Power Supply rated at 1.5A shows 2.9A on multimeter

Discussion in 'General Electronics Chat' started by michaelbails, Feb 24, 2012.

  1. michaelbails

    Thread Starter New Member

    Aug 2, 2011
    1
    0
    Hi,

    I have a 24V, 1.5A output power supply. This is driving a stepper motor with some control circuitry inside a box. The motor is a 2-phase stepper motor with max 2A per phase (as per the datasheet). There are 4 wires coming from the stepper motor (A+, A-, B+, B- labels). I took one of the phase wires and ran it through a digital multimeter to measure the current and it shows the DC current max as 2.9590A and the min as -2.9700A. Can someone tell me how this is possible? I though a 1.5A power supply will allow at most 1.5A in a circuit.

    Thanks for your help.

    P.S. we tried the same test with a 24V 3.0A power supply and got identical results as the 1.5A supply.

    Best,

    Mike
     
  2. #12

    Expert

    Nov 30, 2010
    16,257
    6,757
    Power supplies aren't guaranteed to stop at exactly the rated amps. They are guaranteed to supply at least the labeled current.

    What you are measuring is 24 volts divided by the resistance of the motor winding at DC conditions..which the motor wasn't designed to do.
     
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  3. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    The motor driver is a switcher so the voltage in is much higher than the voltage out--the same ratio also applies to output /input current so the motor phase current can easily exceed the power supply current.
     
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  4. K7GUH

    Member

    Jan 28, 2011
    191
    23
    Power supplies will put out whatever current you draw from them, until the blue smoke escapes. Then they take up a career as paper weights.
     
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  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,769
    969
    This is why a fuse is always recommended and should be used... The fuse should prevent your supply from going up in flames when abused.
     
  6. Lundwall_Paul

    Member

    Oct 18, 2011
    220
    19
    If you really want to characterize the supply I would do a load regulation test.
    Connect the supply to an ELoad (or a resistor bank) Set to desired voltage 24V in your case.
    Monitor the voltage output and the current slowly increment the load up at some point while increasing the load you will see the 24 volts starting to drop. When it drops you have reached the true max current rating of the supply. If your meter drops from 24 volt to 23.95 that’s it. It is no longer regulating.


    But I would be cautious by not exceeding the manufacturing ratings. Manufactures typically design the supply above the rated output to guarantee the desired output current.
     
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