Power On Lamps

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Flyboy

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Does anyone know of a simple circuit I can use to avoid stepping down 240v to 12v power 'ON' lamps?

I want to mount a panel of 12v lamps or LEDs that show when certain 240v equipment is receiving power.

Thanks
 
A neon bulb would require a ballast resistor. You can purchase indicator lights that run on 240, just search around shouldn't be that hard to find. All of the equipment I have at work uses standard mains supplied indicator bulbs, though they typically run off 120 volts. 240 is split phase so the indicator would be run from one of the phases to ground for 120V, same thing with the there phase equipment we use, they only use one phase to power the indicator lights. The current draw is extremely low so it doesn't unbalance the multi phase systems.
 
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Hi mate,

Not certain if the OP is in the UK but if he is then the 240v AC is not split phase. Our 3 phase is 440v( though it is actually about 415) so grounding any single phase always gives 240v (now actually a European standard 230v)

Al
 
why dont you power the LED directly? just connect it in series with a 100k resistor and connect it to 220/240 mains/ as you require and it would work as a good indicator.
 
mbarazeen,

1. LEDs have very low reverse voltage limits. With out an anti-parallel diode across the LED, it will likely smoke.
2. 100K resistor in series with the LED will only allow 240uA RMS (336uA peak) for 240VAC. Not enough current to get much light out of the LED.

Ken
 
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You could just use two LED in anti-parallel and a ballast resistor, I've done that on mains power before. But if you get an transient spikes kiss your LED's goodbye.
 
LED, 1N4007 and two 33 k.ohm 1W resistors in series,
Then a 1N914 anti-parrallel with the led.
Have used this circuit for 20+ years with no failures in potential indicators on 240 Volts kWh meters and control relays on overhead power lines to indicate the status of the relay.
 

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If you want less flicker and higher brightness don't use the 1n914 in anti-parallel simply use the same LED in anti-parallel.
Rodalco what's the reasoning for using two resistors? Power dissipation? A single resistor of twice the rating of your one resistor would be identical unless the components are cheaper or readily available.

I don't like this solution just because it upsets my personal sensibilities to see how much of the power going through the circuit is being wasted as heat. Personally I would add a modest capacitor in parallel to short transients. You're already throwing a huge amount of power away so little more from a cap isn't gonna hurt, just keep well under the ripple current handling/dissipation of the cap you use and it'll eat spikes for lunch.
 
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1 and 2W carbon-film resistors are only rated at 500V (Stackpole CF/CFM series). I use two in series even on 120V just to be safe.

Ideally, series resistors connected to the mains ought to be flameproof, too.
 
mneary, beaten me and he is correct there, less voltage stress per resistor.

1 Watt R's are easier to get and smaller compared to 5 Watt R's.

Sceadwian, I have built at least 10,000 of these indicator lamps with NO faillures. Try to beat that with a Capacitor, Not in a million years ( just joking < led's have only been around for about 40 years).

1 Watt wastage is nothing for reliabilty AFAIC.

If flicker bothers you could put a small cap in parrallel with the LED.
 
Less voltage makes sense, no wonder they don't fail, though I'm a little surprised transients don't take them down, LED's are sensitive to those unless something else before the indicator eats the transients or some other inherent part of how they're used limits transients. I also don't really have any kind of concept of how common and severe transients typically are.
 
then why not to use a capacitor instead of a resistor to limit AC? a small capacitor (400V) to keep 20mA can work. may be costly?
 

Overhead lines are subject to large trainsients, especially during thunder storms and high voltage switching.
The indicator light are in parrallel with a 240 V relay coil or metering coil, which probably even have a switch on surge as well.
As said before there is no need to run a small indicator light at 20 mA when directly running of 120 or 240 V mains.
5 to 10 mA is ample especially with a high efficient LED.
Reliability is my issue so a capacitor is out of the question.
 
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