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Voltage Presence Indicator

Discussion in 'Electronic Design' started by jj, Oct 5, 2003.

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  1. jj

    jj Guest

    Hi,
    I am looking at adding a voltage presence indicater to a 220 V ac circuit.
    I am limited with space so I put an LED (3mm) with diode and resistor in
    series and this seems to be ok.
    What I want to know is if this is correct and if not what is the prober way
    of doing this.
    Please recommend some components.
    Thanks in advance
    John
     
  2. John,

    Wow, that's a lot of power dissipation for the resistor (~1.1W for
    10mArms LED current). Make sure that your resistor can handle this much
    power continuously.

    For 220V, I would actually use a neon lamp. Yes, they are old fashioned,
    but they use a lot less power. With a typical neon lamp at 120V, I
    usually see 110K resistors (with a range of 47K to 220K depending on
    lamp brightness). So at 220V, I would recommend starting with somehting
    around 220K. This would dissipate just under 0.25W compared to the 1.1W
    for the LED circuit.

    YMMV,
    Marc
     
  3. John Fields

    John Fields Guest

    ---
    That's fine, just make sure that the series diode can handle the reverse
    spikes on the line and that the LED ripple isn't annoying. Something
    like a couple of 1N4007's in series might be overkill, but it's a cheap
    way to make sure that you can (almost) stop worrying about the LED's
    survival. If you use a high efficiency 2mA LED your series resistor
    will need to be 110k ohms and will dissipate 220mW, so you could use a
    1/2 watter, but you need to make sure that the resistor can handle the
    peak voltage (311V). If you wanted to you could forget about the spikes
    by blowing off the series diode(s) and putting a diode in inverse
    parallel across the LED, but your resistor's dissipation would increase
    to 440mW, so using a one watter would be prudent, and you'd still need
    to make sure that 311V across the resistor wasn't a problem and that the
    visual ripple was acceptable. If the ripple was a problem you could use
    a full-wave bridge with the LED on the DC side of the bridge, and the
    110K resistor in series with the either the AC or DC side of the bridge.

    I'd use the bridge, but that's just me...
     
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