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  • Help, I need some DC power supply help

    I just read Mcostello's post on his DC motor problem and you all solved his problem fast.
    I have a HD 24-4.8 A power supply that is suppose to put out 24 VDC at 4.8 amps. I want to use it to power a 24vdc motor on the table drive unit I built for my little Griz X3 mill. When I hooked it up today for it's maiden voyage it would not run the motor. On checking the voltage with my tester is only shows 2.4 volts. I know the tester works ok because it shows the correct voltage on the other 31 vdc power supply that I have.
    Why would the power supply only put out 10% of it's rated voltage?
    Thanks
    Mel
    _____________________________________________

    I would rather have tools that I never use, than not have a tool I need.

  • #2
    One possible reason is "foldback limiting". The load is too low a resistance and is trying to draw too much current. It may be a short in the motor, or the motor may be stalled for some reason. A DC Motor draws maximum current at stall. There may be a sensing circuit in the output of the supply that senses this and limits voltage / current of the supply to keep it from being damaged. Measure the resistance of the load (motor) and using Ohm's Law figure out how much current it is trying to draw at the rated output voltage of the supply. If that is a very large number, you are into foldback limiting and just about to let all the magic smoke out of the supply.

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    • #3
      Try it on a 200W or higher (up to 500W) light bulb. Doesn't matter that they're 110 VAC, it will put a load on the power supply just fine. A 500W bulb will draw 4.5A max so won't overload your circuit.

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      • #4
        Originally posted by dp
        Try it on a 200W or higher (up to 500W) light bulb. Doesn't matter that they're 110 VAC, it will put a load on the power supply just fine. A 500W bulb will draw 4.5A max so won't overload your circuit.
        It'll draw a lot more than that at 24 volts. Have you ever measured
        the cold resistance of a tungsten bulb? :-)
        ...lew...

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        • #5
          Try a pair of tail light bulbs in series for a start. If that works then a pair of headlights in series will draw 4 amps.
          Free software for calculating bolt circles and similar: Click Here

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          • #6
            Mel,

            I wish you the best of luck on this, as you know I am DC impaired.

            Jay
            "Just build it and be done"

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            • #7
              I would measure the open circuit voltage first to see that you are getting 24 volts out. If okay, I would then hook it up to the motor (unloaded).

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              • #8
                I know so little about this stuff that I really shouldn't be messing with it. But I do have a volt meter and it only shows 2.4 volts directly out of the supply unit. This is the unit I'm trying to use.
                Mel
                _____________________________________________

                I would rather have tools that I never use, than not have a tool I need.

                Comment


                • #9
                  DC motor and power supply

                  I agree, the stalled or stopped motor is exxceeding the rated output of the power supply thus locking it down. Your might try to hand start some rotation then kick in the power supply.

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                  • #10
                    That looks like a pretty old supply. I am assuming it is reading 2.4 vdc with no load. Since it is using three 2n3055s in parallel to regulate the load I suspect it isn't very sophisticated. One quick check to do is to measure from the case of any of those three transistors on the front to the panel they are mounted on. Based on the scratch next to the lower left one somebody has already done that. You should see a voltage in excess of 24 volts dc. If not then the supply is toasted.
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                    • #11
                      Thanks Evan, And I think your right, It's toast. I bought it at a recycle place that sells a lot of this kind of stuff and I only paid $15 for it. I will find another one later. With no more than I know about this sort of stuff I would be better off forgetting it. That scratch on the front came from a arc that one of the output wires made after the unit was unplugged and setting on the bench. No power to the unit. scarred the hell out me. Was not expecting like that happening. See I know nothing. One more thing: I hooked up a old HP printer transformer/power supply to the motor and it works fine. 31Vdc but it runs, for how long ????
                      Thanks for your help guys
                      Mel
                      _____________________________________________

                      I would rather have tools that I never use, than not have a tool I need.

                      Comment


                      • #12
                        Lugnut,
                        Turn the power supply over. There should be four terminals where you hook up the output wires. There should be V+, V-, S+ and S-. Make sure that the S+ is connected to V+ and the S- is connected to V-. These are used to sense and control the output voltage, and are usually just jumpered right at the supply. If a long length of wire is used between the supply and the load, a voltage drop may occur. If necessary a second pair of wires are run from the sense terminals (local jumpers removed) and connected to the V+ and V- leads at the load. That way the load actually gets the rated supply voltage. The voltage at the supply will be somewhat higher to compensate for the voltage drop between the supply and the load.

                        'Course, maybe it is toast already, but check the jumpers. These linear supplies can take a lot of abuse.
                        Last edited by Weston Bye; 03-21-2009, 07:10 PM.
                        Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
                        ~Practitioner of the Electromechanical Arts~

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                        • #13
                          Running a 24 volt motor on 31 volts won't hurt it unless it is running at maximum load for an extended time. I run my 12 volt bike motors at 24 volts and I have only burned out 1 so far. I wasn't the voltage that did it though.

                          FYI, if you pulled a strong arc from the transistor case after turning it off I suspect the supply is at least working well enough to power the motor. Try hooking up the motor to the round case one of one of those 2N3055 transistors and the other wire to the panel itself. Note that there is an insulator between each of those transistors and the panel. Don't mess it up. The screws holding the transistors to the panel may have regular nuts on the other side or they may go into tinnerman or similar nuts so if it works you might have to open the case so you can loosen one of the mounting screws to make a permanent connection. When the supply is on if it is working on the unregulated side those transistors will have as much as 30 volts on each of the round top cases. Don't short them to the panel.
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                          • #14
                            I'm not real sure what your telling me Evan but I will check it out tomorrow. Here is a couple more photos of the inside of the supply.



                            thanks for trying to help
                            Mel
                            _____________________________________________

                            I would rather have tools that I never use, than not have a tool I need.

                            Comment


                            • #15
                              I must not be explaining it well enough. Here is a picture. It's really simple. All this does is bypass the part of the power supply where it may have failed. The failed part, if it is as I think, poses no hazard.


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