LED pilot light for 220 extension cord

[frank70]

If I may point out the obvious here: An LED typically requires about 20ma. and has a voltage drop of about 2 volts when forward biased. Assuming 240 volts across the line, that means the resistor would need to dissipate 238*0.02=4.76 Watts, far more than that puny resistor in the photo could handle, not to mention a tremendous waste of energy. LEDs also don't take well to being back biased with up to 340 volts peak. What I'm saying is that you can't just put a resistor in series with an LED and plop it across the AC line voltage (though that's exactly what you can do with a neon bulb). I've built line-powered LED nightlights, and it requires at a minimum a capacitor, resistor, and diode in addition to the LED.

Because of the glare, I can't make out the value of the resistor, but it looks like 820 ohms which certainly won't drop the bulk of 240 volts without exploding. Resistors used in series with neon bulbs are in the meg-ohms range.

What you have is not what it appears - beware (and make sure you have your life and fire insurance paid up.)

 

[qguy]

hi Frank,

Thanks for pointing that out, there are no other components that is visible. I will check the switch if the casing has other components inside. The device has been in use for a couple of years and 4 devices are plugged into it, basically I am wasting 5 watts per outlet, thats 20 watts 24/7 down the drain, not good !!! Will the capacitor, diode resistor combo help reduce the wastage ? if so, how much energy will I be saving ? also can you please please provide instructions on how to assemble the parts.

NOTE : I can not do away with the LEDS, I need the LEDS to tell me if the pumps in the aquarium is working or not

Thanks

If I may point out the obvious here: An LED typically requires about 20ma. and has a voltage drop of about 2 volts when forward biased. Assuming 240 volts across the line, that means the resistor would need to dissipate 238*0.02=4.76 Watts, far more than that puny resistor in the photo could handle, not to mention a tremendous waste of energy. LEDs also don't take well to being back biased with up to 340 volts peak. What I'm saying is that you can't just put a resistor in series with an LED and plop it across the AC line voltage (though that's exactly what you can do with a neon bulb). I've built line-powered LED nightlights, and it requires at a minimum a capacitor, resistor, and diode in addition to the LED.

Because of the glare, I can't make out the value of the resistor, but it looks like 820 ohms which certainly won't drop the bulk of 240 volts without exploding. Resistors used in series with neon bulbs are in the meg-ohms range.

What you have is not what it appears - beware (and make sure you have your life and fire insurance paid up.)

 

[frank70]

You're not wasting 5 Watts per outlet because, simply put, if that resistor were dissipating 5 Watts, it would be a black piece of charcoal, totally burnt up! Like I said, you do NOT have an LED in series with a resistor across the AC mains. If you attempt to replace it with such, it cannot work and something will die or burn. I don't know exactly what you have - I have a power strip here in the US with what appears to be a LED light that is on when the switch is on. I'm not about to take it apart to do research on what's driving it, but I suspect is is an integrated circuit chip with an LED that was designed specifically for this application. Here's an example (though much larger than what you have): http://www.westmarine.com/buy/blue-sea-systems--led-indicator-lights--P009_273_003_509

 

[jason 77]

Could the OP take more pictures of the led and surrounding wiring please?

 

[poiihy]

You're not wasting 5 Watts per outlet because, simply put, if that resistor were dissipating 5 Watts, it would be a black piece of charcoal, totally burnt up! Like I said, you do NOT have an LED in series with a resistor across the AC mains. If you attempt to replace it with such, it cannot work and something will die or burn. I don't know exactly what you have - I have a power strip here in the US with what appears to be a LED light that is on when the switch is on. I'm not about to take it apart to do research on what's driving it, but I suspect is is an integrated circuit chip with an LED that was designed specifically for this application. Here's an example (though much larger than what you have): http://www.westmarine.com/buy/blue-sea-systems--led-indicator-lights--P009_273_003_509

Well i've seen LEDs used many times connected directly to mains with just a resistor... :thinking:

 

[broadgage]

LEDs are widely used as indicators on 240 volt circuits.
The current is usually only a couple of ma in order to reduce heat in the series resistor, 20ma would be several watts wasted, 2ma at 240 volts is only about half a watt.

However you cant connect a standard LED to line voltage with just a resistor in series to limit the current, consider what happens on each half cycle of the line supply. On one half cycle the LED will light, but on the other half cycle it wont conduct and will therefore be subjected to full line volts in the reverse direction. Whilst LEDs do in practice withstand a lot more than the usually claimed 5 volts, they will fail rapidly if reverse biased at line voltage.

There are 3 ways around this problem.

Firstly, use a special LED that consists of two chips connected internally in inverse parallel such that one lights on each half cycle.
Secondly, shunt the LED with a reverse biased silicon diode, this adds an extra component and also doubles the loss in the series resistor for the same light output.
Thirdly, connect in series with the dropper resistance a silicon diode so as to block the half cycles that are of the wrong polarity to light the LED. This also adds another component, and the diode may pass enough leakage current to kill the LED.

The first approach is the most common IME although the LED is a special, they are very cheap in bulk, and only two components are needed, neither of which is polarity sensitive.

 

[broadgage]

Here is a typical bi-polar LED, http://www.ebay.co.uk/itm/KINGBRIGH...098?pt=LH_DefaultDomain_3&hash=item2a30f0ba2a

 

[poiihy]

Here is a typical bi-polar LED, http://www.ebay.co.uk/itm/KINGBRIGH...098?pt=LH_DefaultDomain_3&hash=item2a30f0ba2a

Is that listing for ONE led?
Wow, expensive!

 

[broadgage]

Yes it is very expensive, that is the penalty paid for purchasing just one and not looking around for the best price, I provided the link to illustrate the type of LED used for line voltage indicators, not as a suggestion that the example was good value.
Manufacturers of say extension cords would buy by the thousand and pay only a few cents.

 

[Illum]

Neon is glass, and without a proper housing is prone to breaking. LED will NOT fracture the 1st time it's dropped.
There are cords like this with neon lights in the ends but they are incased in plastic.

Good point, most Neon lamps are built into a retainer.


LEDs are a bit fragile in high temperature environments though, its not possible for a Neon to be killed by its own heat. One too many power spikes then yes for an LED

 

[PointyOintment]

The existing circuit looks terrible. I would replace the whole thing with the innards of a cheap LED nightlight. They are quite efficient. Just replace the photocell with a fixed resistor of appropriate value to make it turn on (or just enclose the photocell in the opaque enclosure of the power strip). And make sure you solder and insulate it better than it looks like the last person did.

 

[Illum]

PointyOintment,

No need to over complicate this.