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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.

See this thread in this forum. The same principles apply.

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