
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