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  • #6089
    Solideo
    Participant

    Ciao,
    I have a question / request, it is possible to limit the output current from a power supply without modifying the latter.
    I would like to use an atx power supply, but also to be able to adjust the output current from the 12v and 5v line.
    E’ a crazy request or it is possible.
    Thank you.

    #6091
    Amilcare
    Keymaster

    It is possible to limit the current simply by stopping the flow when a predetermined threshold is reached. To do this you have to take into account the loss of a few tens of mV to measure the absorbed current and for the semiconductor that physically interrupts the voltage once an excessive absorption is detected.
    In practice, when the threshold is exceeded, the power supply transforms into a current generator that limits the output voltage so as to have the maximum current set. The ohm law cannot be circumvented V = R * I and if you consider the current to decrease of R constant (load), the tension will act accordingly.

    • This reply was modified 3 years, 4 months ago by Amilcare.
    • This reply was modified 3 years, 4 months ago by Amilcare.
    #6096
    Solideo
    Participant

    Excuse the ignorance.
    but the semiconductor must withstand the maximum current I intend to make it supply ?

    #6098
    Amilcare
    Keymaster

    It is advisable to use a mosfet with its low conduction resistance causes a minimum voltage drop while the resistance used as a current sensor is the cause of the rest of the total voltage drop.
    If you have both 12V and 5V available at the same time, it is better to use the first voltage to drive said mosfets to have a smaller conduction resistance.
    The rest of the circuit is simple, an operational used as a comparator in which in one input has the tripping voltage of the other in the other the drop voltage in the current sensor resistance and the output to drive the mosfet.
    If you give me more specifications than the currents in play I can be more precise

    #6102
    Amilcare
    Keymaster

    This is the 5V section but it is identical to the 12V section so I didn't draw it. The resistances from 0,1 are from 5W in principle each resistance leads to an increase in the maximum current of approximately 4.5A.
    For the operational it is not critical a single LM358 is fine or similar for both sections. the -5V line corresponds to the white wire of the ATX, it being understood that the green wire must be connected to ground to turn on the ATX itself.

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    #6104
    Solideo
    Participant

    First, a THANK is only right.
    I'm happy because’ I too had thought of a comparator, but clearly I certainly don't have the knowledge to bring it back to a circuit.
    And now we come to a few trivial questions, but I have to do them.
    If I understand correctly, each sense resistance increases the maximum of the adjustable current by 4.5 A ,honestly I consider it too high as a maximum threshold, I thought more of a value of 2 maximum 3 A, so I think a resistance of 150 mOHM should be fine.
    The second question is about resistance from 470 Ohm in series with the 4k7 and the potentiometer, if it is not asking too much, because'?

    #6109
    Amilcare
    Keymaster

    Put 0,1 and adjust the current to the desired value be it 1A or 3A or any intermediate value little changes.
    The resistors in series with the potentiometer are used to guarantee the current excursion from a few mA to the desired value up to the max for each resistance of approximately 4.5A. The calculation method is to have a voltage lower than the inverting voltage on the non-inverting input until the power resistance with its voltage drop causes the voltage on the inverting pin to drop below the threshold does not invert and bring the output of the operational level one.
    At this point the mosfet stops conducting and the current drops. When the latter drops, the cause of the change of state also ceases, bringing the mosfet back into operation. In a continuous cycle that adjusts the max current to the desired value.
    Practically only the ohm law is used to set the current limits, do the calculations with the potentiometer at the two extremes and you will see that then it becomes easy to understand the meaning of the resistances.

    • This reply was modified 3 years, 4 months ago by Amilcare.
    • This reply was modified 3 years, 4 months ago by Amilcare.
    #6117
    Amilcare
    Keymaster

    correct Scheme, for my mistake, an N-channel mosfet had been inserted instead of P

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    see source fidocadj Zoom fidocad
    • This reply was modified 3 years, 4 months ago by Amilcare.
    • This reply was modified 3 years, 4 months ago by Amilcare.
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