12 V Lamp On 240 V

What's the current requirement for the lamp? (alternatively, what's its wattage rating?)

Website doesn't say. It's a 12 V LES style lamp. I can probably get a

5 watt wirewound resistor for it.

I looked on the page for the replacement lamps. 50 mA

I make that to be 4560W. Be advised though that the resistor will dissipate about 11.4W.

The calcs, like an LED.

R = (240-12)/0.05 gives 4560R.

Power rating is P = 0.05^2*4560.

I would round it up to a 4k7 resistor.

May your luck be in. Don't blame me if you blow yourself up.

Rob.

So worst case is you'd want to drop 250 - 12 = 238 Volts at 50 mA across the resistor. Right away you can see that it'll have to dissipate 238V*50mA = 12 Watts. That's a lot of heat to shed just to support an indicator lamp.

The resistance would be 238V/50mA = 4700 Ohms near enough.

Maybe you should look at finding a 240V indicator lamp?

Thanks. Found a 4.7k 5 watt, wirewound, ceramic cased resistor on the Dick Smith website.

I have a neon indicator lamp installed at the moment, it's just about dead, and there's no way to hold it in place. I don't have a hot glue gun.

"Dave.H" schreef in bericht news: snipped-for-privacy@d4g2000prg.googlegroups.com...

Well, according to Ohms law:

R=(240-12)/50=4,56k so take 4k7. Wattage (240-12)*50=11400mW so you will need at least a 12W resistor. The thing also will become pretty hot. Generally speaking, this is not the best way to use a 12V lamp.

A capacitor of about 68uF might do a better job. No need to say it should be able to handle at least 250Vac.

Electronics may do even better but are more complicated. Not the thing I'd advise to a beginner.

BTW. Why do you need this extraordinary application? Did you try to find other solutions?

petrus biybyter

Perhaps an LED instead? A regular red, 20 mA one will do.

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You can buy one. Small types are dead cheap overhere. Guess the resistor you need may be more expensive. Especially as it will frie your equipment. Otherwise, you can use silicone kit or construction kit. They're available in small tubes.

Neon indicator lamps are for sale in many styles. A lot of them with build in resistors. Whats the current lamps type? Has it a thread or a bayonet fitting? Has it a resistor build in? I should not be surprised if the type is available if only you know where to look for it.

petrus bitbyter

Why don't you use a 12.6 volt "Filament" transformer?

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

"Dave.H"

** Only if you intend to kill yourself or the next person who tries to change that bulb.

What an extremely stupid question.

....... Phil

A LED lends itself to the "wattless dropper" trick, at the mains frequency the impedance of a capacitor is much higher than the load so it acts similar to a constant current source. The capacitor must be rated for mains - at least 400V, preferably 600V, and you need a resistor in series, about 1k2 to limit current surge and the LED must have an inverse parallel diode to stop its PIV being exceeded, try a 0.047uF to start with and work up to the required brightness with successive higher preferred values.

Are you aware that "Omron" do a 240v/12v panel lamp. Its basically a low voltage transformer and lamp in a single hole fitting. Not a cheap as putting a new neon lamp in though. You can buy those for a few pence.

--
Best Regards:
                     Baron.

Use a transformer.

What an extremely stupid answer.

heh

Well he was raised by dingoes!

Why not rely on the impedance of a capacitor? A good textbook will give the the formula relating impedance to frequency and capacitance.

R

He can do this with his 12V incandescent. Use a 0.68uF cap and a 10 ohm 5W resistor in series with the 240V and the bulb. As you pointed out, the cap must be rated for 400VAC, which means it won't be cheap.

If he uses an LED, he'll need another diode in parallel, backwards, and probably a zener in parallel to prevent overvoltage. The incandescent doesn't care about this, so it may be a better choice.

As others have pointed out, the best and cheapest choice may be a glue gun and a neon indicator bulb which is rated for 240VAC.