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Directly powering an L.E.D from the mains

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M.Joshi

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Hi,

I have seen circuits with L.E.D's being powered directly from the mains (240V A.C) without a transformer. What does the circuit consist of?
 
It generally consists of a high value resistor (near about 100k @ 230V) in series with the LED.
 
kinjalgp,

Could you explain how this works as I thought that the role of a resistor in series with an L.E.D was to limit the current :?:
 
Yes the resistor is to limit current through the LED.

The resistance value can be calculated using the equation
R = (Vs - Vf)/If

Where
Vs = AC RMS Supply Voltage Or DC Supply Voltage
Vf = Forward Cut-In Voltage of LED (Generally >= 1.5V)
If = Forward Current of LED (depends on LED type & intensity >= 1mA)
 
Re: Re: Directly powering an L.E.D from the mains

kinjalgp,

Just a few more questions:

Wouldn't the L.E.D be damaged since the forward voltage is being exceeded?

How is the wattage of the resistor calculated?

Is a reverse-biased diode required in parallel (For A.C) to prevent damage due to the reverse breakdown voltage being exceeded?

Thanks :)
 
No, LED won't blow since almost entire voltage is dropped across the resistor leaving only a few volts for the LED.

Power of resistor = If*If*R or ((Vs-Vf)^2)/R

A PIV protection diode may be used if required.
 
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