Modifed Power Supply

Discussion in 'General Electronics Chat' started by howied42, Oct 11, 2016.

  1. howied42

    Thread Starter New Member

    Oct 11, 2016
    I'm attempting to convert a Mac power supply 18V 6 Amp power supply to push out full Voltage/Current to power a power tool.

    The problem i'm having is, I'm able to get the power supply to output full current after the first time it's plugged in. The second attempt at pulling power from it fails.

    It appears to have a current sensing circuit that drops gates the current after the initial draw drops to 0 and doesn't come back.

    There is 3'rd wire in the cable that i've found is the sensing wire. I'm using a 10k resistor and .1uf capacitor to fool the circuit initially to get full voltage, but after the first pull of the power tool it doesn't allow another.

    I'm thinking it might need some kind of constant load even if the tool is not pulling to keep the sensing gate always open but i don't know what that would look like.

    Any help would be appreciated, and if i haven't explained the problem well please let me know and i'll try to answer the best I can.

  2. AlbertHall

    Distinguished Member

    Jun 4, 2014
    PC and MAC power supplies are very well protected against almost all possibilities. If anything bad happens they shut down until power cycled. It sounds like you may be hitting one of the protections, maybe the current limit.
    However, I suspect it may be the overvoltage or undervoltage protection which trips if the output voltage is too high or too low. Your power tool is an inductive load and when you switch it off it will generate a back emf spike when switched off which will be fed back into the power supply. If that is what's happening it may be quite difficult to eliminate.
  3. howied42

    Thread Starter New Member

    Oct 11, 2016
    Thank you very much for your quick reply. If it's an EMF spike there isn't much i can do. Below is some more details.

    I'm definitely not going over voltage or over current. For the moment i'm attempting to power a "Cordless drill". When i slowly press the button for power it gets up to full speed. After i slowly release the button the drill stops. The next time i slowly depress the button sometimes it starts again for a short moment then quickly stops or it doesn't work at all.

    I feel like if the power supply still retains a minor load after the drill stops looking for power it should keep the sensing circuit engaged. I just don't know what that part of the circuit would look like.