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Thread: Triggering a relay using a transformerless power supply

  1. #1
    Thread Killer Illum's Avatar
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    Apr 2006
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    Default Triggering a relay using a transformerless power supply

    Following the calculations here: http://www.daycounter.com/Circuits/T...Supplies.phtml

    With the circuit as follows:


    Using:
    Vrms: 120V, 60Hz
    R1: 6.8 ohms
    C1: 1uF 250V Metal film
    C2: 470uF 25V Electrolytic radial
    D1: 1N5818
    ZD1: 1N4733 5.1V 1W Zener

    The calculations yielded:
    Iout: 25.8ma

    I have a couple Kest# KS2E-M-DC5 relays, 5V coil with 130 ohm resistance, or about 38ma max. My question is very simple: Must I attain 38ma in foward current in order to trigger the relay? or will 25.8ma suffice?

    Building a couple blackout lights that uses a relay to monitor line voltage that closes a switch to an LED when power fails

  2. #2
    Flashaholic
    Join Date
    Aug 2011
    Location
    MD
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    336

    Default Re: Triggering a relay using a transformerless power supply

    Ideally, a 5v relay will pull in at 5(2/3)v and drop out at 5(1/3)v.

    It's like a car clutch. The top 1/3 of pedal travel is fully engaged and the bottom 1/3 is fully disengaged.

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