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  • DC motor power supply/speed control

    Please assume I'm naive and ignorant when it comes to many things e-leck-trickle because it's mostly true.

    I have a couple milk crates up in the attic full of various PC power supplies I've accumulated here & there. There's several types ranging from XT/AT to the ones that came out of 386/486, PI, II & III's.

    I also have a few small 12V DC motors that came out of auto window regs., elec. park brake actuators and the like. I haven't measured the current draw under load but just for argument's sake let's say it's 5A max.

    My questions:
    A) Will a PC power supply produce 5A @ 12V for intermittent use, like for a power crossfeed on a small lathe or a power feed on a small mill?

    B) Is it possible to hack some type of inexpensive speed control device or does anyone know of a simple design to build one cheap?

    This is a back-burner project but I thought I'd axe you fellers since there seems to be no shortage of really brilliant electrical wizards on board here.

    Thanks for looking.
    Milton

    "Accuracy is the sum total of your compensating mistakes."

    "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

  • #2
    Most if not all computer power supplies MUST have a certain minimum
    load on the 5 Volt output to even run.
    Now as to the current available on the 12 V output that is a big
    variable. I havent checked those supplies in quite a few years but
    it should be marked on the sticker on the supply.
    Had one I just salvaged on my desk up till yesterday but took it to
    the shed so can't look easily.
    ...lew...

    Comment


    • #3
      You should be able to get 5A out of one but you'd have to check the rating label. Mine are out in the barn so I can't look at one either. It doesn't take much to get one to power-up. In most cases, all it takes is grounding one of the leads. If a 5v lead needs a load, add a pilot light. You can also combine the output of various voltage levels to get a range of volts. Like: 5, 7, 10, 12, etc. If you could live with stepped speeds on a motor instead of variable, a rotary switch will work.

      Comment


      • #4
        These are almost invariably switching type power supplies with tightly regulated outputs. They are not going to be variable, as would be required for any kind of speed control. They are good for running fairly stable electronic loads, and little else. Too specialized.

        Comment


        • #5
          a lite dimmer switch makes a cheapo speed control dumps out chopped ac so if you stuff it thru a bridge rectifer its kinda passable

          Comment


          • #6
            No way a dimmer switch should be anywhere near a switchmode supply, or a DC output.

            Only way that will work is on a transformer primary.
            Just got my head together
            now my body's falling apart

            Comment


            • #7
              A window motor system is designed for very intermittent use, and will draw a lot of current during the short time it's operating the window. That can range up to about 30 amps for some of the older types. If they have wound fields, don't bother with those. If they are permanent magnet, some of those will draw up to about 20 amps- then there are the more efficient types which can actually run a window up or down with 10 amps or less average. There are many of these types on the surplus market, but you might want to make sure you're going home with the suitable ones. I would want to do a test before leaving the store with it- if it doesn't draw less than about 2 amps with 12 volts and no load, it probably won't be suitable for a leadscrew application. Even if your power supply can run it, you might find it overheating if it runs for very long at a time. Most of the more modern ones should be fine, incxluding rear window wiper motors and sunroof motors.

              I don't know of any older computer power supplies which are capable of delivering the higher levels of current on the 12v output. I think an AT450 will, though. I'd have to check the specs.

              To run a leadscrew may not require as much current as winding a window, and if not, then the motor won't draw as much, as long as it's not saddled with high internal friction. The no-load current draw will tell the story there, so that becomes about the most significant test you can do to determine the motors suitability. One of the stores I go into has a 12 power supply with voltage and current meters on it, and they are happy enough to do such a test for me.

              Another factor is speed control, obviously. There is external sensing using hall effect sensors, photo interrupters, or pick-up coils, then there is direct motor speed control using a feedback circuit as is done with tape decks and some turntables. I have not yet seen this latter type of circuit used with higher current motors, so there may be some problems involved, possibly to do with controlling over a wide range of speeds. This is something you would want, so it might be best to go with one of the external sensing systems. I'd be interested to hear what others think or know about getting precise speed control from these low voltage, high current dc motors. Simple can be good, and is usually more reliable, but it has to work as required so that might take an electronic package that's a little more sophisticated. The final current pass element has to be able to handle a lot of current of course, and the speeds kept under control even as the load may vary from light to heavy. Can someone point to a suitable circuit?
              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

              Comment


              • #8
                There are many useful scrapyard motors.
                Continuous duty include wiper motors (with gearbox)
                heater fan motors.....
                Just got my head together
                now my body's falling apart

                Comment


                • #9
                  Thanks for the input everyone. If the PC power supplies aren't powerful enough, I suppose I could use a car battery with a trickle charger as the power source which leaves me needing a gearbox and a speed controller. I've looked around on the web this morning and found a few treadmill motor controllers pretty cheap but they're up around 90 - 120 vdc.

                  Seems to me that with the plethora of r/c car speed controllers and variable speed cordless drills on the market these days, there oughta be some surplus chips & circuits posted on the web somewhere for frugal sorts like us to build sumthin' usable for very little cash outlay.
                  Milton

                  "Accuracy is the sum total of your compensating mistakes."

                  "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

                  Comment


                  • #10
                    DB, what are you trying to drive?
                    Would help if we had some idea what you want to do.
                    Just got my head together
                    now my body's falling apart

                    Comment


                    • #11
                      Originally posted by DICKEYBIRD
                      Please assume I'm naive and ignorant when it comes to many things e-leck-trickle because it's mostly true.

                      I have a couple milk crates up in the attic full of various PC power supplies I've accumulated here & there. There's several types ranging from XT/AT to the ones that came out of 386/486, PI, II & III's.

                      I also have a few small 12V DC motors that came out of auto window regs., elec. park brake actuators and the like. I haven't measured the current draw under load but just for argument's sake let's say it's 5A max.

                      My questions:
                      A) Will a PC power supply produce 5A @ 12V for intermittent use, like for a power crossfeed on a small lathe or a power feed on a small mill?

                      B) Is it possible to hack some type of inexpensive speed control device or does anyone know of a simple design to build one cheap?

                      This is a back-burner project but I thought I'd axe you fellers since there seems to be no shortage of really brilliant electrical wizards on board here.

                      Thanks for looking.
                      Hi,

                      Funny you should ask. I'm no wizard but I do have a Ham license and some background as a technical writer. One of my hobbies is model railroading. I have a "G" Scale (1/2" to the foot) model railroad that runs around the wall of my barn on the second floor. I needed a power supply to run two locomotives on separate tracks at the same time. Some folks pay as much as $500 for a fancy dual "power pack" with a fancy case. The link shows you a schematic that will give 1.2 to 30 VDC.

                      I got the attached schematic and parts list link from the internet about 5 years ago and decided to build a power supply from it. It's based on the LM338K Power Regulator, an inexpensive unit available from electronic surplus shops. Radio Shack used to have them for about $2.00 but they have gone "Botique" in the past few years and their open stock for electronic parts has been suppressed.

                      That being the case, I went to a larger electronics surplus store in Sacramento and got everything I needed at once. I built a dual pack with an "A" and a "B" side. The design puts out a nice, stable DC output which is variable from 1.2 to 30 VDC and will give up to 5 amps. I had some left over ammeters and voltmeters from a prior model railroad so I integrated those into the top of the case with the on-off switches, variable power knobs and some power lights, etc.

                      The parts for a dual pack cost me about $30 with all of the capacitors, two transformers, some bridges, resistors and a metal case to hold it all.

                      The only negative issue (no pun intended) is that the LM338K's start at 1.2 Volts. That caused the locomotoves to "crawl" even when the power was set all the way down. This was cured with an external ceramic resistor on each side to soak up the excess voltage and bring the idle voltage to 0 (zero). This unit has been used quite a bit in the past five years and has proven to be absolutely stable. The signal is almost perfectly flat DC. I was even able to tap off for a separate 12 VDC, 5 amp fixed supply to operate switch machines which were also homemade from small DC slot car motors and two diodes, also surplus.

                      One of these supplies will easily operate your motor at up to 5 amps with variable voltage up to around 25-30 VDC depending on your transformers. No charge for this info.

                      http://www.aaroncake.net/Circuits/supply.asp
                      Last edited by gnm109; 06-05-2008, 10:41 AM.

                      Comment


                      • #12
                        Now we're getting somewhere! That looks simple enough for even me to understand. I'll have to do some checking but as others have said, 5A may not be enough but I'll bet there are other similar circuits that are more powerful but still in the range of my cheap-o budget.

                        Swarf my plan is to try to build a power cross-feed for my little Taiwanese 8x16 Emco clone. If it works, I'd like one for the X-axis on my X3 mill as well.
                        Milton

                        "Accuracy is the sum total of your compensating mistakes."

                        "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

                        Comment


                        • #13
                          http://customers.swcp.com/~tandberg/...pplyATX_03.htm is just one of a raft of similar mods to
                          convert comp PS to general use. I did this several years ago to an
                          old PS which was rated at 5V 20A and 12V 12A. By not loading the
                          5v buss down, the 12v rail should be good for at least 10A. There
                          are circuit board pots inside the supplies that are used to adjust
                          output voltages +/- 10%. Such supplies are very well regulated
                          and make good (but NON isolated from the AC line) power supplies.

                          PWM motor controllers are available in kit form for $12-20 and can
                          be used to drive the motor. By subbing different FET outputs
                          with appropriate heat sinks you can get current/voltage ratings
                          from 30V/50A to 400V/10A. www.mpja.com has a motor controller
                          kit but a google on 555 or 556 PWM motor controller will bring up a
                          bunch of writeups.

                          Evan published a motor control ckt and pictorial for a lead screw
                          driver for his SB9 1-2yrs ago but it didn't pop up on a cursory
                          search. A search on Evan should turn up his website where this
                          was stored. It did pop up a thread showing his cross feed power
                          drive for the SB9. Some of Evan's posts give his website.
                          Last edited by sch; 06-05-2008, 11:02 AM.
                          Steve

                          Comment


                          • #14
                            Dickeybird,

                            Here's the link to Evan's PWM motor controller that sch was talking about:

                            http://vts.bc.ca/metalshop/pwm/pwm.htm

                            Comment


                            • #15
                              GREAT stuff guys; thank you very much!
                              Milton

                              "Accuracy is the sum total of your compensating mistakes."

                              "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

                              Comment

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