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PWM lead screw drive info

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  • PWM lead screw drive info

    See here:

    http://vts.bc.ca/workshop/pwm/pwm.htm
    Free software for calculating bolt circles and similar: Click Here

  • #2
    Well done.

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

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    • #3
      Thank you very much Evan.

      Comment


      • #4
        Great job Evan! That is just the circuit I need to upgrade the drive on my homemade sinker edm. Thanks for posting it!

        Jim
        Jim Koper
        J&R Machining

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        • #5
          Thanks guys. I left out something important in the construction notes which I have now added:

          IMPORTANT NOTE: The FET mounting tabs are at drain potential. Either the devices must be mounted with insulating spacers and washers or the entire heat sink must be insulated from the chassis.
          Free software for calculating bolt circles and similar: Click Here

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          • #6
            Noted.

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

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            • #7
              Evan,
              Thank you for taking the time put together the info, much appreciated. All the best.

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              • #8
                I shouldn't have been suprised! But I was.

                Evan, your publishing is as nice and high quality as your machine work!!

                Thank you.
                uute

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                • #9
                  Thanks for putting that together for us, Evan. I wish I still had the urge to do projects like that. Maybe my enthusiasm for electronic projects will return- this is the exact project I need for my tadpole, a control circuit for the motor. I will have to go multiple fets for sure, as you mentioned. Probably six 50 amp devices. I'll have to pay attention to the combined gate capacitance as far as switching times and the available output current from the timer chip.

                  I got a chuckle out of seeing those snubber parts- I don't think I've seen an old style carbon resistor for years now. Rescued from a certain death in the 'assorted parts' drawer-
                  I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                  • #10
                    thanks very much Evan - nice job.

                    I've a long way to go on the learning curve and wonder if you could answer a couple of questions?

                    if you moved to an irf740 (400V 10 amps) wouldn't that work for the treadmill motor?, they are also cheap and common. you mention a dozen ganged up but do you think one would one suffice?

                    I understand that the first FET, when on, allows current to flow through the motor to common. but what does the second do? it seems connected to itself with an rc in the middle

                    Is there a way to get the shematic in a higher resoltion?

                    thanks again
                    .

                    Comment


                    • #11
                      Nice job Evan!

                      Darryl, I have a Motor Guide Pulse Width 50 footlbs thrust trolling motor that "may" work in your application. They are a bit pricey but it already comes with the foot control for gas pedal!!

                      This motor will produce enough torque to damn near put you in the drink if it's in reverse and you're leaning way over the bow when somebody punches it
                      - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
                      Thank you to our families of soldiers, many of whom have given so much more then the rest of us for the Freedom we enjoy.

                      It is true, there is nothing free about freedom, don't be so quick to give it away.

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                      • #12
                        <font face="Verdana, Arial" size="2">JT,

                        I cobbled up the circuit on a breadboard first and swept the frequency to see how the motor behaved. It really lost torque at higher frequencies. I have no specs on it so I went with what worked best. I also am working with one hand tied behind my back as my good triggered sweep scope is down right now (until I repair it). All I have for a working oscilliscope is an old POS RCA educational kit I picked up at a garage sale. It's good for at most low sonic frequencies. I guess I should fix the good one.
                        </font>
                        The 555 is somewhat limited in current.... At higher frequencies the 0.1 uf (C8) will load the output enough that it may not fully turn-on the fets. (The fets have a low gate charge requirement, so that is likely not the problem.)

                        The R-C time constant of C8 and R11 is good to around 700 Hz, so much higher than that and you will no longer be discharging C8 before the next pulse. Maybe lower than that depending on duty cycle.

                        If you get rid of C8, or drastically lower it, you should be able to work to higher frequencies. It shouldn't be required.

                        That circuit depends on R11 do discharge both the gates and C8 before the next pulse. A more robust driver would be able to pull down the gate fast and allow higher frequency operation.

                        Even a number of paralleled sections of CMOS inverter would help. I have seen a number of drives use that, although a "real" gate driver has more drive. You don't need a lot with 20 nC of gate charge, the inverters would work, especially at a few kHz.

                        You could still isolate the PWM portion from teh inverter using a diode and resistor as now, and also a diode for its power supply. That would give you the drive and the isolation, both.

                        A higher frequency could reduce vibration a lot, and also move the noise out of the audible range.

                        The motor shouldn't be a limit. Remember, VFDs do use higher frequencies, up to and above 20 kHz, and they have relatively high inductance motors to run.
                        1601

                        Keep eye on ball.
                        Hashim Khan

                        Comment


                        • #13
                          Mcgyver,

                          The FETs are in parallel if you study the circuit. FETs have a positive temperature coefficient so that as they warm the on resistance goes up. As long as they are on the same heat sink this means they share load automatically.

                          Right clik the link below the schematic image and save it to your computer for the high res schematic.

                          JT,

                          The 555 can source and sink about 200 ma at 12 volts. Of course in this application it is only sourcing but it should be enough to very firmly put the FETs into full conduction as the 720 exhibits full turn on at about 8 volts on the gate at full current. For higher frequencies the value of R8 can be reduced considerably and C11 reduced as well. I like to keep a bit of time constant as the 555 has a turn on/off time on the output of only 100ns. Slowing that down a bit will reduce the chance of ringing in the gate circuit as well as soften the motor pulses and noise.

                          If more drive is needed then the 555s can be operated at 15 volts.

                          [This message has been edited by Evan (edited 10-21-2005).]
                          Free software for calculating bolt circles and similar: Click Here

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                          • #14
                            [QUOTE]Originally posted by Mcgyver:
                            [B]thanks very much Evan - nice job.
                            if you moved to an irf740 (400V 10 amps) wouldn't that work for the treadmill motor?, they are also cheap and common. you mention a dozen ganged up but do you think one would one suffice?
                            I understand that the first FET, when on, allows current to flow through the motor to common. but what does the second do? it seems connected to itself with an rc in the middle

                            Is there a way to get the shematic in a higher resoltion? QUOTE

                            There is a download link for the larger resolution schematic just below the schematic itself. The result prints nicely on an 8x10 sheet. The FETS are paralleled for increased current capacity. Nice thing about power fets is that they can be paralleled without any concern over one device hogging all the current. They play together nicely and linearly increase current capacity with number though there are some minor limits.
                            I have driven an "18amp" treadmill motor from 110vac with two paralleled IRF740 FETs with no temperature rise in the FETS, until the uncooled motor got too hot to touch. Speed control was smooth over the range of the DC motor controller I used (a www.mpja.com
                            6067-kt dc motor controller with a similar ckt to Evan's sold originally with a FET now with a power darlington. Just sub the FETs of choice.)
                            Steve

                            Steve

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                            • #15
                              Careful with that spec.....


                              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. 12-8=4, and at 4 volts drop, it can only supply a lower current, which isn't specified.

                              I have reason to think that may be a misprint in this copy of the catalog, but there is some drop, and the pull-up is not as strong. And, teh current data sheet at National's website also shows the same data. And their graph vs tabular data are inconsistent.

                              However, it isn't unusual for pull-up to be weaker......

                              So you probably won't get as strong a pull-up as you might expect.

                              The multi-inverter drive is cheap and very effetive.
                              1601

                              Keep eye on ball.
                              Hashim Khan

                              Comment

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