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Thread: Thoughts on an ITPS Override Switch

  1. #21
    Variable Bitrate wkolodny's Avatar
    Join Date
    Apr 2004
    Quote Originally Posted by STI_FFY
    DPDT W/center-off. The three examples I gave are precisely this type, You just would not have used the second gang in the previous example (now you will use it).
    Cool. Thanks. Got any resistor/diode recommendations/Specs?
    Don't Fear The Gurkey
    Click here to check out My Setup

  2. #22
    Constant Bitrate STI_FFY's Avatar
    Join Date
    Apr 2004
    New Market, MD
    Quote Originally Posted by wkolodny
    Cool. Thanks. Got any resistor/diode recommendations/Specs?
    Most any LED will do, biggest decision will be what size and mounting style you want to utilize. There are panel mount types at RS also.

    Their part # 276-068 for example, has a forward voltage of about 1.8V.

    ~13.8V-1.8V = ~12V the resistor must drop.

    This LED will tolerate 20mA, so the formula to determine optimal resistor value for maximum brightness is 12V/20mA = 600 Ohms. Resistors aren't readily available in that value, and lower value = higher current and is less tolerable so next common value of 670 Ohms is about right. A 1K would work fine, but be a bit less bright (12V/1K=12mA). Less bright might be a good thing depending on how you mount it.

    I'd recommend experimentation with several types to find the right orientation and brightness. There's nothing more annoying than a "laser beam" lookin' LED in the eye when you're driving.

    Don't forget to spec the power dissipation of the limiting resistor. 20mA X 12V = 240mW. This would be tolerable for a 1/4 watt resistor indefinitely. If your chosen LED is much brighter and requires more current, you may have to go to 1/2 Watt resistor.
    SeeYa! -Jim-

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