Can we use 5VDC LED in the AC line to show indication

Discussion in 'General Electronics Chat' started by strameshkumar, Aug 30, 2016.

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  1. strameshkumar

    strameshkumar

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    [​IMG] upload_2016-8-30_17-46-3.png
    Hi All, I am creating this design . The LED is 5VDC and take 40mA current. 10K Resistor is 0.25W. can this LED glow when the fuse blown ? Or It will glow in all time ?
     
    strameshkumar, Aug 30, 2016
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  2. strameshkumar

    shrtrnd

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    If that fuse blows, you'll have 208VAC across a 10K Ohm Resistor and an LED.
    What do you think is going to happen?
    Why don't you just use a neon bulb?
     
    shrtrnd, Aug 30, 2016
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  3. strameshkumar

    duke37

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    The current will be about 20mA AC.
    The power in the resistor will be 200*200/10000 = 4W so a >5W resistor will be appropriate.
    The led will pass current in one direction and will burn out when the polarity changes. One way around this is to use two leds in inverse parallel.

    If the fuse blows, then the circuit will not be isolated and will remain a danger.
     
    duke37, Aug 30, 2016
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  4. strameshkumar

    Minder

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    As per post#2 use a neon, this is how it is done in industrial fuse holders to indicate blown fuse.
    M.
     
    Last edited: Aug 30, 2016
    Minder, Aug 30, 2016
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  5. strameshkumar

    AnalogKid

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    In place of a 10 W resistor you can use a 0.5 uF film capacitor rated for the AC voltage. But you still need either another LED or a 1N4007 diode in reverse parallel with the LED. And the downstream devices still will be dangerous when the fuse blows.

    ak
     
    AnalogKid, Aug 30, 2016
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  6. strameshkumar

    davenn Moderator

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    If you have to ask this, then you probably shouldn't be playing with mains voltages


    thread closed for everyone's safety


    Dave
     
    davenn, Aug 30, 2016
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