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Check My Math

Discussion in 'Bad Dog Cafe' started by Jeru, Feb 21, 2018.

  1. Jeru

    Jeru Tele-Holic

    Nov 17, 2006
    I'm converting an old console radio into a bluetooth stereo amp
    for a dear friend's brother in law. I'm ditching the tube guts and
    putting in a Bluetooth-equipped stereo board. Thing is, the LEDs
    that the board will power just aren't up to the task of lighting up
    the radio dial -- it's got to be an incandescent bulb.

    For reference - one of my builds. Scroll to near the end to see
    what I'm talking about re: lighting the dial:


    So -- help me do this safely and check my math re: power resistor
    needed to safely lower the voltage and use a #47 bulb to backlight
    the dial. I really want this thing to be SAFE and dependable.

    The wall wart for the bluetooth board puts out 19vdc. I'm going to use
    a DPDT switch to connect hot/ground to 1) the BT board and 2) the
    bulb circuit. #47 bulb draws .15A of current.

    The Bulb circuit is: 19v+ --> resistor --> Bulb --> 19v neutral.

    I figure that a 10W (ceramic white block) resistor ought to be just
    fine. 75 ohm resistor gives me about 7vdc to the bulb, a voltage drop
    across the resistor of ~12vdc.

    Using Ohm's law to calculate the power dissipation of a 75 ohm resistor
    dropping 12v, P = V(squared) ÷ R.

    So, 12v squared = 144 / 75 ohms = 1.92 watts power dissipation.

    So -- at five times the dissipation, even though the 10W resistor the resistor
    gets warm/hot to the touch, it should be fine/safe in this circuit?

    Is there anything that I'm missing?

    Many thanks.
    Last edited: Feb 22, 2018
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