Incandescent pilot lamp off mains. Bulb rating question. - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: December 11, 2013, 08:28:30 pm
content: I know I should be able to work this out for myself, but my calculations don't tally with the parts list.Electronic unit, 230V AC mains powered. Mains "ON" pilot lamp is a small bayonet bulb in a holder, across the switched side of the mains live and neutral, via a biggish wattage ceramic wore wound resistor of 20K Ohms in the live lead. The bulb in it is 28V @ 70mA. It doesn't even glow dully. All connections and bulb are sound. Cold filament resistance is about 38 ohms. Schematic calls for a 14V bulb, but parts list shows a 28 V bulb...... Tempted to convert to a neon or a LED and rectifier, but also curious as to what it SHOULD have had in it originally. Thanks.

author: Andy Watson
author_rank: Super Contributor
timestamp: December 11, 2013, 08:40:44 pm
content: I would ignore the voltage of the bulb and go with 240V / 70mA. About 3.4k I reckon. The voltage of the bulb will mean that you're slightly under-running the filament - but that's no bad thing for a pilot lamp. But it's a whole lot of power being thrown away. I would definitely look at the neon option.

author: minime72706
author_rank: Frequent Contributor
timestamp: December 11, 2013, 10:16:08 pm
content: a 20k resistor in series with such a thing seems pretty absurd given what the resistance of the filament would be when actually emitting a reasonable amount of light. I would definitely grab a neon or LED. Even with a resistor and diode, the LED may not tolerate transients (talking out of my ass just to be safe) on the mains input; consider some sort of protection in parallel with it. (X capacitor, TVS diode, MOV, etc.) either that or don't bother since it's just a damn LED.

author: c4757p
author_rank: Super Contributor
timestamp: December 11, 2013, 10:38:09 pm
content: Quote from: minime72706 on December 11, 2013, 10:16:08 pmThe exponential I/V curve of the LED will make sure that it protects the TVS or MOV. A capacitor might help though.

author: grumpydoc
author_rank: Super Contributor
timestamp: December 11, 2013, 10:52:50 pm
content: Well, 20k across the mains is going to be 12mA and the voltage divider formed from a 38ohm filament and 20k ohm resistor is going to leave 0.45V across the bulb so I'm not surprised it doesn't glow. Even the 400ohm hot resistance would only leave 4.7V across the bulb.Assuming that the resistor is original we'd need about an 0.15W 12 or 14V bulb with a hot resistance of something in the 1100-1300ohm range. The problem there is that I can't see the cold resistance causing enough voltage drop in the bulb to get it up to "working" temp.For a 28V bulb you'd need an 0.33W bulb with a hot resistance of 2333ohms.I can't find a 12 or 14V 0.15W, nor a 28V 0.3W bulb listed by Farnell so you might have a hunt.A LED and a .15uF cap in series should do it (plus a diode if you don't want to reverse bias the LED every half cycle).

author: Zero999
author_rank: Super Contributor
timestamp: December 12, 2013, 02:26:35 pm
content: Have you looked into replacing it with a 240V bulb or neon?Or what about a small transformer? Even Maplin sell a 230V to 12-0-12V 100mA wire ended transformer which would work.http://www.maplin.co.uk/p/12v-100ma-sub-miniature-transformer-wb02c

author: Chris Wilson
author_rank: Super Contributor
timestamp: December 12, 2013, 02:54:51 pm
content: I think I will just remove the wirewound and use a drop in bayonet based miniature neon. I have also found the schematic for the panel is from a 3 phase unit, that would have the existing lamp across two phases. The neon is a drop in fix, and cheap. Thanks for the replies everyone, and may I wish all a very happy Christmas and a healthy and prosperous New Year.

author: minime72706
author_rank: Frequent Contributor
timestamp: December 12, 2013, 03:48:30 pm
content: Sounds like a good decision; grand holiday wishes in your direction as well!

author: SeanB
author_rank: Super Contributor
timestamp: December 13, 2013, 05:40:26 pm
content: The lamp that you need to use there is a 110v incandescent lamp. It is run via the resistor and will have about 100VAC across it when hot, with the resistor dropping the rest. The lamps typically will have a 5 year or so life in this application, left on 24/7. Better will be to buy one of the LED retrofit lamps available from eg RS or Farnell. There are also industrial lamp units with an integrated 220VAC to 6V3 transformer, designed to have a common 6.3V 1W lamp installed in the holder. I have some which run a 36V lamp off a 28V rail, via a large 5R power variable resistor ( really only there as short circuit protection) as indicators. Those last decades in this application.Neon lamps are not bright enough, and if you run them at a higher current to get them bright they have a short life. Typically you want under 100uA current, which corresponds to a very dim light. High current sputters the envelope black in a very short time. LED lamps can be very bright, and you can have them with very long life at low current.