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Series resistor.

Discussion in 'Electronic Repair' started by klem kedidelhopper, Nov 15, 2012.

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  1. I think I know this but I'd like to be sure. I need to affix a small
    neon indicator on to a 240V line. Would 80K be an appropriate resistor
    to use? Thanks, Lenny
     
  2. N_Cook

    N_Cook Guest

    usually a bit more , 150 K perhaps
     
  3. Guest

    82K is correct if the neon indicator has an internal resistor. If
    it's a 'bare' NE-2 lamp, use 130K.

    PlainBill
     
  4. Phil Allison

    Phil Allison Guest

    "klem kedidelhopper"
    ** The value of 80k is fine, but needs to be rated at 1 watt to have a long
    life.

    The usual values found inside 240V neon bezels and illuminated switches is
    150k to 220 k ohms - chosen mainly because the resistor needs to be of
    small physical size it and this dictates one of 0.5 or 0.25 watt rating.

    The voltage across the resistor is about 200V so 150k dissipates 0.27 watts.


    ..... Phil
     
  5. Robert Macy

    Robert Macy Guest

    bare bulb?

    assume neon turns on at 90V and extinguishes around 60V.

    Sinewave are 'so fast' just assume the neon is like a bidirectional
    zener of 60V:

    First what power idssipates if there were NO neon bulb:
    to find R try 240*240/R=Power
    therefore, R=240*240/Power
    rule of thumb for resistors is use half their rating, so change
    formula to
    R=2*240*240/Power
    so a 1/4W means R = 460k, close value 470k 5%
    and 1/2W means R = 230k, close value 220k 5%

    Now let's 'clip' the voltage that goes to the neon:
    as a simplistic estimate, 240-60=190, and try again
    R=2*190*190/Power
    1/4W R = 289k
    1/2W R = 144k

    use 1/2W 220k, looks like won't hurt anything.

    just as a check, how much power is going into the neon?
    current is (240-60)/220k = 0.86 mA
    power into the neon bulb is 60*0.86mA = approx 50 mW, so neon is not
    likely to burn up.

    Plus, dopn't those GFI outlets limit AC current to something like less
    than 1 mA, so even if you get your fingers in there you're not likely
    to be killed.
     
  6. Phil Allison

    Phil Allison Guest

    "Robert Macy"

    use 1/2W 220k, looks like won't hurt anything.


    ** Correct.


    Plus, don't those GFI outlets limit AC current to something like less
    than 1 mA,


    * Trip current is normally 10mA and time to trip is 30mS for units that can
    be plugged in.

    This is for a 240V country.



    ..... Phil
     
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