for testing shorts need idea to draw amps please

Discussion in 'General Electronics Chat' started by John626, Oct 24, 2014.

  1. John626

    Thread Starter New Member

    Oct 24, 2014
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    I am testing a motherboard that has short to ground.

    I want to use a ATX psu
    So in this order PSU 12v rail to - mosfet on mobo tells the board it needs to be at 12v
    ground on board -to positive end of (something that draws 5 10 or 15 amps)
    ground wire of ---the something
    back to the PSU ground. The motherboard will be the middle man.
    I need to be able to put a demanding load on the output of ground wire on the motherboard. I should be able to feel the heat where it's shorting to ground ie GPU
    The led strip I have is too small I need something more demanding I need something 12v that will draw the type of amperage.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Lightbulb? I use an old auto headlight. It'll draw ~4A at 12V.
     
  3. John626

    Thread Starter New Member

    Oct 24, 2014
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    So the ground on mobo to positive bottom of bulb and the wire to the side of bulb ground back to psu ground?
    I 've seen this done before with a Whole bunch of LED strips on the output of the ground on motherboard I think it has be something load demanding to inject the 12v through the board. I don't have a whole bunch of leds.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Parallel or serial configuration is up to you, depending on what you want to accomplish.
     
  5. John626

    Thread Starter New Member

    Oct 24, 2014
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    Does the power supply need to see 2.4ohms to draw 5amps? so it needs to go before the motherboard? The bulb is actually a current limiter so its not shutting off the PSU when it sees the shorts and wants to shut down right?
     
    Last edited: Oct 24, 2014
  6. John626

    Thread Starter New Member

    Oct 24, 2014
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    I think utimatley I'm need a way to have it where the ATX psu 12v does not want to shut down when it sees the short on the board. I need it to feel stay hot where the short is. So I think i'm needing something with alot of resistance on the output of the board right?
     
  7. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Borrow an auto battery tester from a mate in a garage.

    But beware some (the lorry type) can draw a frightening amount of amps.
     
  8. profbuxton

    Member

    Feb 21, 2014
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    Really need to use a variable current limited supply. Set volts to 12v or so, connect and slowly raise current limit to required level. You may be able to track voltage drops on tracks to determine short location or if possible use a current tracer(if available). Other option if is to use calibrated fingertip to feel for hot parts.
     
  9. John626

    Thread Starter New Member

    Oct 24, 2014
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    There is no idea to get variable current limiting from the 12v psu I have?
     
    Last edited: Oct 24, 2014
  10. wayneh

    Expert

    Sep 9, 2010
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    In series, lightbulbs of different wattages will limit current to the amount the bulb uses at 12V. Placing an identical bulb in parallel to the first will double that current. Not elegant, but cheap if you already have the bulbs.

    You could certainly build yourself a constant current supply from a computer PSU but it's not the easiest project for a newbie because of the power involved.
     
  11. John626

    Thread Starter New Member

    Oct 24, 2014
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    The Psu I have is 12v @9.2amps I need to bring it down to 1 amp or little higher can I use a resistor?
     
  12. wayneh

    Expert

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    Well, a 12W bulb will allow 1A thru itself. ;)

    A resistor will sort of work. Suppose you want to drop 12V at 1A. That means a 12Ω resistor. The power it will dissipate is I^2 x R, or 1 x 12 = 12 watts. To keep it from overheating, you'll want one rated to at least 24W.

    Neither of these simple solutions is the same as a constant current supply, which will self-adjust the voltage (up to a limit) to reach the current target.
     
  13. John626

    Thread Starter New Member

    Oct 24, 2014
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    Thanks for all your input greatly. I will try with a 10w 1.0hm resistor first.
     
  14. wayneh

    Expert

    Sep 9, 2010
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    It will overheat. Be careful.
     
  15. John626

    Thread Starter New Member

    Oct 24, 2014
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    What the psu ? it will shut itself off right ?
     
  16. wayneh

    Expert

    Sep 9, 2010
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    The resistor is not rated to handle the power and will get very hot, and then fail.
     
  17. John626

    Thread Starter New Member

    Oct 24, 2014
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    I can't handle all this i'm going to buy a long led strip off amazon overnight and connect the the positive to the ground on board and the negative from led to the psu and I should be in business.
     
    Last edited: Oct 24, 2014
  18. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Why don't you spend your money on two 5 ohm 10 watt resistors. I think you may :D instead of :(.
     
  19. John626

    Thread Starter New Member

    Oct 24, 2014
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    na
    I've seen this done before actually the two mosfets on the board one has 18v and the other turns it into 12v the board can use to boot , by making the board the middleman the load of the led strips will continuously pull the 12v need to find the shorted component by heat.
     
  20. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    Use a milli-ohmmeter or and audible one. No high currents involved.
     
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