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PWM lead screw drive info

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  • #16
    "The National spec shows that for pull-UP, the 200 mA corresponds to typical 12.5V DROP on the output. In other words with a 12V supply, it can provide that 200 mA only into a short."

    My understanding (which agrees with the graph) is that is what the output drops to, not how much it drops. That also agrees with my experience.
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    • #17
      was that hi-res link there before or am I really that blind thanks guys for the help. i'll continue the study until the ball drops on the fets in parallel

      I actually surprised myself by designing one that was very similair to this and almost worked, but with no transformer and a bigger cap. used a dual 555 to drive a 740. worked pefectly on a light bulb load but would blow up with the motor. only recently discover i wasn't using fast a enough diode - i'd used a 4001 iirc instead of the Schottky

      anyway, thx, the design as encouraged my to try again
      .

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      • #18
        I just added a bit more info to the web page and am posting it here as well.

        ----------------------------------
        Scrounging Parts

        Dead computer power supplies are a good source of parts for a project such as this. In particular, heat sinks and diodes as well as mounting hardware are easily obtained. Just go to your nearest computer repair shop and ask for a few dead power supplies, I am sure they will be happy to give you some.

        Here is an example of what is often available in a dead supply. These parts will not usually be damaged. Especially useful is the dual schottky diode in this picture. It is rated at 2x15 amps at 100 volts DC.



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        • #19
          Those old power supplies for parts? Just make sure you can and know how to discharge the filter caps. Man that hurts.

          ------------------
          Gene
          Gene

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          • #20
            Nice looking project! I noticed the mid-project workbench behind the lathe has been blurred out.

            I think I noticed a typo on the output parts list:
            R3, R4 100 ohm gate resistor for FET. See note 1.
            R3 1k, 1/2 watt

            Is the second R3 supposed to be R5?

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            • #21
              Yep, typo. Thanks, I have fixed it.
              Free software for calculating bolt circles and similar: Click Here

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              • #22
                <font face="Verdana, Arial" size="2">Originally posted by Evan:
                "The National spec shows that for pull-UP, the 200 mA corresponds to typical 12.5V DROP on the output. In other words with a 12V supply, it can provide that 200 mA only into a short."

                My understanding (which agrees with the graph) is that is what the output drops to, not how much it drops. That also agrees with my experience.
                </font>

                That's why I mentioned that I have reason to doubt the actual words used in the data sheet description ....

                In any case, a driver with bidirectional drive would reduce mosfet dissipation and allow you to reach higher frequencies easily without reducing the resistor to impractical values.

                And you could still maintain your diode isolation at a point prior to the driver.

                For higher voltage drives as has been asked about, an optocoupled gate driver such as mentioned in teh original thread, will allow control via a control circuit which is at low voltage and NOT directly connected to the line voltage (for 90V DC motors, etc). Optos with UL /VDE isolation and the requisite speed are available.

                A transformer also will do it, but has a serious problem with wide range PWM unless a lot of trouble is taken. The opto has no such problems.
                1601

                Keep eye on ball.
                Hashim Khan

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                • #23
                  Yes, if I were going to drive at a higher voltage I would use an opto-iso. They are cheap by the bagfull and fast. As far a xformers for this sort of application, that is beyond my level of expertise to design.
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                  • #24
                    Thanks for taking the time to put that together Evan. I have a few projects in mind that will require controlling motor speed and I may try my hand at some electronics work.
                    Techno-Anarchist

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                    • #25
                      Just picked up MEW 110 today from the newsagents.
                      In there is an article By Peter Rawlinson who goes on the describe a power feed for the tailstock as hes'got problems with his hands,

                      He goes about this slightly different to Evan in that he uses a stepper motor and driver to achieve the motion.
                      Using this method he gets forward and reverse, variable speed and jog.

                      By playing with the step multiplier on the stepper driver and the pot you can get a very wide range of speeds.
                      Going to the extreme Peter found that he could get 1 motor rev in two minutes, hardly viable but it shows what is achievable.

                      Sir John .
                      .

                      Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



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                      • #26
                        And I just got MEW 109 last week. Must come by container via Suez..
                        Steve
                        Steve

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                        • #27
                          John,

                          I am trying to imagine using a power feed on the tailstock. You do mean the quill feed, right? I can see it for some drilling jobs but not others. I change tooling so often in the tailstock it seems that it would be in the way a lot.
                          Free software for calculating bolt circles and similar: Click Here

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                          • #28
                            Evan,
                            This was done on a Myford that had the capstan handle conversion fitted.
                            These have a thru hole so you can drift tooling out without having to wind back to auto knock out.
                            The guy was saying that with his hand problems he couldn't press on anything with his palms hence the conversion.

                            I have often thought that it might be nice to have power feed on a tailstock on the larger lathes as punching a 3" drill thru takes a bit of grunt but I don't know without doing it how useful it would be as a fixed attachment.

                            Some lathes have a limk that can fit between the carriage and tailstock to allow the carriage to tow the tailstock but we are getting off the subject.

                            The purpose of mentioning the article was to bring attention to the use of stepper motors to the same application.
                            This is only then one step away from CNC threading with a single slot opto switch on the spindle.

                            John S.
                            .

                            Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



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