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Thread: What did I do wrong?

  1. #1
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    What did I do wrong?

    I did my Mac mini install as described. Using a 19V 4.2A laptop car power supply and a 3.2K 1/2w resistor to the i-sense wire. Everything worked fine for about eight hours, then it started re-booting over and over again, every five seconds. I unplugged it, checked and the voltages, everything seemed okay. Plugged it back in and an hour later it started again. I think the resistor is overheating and failing, but I don't know how to check it. What should I look for? I'm so close. I will share my $20 build here as soon as it's finished. Please, someone, help me out.

  2. #2
    MySQL Error soundman98's Avatar
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    i'm not a mac guy, but i'm willing to try to help.. what guide did you follow? i can't seem to find any references to the specific wattage of resistor, just that everyone uses a 2.2-3.3k ohm one...

  3. #3
    Fusion Brain Creator 2k1Toaster's Avatar
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    I doubt the resistor is overheating. You used a 1/2W? Well you are only pulling about 1/10W if you pulled that pin up to 19v. I don't do macs, so that's the extent of my help.
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  4. #4
    Raw Wave
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    Ditto. resistor power should be ~1/8W = ie, P=VV/R = 20V * 20V / 3200Ω = 4000/3200 = 0.125W

  5. #5
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    Thank you gentlemen, one and all. You've been quite helpful. It seems the census of opinion is that I used too "strong" a resistor? I'm not sure "strong" is the proper technical reference when referring to the wattage of a resistor. but I went with the 1/2w thinking it was "stronger", and that it was desirable. Now I see, I don't know nothin' about resistors. 1/8w it is then. An update, now I get a power light on my mini, but it won't boot at all, not even a "GONG". Odd, that it would work for ten hours, then one hour, then not at all? Also, what's up with the range? 2.3-3.3K, should I use a 3.0K and split the difference? What IS the difference? Suggestions?

  6. #6
    MySQL Error soundman98's Avatar
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    no, the resistor rating is fine-- just use the 1/2w

    resistors are rated based on the amount of power that they can pass-- look at a 1/2w as being a little larger water pipe then a 1/8w resistor-- it still allows the power to flow through, but can handle more power then the 1/8w.. it never hurts to use a larger-wattage resistor then needed, just don't use a smaller-wattage resistor then needed..


    are you able to use the old mac power supply to check that the pc is working correctly with mac-supplied gear?

  7. #7
    Raw Wave
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    I agree. In fact IMO you have chosen the correct resistor...
    1/2W resistors tend to be the most reliable... Maybe it's the combination of being physically big enough yet not too big (unlike 1W, 2W, 5W etc) or that they have been the highest numbers produced (until perhaps recently?).

    But excluding such physical issues, in this case bigger is better.
    If the resistor (or component) is designed to dissipate 1/2W of heat, then 1/8W is no problem.
    It will run cooler and hence live longer.

    It isn't "too" big - it is not as if is passes "too much current".

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