replacement neon (or LED) indicators

[JohnD]

I have a couple of these

which are working fine, but the neons have gone dim. To save the expense of new ones (they are a non-standard size), is it possible to get replacement mains voltage neons? I have an idea they have a resistor in the connecting wire to drop the voltage.

 

[JohnW2]

I have a couple of these ... which are working fine, but the neons have gone dim. To save the expense of new ones (they are a non-standard size), is it possible to get replacement mains voltage neons? I have an idea they have a resistor in the connecting wire to drop the voltage.

I've personally never seen any for sale, but that doesn't prove much. As you say, they have a very high value series resistor in the lead (usually under shrink wrap) to limit the current,

Don't you have any discarded accessories with neons that you could cannibalise? - or failing that, buy the cheapest and nastiest accessories with neons that you can find on eBay.

Kind Regards, John

 

[SFK]

Not as many as I would have expected on ebay "Search: Neon Bulb 240":
https://www.ebay.co.uk/sch/i.html?_from=R40&_sacat=0&_nkw=neon+bulb+240&_sop=15

Or buy two of cheapest Switches with Neons and remove the neons (not a cheap solution though):
https://www.screwfix.com/search?search=neon#sort_by=price

SFK

 

[bernardgreen]

https://uk.rs-online.com/web/p/neon-indicator-lamps/6559413/
250 uA so fit 1Meg Ohm resstor in series

https://uk.rs-online.com/web/p/neon-indicator-lamps/6559407/
600 uA so for 500 uA fit a 470K Ohm resistor in series.

 

[JohnW2]

250 uA so fit 1Meg Ohm resstor in series
600 uA so for 500 uA fit a 470K Ohm resistor in series.

There will be something like 90-100V across the neon once it has struck, so only around 130V - 150V across the resistor. I would therefore thing something like 560 kΩ and 220 kΩ respectively would probably be appropriate, wouldn't it?

Kind Regards, John

 

[bernardgreen]

You could be right, memory fade this early in the morning.

 

[JohnW2]

You could be right, memory fade this early in the morning.

Well, my (approximate) arithmetic is certainly right .

Although I obviously don't know the characteristics (working current) of the neons concerned, I seem to recall that 220 kΩ or 270 kΩ is not an uncommon value of resistor to find.

Kind Regards, John

 

[JohnD]

thanks
I have ordered some of the ones with resistor, and have some heat-shrink sleeving.

I saw a reference that they have a life of 20-25,000 hours, which doesn't seem much. About 3 years if continually on (which they will be).

Why don't they use LEDs now?

 

[JohnW2]

I saw a reference that they have a life of 25,000 hours, which doesn't seem much. About 3 years if continually on (which they will be).

That corresponds roughly with my experiences.

Why don't they use LEDs now?

Interesting question. For a start, if one was just going to use an LED and a resistor, one would have to try to find an LED which could tolerate the ~325 peak reverse voltage during alternate half-cycles.

Even if one could find such an LED, I suspect that the main issue would be power consumption. As bernard has illustrated, the neons usually draw well under 1mA, whereas I doubt that you'd get enough light out of an LED with less than about 5mA, possibly more, particularly since they would only be conducting for half of the time.

The main problem, of course, is that LEDs are much lower voltage devices, with all the 'unused voltage' (much more with an LED than a neon) being used to waste energy in the resistor. If one assumes 90V across it, a neon with, say, 0.5mA going through it would be getting 45mW, with some 70mW wasted in the resistor. With, for example, 5mA through a 3V LED would put 15mW into the LED with about 1.13W being wasted in the resistor. To get the same power into the LED as the neon (45mW) would require about 15mA, with about 3.4W being wasted in the resistor.

Kind Regards, John

 

[JohnD]

I found a few on RS https://uk.rs-online.com/web/c/ligh...&sort-by=P_breakPrice1&sort-order=asc&pn=1and others that were much more expensive than the neons. I suppose the market hasn't yet bulked up.

I bet the LED factories could churn them out for 50p a thousand.

 

[JohnW2]

I found a few on RS https://uk.rs-online.com/web/c/ligh...&sort-by=P_breakPrice1&sort-order=asc&pn=1and others that were much more expensive than the neons. I suppose the market hasn't yet bulked up.

Possibly, but they are all panel-mounting assemblies, hence appreciably more complex that the bare neons.

Most of them seem to be 3mA. That's a little less than I predicted (5mA), but still 5-6 times the power consumption of a neon. As I said before, from the point of view of 'efficiency', it makes sense to use something (i.e. a neon) whose operating voltage is much closer to the supply voltage than is the case with an LED (given that all of the 'unused voltage' is going to result in 'wasted energy' ... even though we are talking about pretty trivial amounts of energy/cost.

Kind Regards, John

 

[JohnD]

when I worked with man-sized computers, they had a lot of panel indicator lights, about 1/4" diameter, with filaments that ran at half-voltage for "off" and full voltage for "on". With vast numbers of them, I suppose they had quite long lives.

 

[bernardgreen]

that ran at half-voltage for "off" and full voltage for "on".

The "keep warm" technique extended lamps lfe significantly by preventing the in-rush current to a cold filament. In-rush caurrent can be 10 times the normal current when the filament is white hot.

 

[333rocky333]

Mk still sell 3 versions of Leads,Lamp,Resister assemblies page 221 in the cataloque.
All 3 types can be found here https://www.mkelectric.com/en-gb/Pr...ryProducts/Neonlampassembly/Pages/5144SS.aspx

 

[JohnD]

thanks!

I wonder where (if?) I might be able to buy some...