Watchdog FET

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Thread: Watchdog FET

  1. #1
    itme's Avatar
    Join Date
    Sep 2018

    Default Watchdog FET

    Hi all.

    I'm currently figuring out my electronics setup. I have a pc power supply (separate from my pc) which gives me a 5v 2.5Amps standby,5v @ 14Amps, 12v @ 25Amps and 3.3v @ 14 Amps. My idea is to have the kflop/kanalog board powered by the standby 5v. so I have a mains switch that I manually switch to begin with, which then puts the psu into standby powering the Kflop/kanalog, then the Watchdog FET switches the psu on by grounding the green cable(see link below), the psu then outputs it's full 5v and 12v lines which activate a relay powering on the mains AC to all motor psu's and drives.

    Does this sound about right? The green cable I measured to be outputting 3.3v, is that okay to wire diretly to the watchdog FET or do I need a resistor to keep the Amps under 1amp? Im not sure what amps that green "On" cable is outputting.

    This is the psu:

    This is the mains relay:

    Also, could anyone confirm that this same relay would be fine running a 1/2 hp coolant pump, though it would be a separate relay to the mains power one.

    Thank you.

  2. #2
    Gold Member TomKerekes's Avatar
    Join Date
    May 2006

    Default Re: Watchdog FET

    Hi itme,

    That sounds reasonable to me. I didn't see a current specification for the green power on signal either but i would expect it to be only a few milliamps. To be sure GND it through something like a 100 ohm resistor, check if the supply comes on, and measure the voltage across the resistor. Current = V / R.

    The SSR looks to be plenty adaquate to me.


  3. #3
    Gold Member john-100's Avatar
    Join Date
    Mar 2007

    Default Re: Watchdog FET

    the green ATX power ON wire will be connected to +3.3V via a resistance of about 15000 ohms

    so the PC motherboard power on or Watchdog FET in your case only needs to pass about 200uA to ground (black wire)

    you can measure the current by using a meter set to 2mA DC and using it to connect the green power ON wire to a black ground wire

    depending on your ATX power supply and the current taken by the Kflop/kanalog
    you may need to connect some wire wound resistors to load the 5 & 12V outputs
    not all power supplies will regulate the outputs correctly without a minimum load on the 5V & 12V outputs

    ( various proportions of the outputs are used to make the control voltage used to adjust the power supply outputs
    the + 3.3 ,+ 5 & +12V outputs should be within 5%
    the [-5v on old psu's] & -12V outputs within 10% )

    Check the +3.3v , +5V & +12V outputs are correct wih your meter if they are out of specification -
    you may need to add load resistors to improve the regulation

    for the 5V output put a load resistor between the red +5v & black ground

    4.7ohm 6W will pass 1.06 A
    2.2 ohm 12W will pass 2.7A

    for the 12V output put a load resistor between the yellow +12V & black ground

    12ohm 12W resistor will pass 1A
    6.2ohm 25W resistor will pass 1.9A

    the resistors will run hot and you may want to increase the wattage of the resistors

    for example
    replace the 4.7 ohm 6W resistor with a 4.7 ohm 12W resistor
    and replace the 12ohm 12W resistor with a 12 ohm 25W resistor


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