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connecting led in 230v ac line

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raman00084

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i am connecting a led with 100k 1 watt series resistor in 230v ac line the led is glowing. my doubt is whether led will fail in ac voltage are it will work fine? led used is normal red to red 5mm led .
 

pic.programmer

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See if you can use a 22k resistor a 1N4007 diode and the LED in series.
 

BradtheRad

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The led should not be exposed to a reverse voltage which is more than a few volts. Therefore put a diode (or a second led) in anti-parallel to the led.
 
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Easy peasy

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The LED current forward or reverse is limited to 2.3mA rms, so you will not kill the led by forcing it to conduct in reverse, its not std practise but at least it should be safe...even if the reverse breakdown voltage was 50V the reverse power in the led is < 57mW.
 

betwixt

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Exceeding the reverse voltage of the LED can cause destructive breakdown, even if the current is limited. Far better to prevent the reverse voltage being given chance to do so.

Methods:
1. High voltage diode in series with the LED/resistor. Prevents risk of damage but the LED will still flicker at half cycle rate.
2. Low voltage diode across the LED. Prevents risk of damage but LED still flickers and overall power consumption is doubled without more light being produced.
3. Two LEDs in anti-parallel. Safe, the LEDs flicker alternately.
4. A bi-color LED. Safe, still one package, may be an intersting color!
5. LED wired across a bridge rectifier made of 4 low voltage diodes. Safe, LED flickers at twice line rate, will appear to be brighter.

Option 5 is probably best, still use the 100K resistor but wire it to a bridge rectifier made of 4 low voltage diodes (1N914, 1N4148 for example). Connect the LED across the '+' and '-' points on the bridge. The voltage is kept to a safe level and the LED will produce about twice the light level compared to your existing system.

PLEASE take note that without isolation, your circuit will be 'live' at all parts and extremely dangerous to touch. Keep fingers well away!

Brian.
 

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@ betwixt, if the reverse power in the led is insufficient to cause die melting, how can it cause destructive failure? explanation please? leds are often used as zeners in ckts...
 

betwixt

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Melting isn't the problem - I agree that would require enough current to cause heating. The issue is crystaline breakdown of the junction, enough voltage to force electrons to make their own permanent pathway through the barrier, distorting the molecular lattice. In other words 'ripping' the junction apart rather than melting it together.

Brian.
 

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electrons thru the junction cannot cause the effects you describe unless there are a lot of them at high energies, they simply cannot distort the lattice in low or moderate concentration under low applied EMF, note avalanche current flow in zeners and leds and BE junctions and normal diodes, fets... - all non destructive.
 

betwixt

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... which begs the question then, "why have inverse voltage ratings at all?" Why not just specify maximum avalanche current?

This is heading off topic...

Brian.
 

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Actually it's very much on topic and directed squarely at the posters question, usually a max dissipation is specified and this along with specified (if it is) or measured Vbr gives the max reverse current tolerable...
We sometimes design with a xtor C-E reversed to get a low gain follower and some commentators have opined that it is not reliable, without giving any reasonable basis for their opinion...
 

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We sometimes design with a xtor C-E reversed to get a low gain follower and some commentators have opined that it is not reliable.

Yes, I heard the same argument related to a reverse operated BJT analog switch, it's in fact not substantiated as long as the actual Vbe reverse breakdown voltage isn't exceeded. But I think the comparison is flawed because it's known that driving BJT repeatedly into Vbe reverse breakdown can result in a degration of transistor parameters, e.g. reduction of current gain.

I'm sure you also know well that the avalanche power handling of MOSFETs is far below it's regular thermal rating.

In so far it's a bit adventurous to claim that the reverse current handling of LEDs would be only limited by it's regular power rating. Is this statement somehow based on experiments or manufacturer specifications?

I don't say it can't be right, but I would be at least cautious.
 

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betwixt

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note avalanche current flow in zeners and leds and BE junctions and normal diodes, fets... - all non destructive.

Just curious, I've seen LEDs used as voltage references in 'cut price' power supplies but only in forward conduction mode. I've never seen one used in avalanche state, has anyone else?

I would expect the voltage to be unpredictable, maybe unrepeatable and possibly prone to ambient light levels.

Brian.
 

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Fwd cond voltage of a led would be very temp dependent, whereas the rev avalanche voltage for fixed current is very zener like for the LED, similar to the E-B breakdown voltage in a normal xtor which is also often used.

Yes, intense ambient light in the region of the led's o/p could be a factor, we'll have to try that and see if it is...!
 

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