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Question about 7.5v from ATX power supply

Discussion in 'Electronic Basics' started by CoogarXR, Jan 19, 2004.

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  1. CoogarXR

    CoogarXR Guest


    I am desigining a PC-based DVR security system. The cameras run at 8.5vdc @
    700ma. I would like to run 2 of them off of a drive controller lead from the
    ATX power supply running the PC. Here is my math, feel free to double-check it,
    as I am just getting back into electronics, and I haven't used my electrical
    math skills in quite some time :)
    V / I = R (Ohms law)

    Voltage Difference 12v -8.5 = 3.5v Divided by .7a (700ma) = 5 Ohm
    700ma x 4.5v = 2.45w

    Now, provided I did my math correctly, could I just use 2, 10ohm 10watt
    resistors in parallel? I only can get 1ohm or 10 ohm in that high of wattage.
    Does this all sound right? Thanks!

  2. cpemma

    cpemma Guest

    Don't use resistors to drop the voltage, very dodgy practice if the load
    varies at all. Use a couple of 317T adjustable regulators, one for each
    camera. One *will* take 1.5A but what you save on the extra reg, you'll
    spend on the bigger HS needed.

    I make 470R and 2k7 about right for the resistors or use a preset and get it
    on the nose.
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