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  • still cant find power supply

    i still am looking for my cnc mill ,but with no luck does anyone have a 40-70 volt 5 amp dc regulated power supply for sale
    thanks

  • #2
    How's this one?
    Todd

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    • #3
      i was pricing out $100 if the power supply says 8 amp output can i step that down to 5 amp

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      • #4
        You dont step down the current. The drives will only pull what they are limited too. A bit more on the current side is not a bad thing. Just means you can drive a fan or two or similar accessories. JRouche

        Also you prolly want a linear supply not regulated? I may be off here....


        http://www.industrialhobbies.com/Mer...ode=PWR_48V_5A
        Last edited by JRouche; 07-21-2006, 11:04 PM.
        My old yahoo group. Bridgeport Mill Group

        https://groups.yahoo.com/neo/groups/...port_mill/info

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        • #5
          For steppers an unregulated supply is recommended. Regulated supplies can't provide enough instantaneous current.
          Free software for calculating bolt circles and similar: Click Here

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          • #6
            You don't need to "step down" the supply's current capacity; the load will draw whatever it requires, and no more.
            Todd

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            • #7
              http://www.plitron.com/ for custom toroid transformers or parts express for off the shelf. www.partsexpress

              capacitor and bridge rectifier and your'e good to go.

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              • #8
                These are not very hard to build. Here is some data on the one I'm building:

                http://www.thewarfields.com/MTLatheCNCDrivers.htm

                The page has parts info, suppliers, as well as pointers to where to get the data to design one that suits your needs. Make sure you have your steppers (or servos) before you dig in as their voltage requirements vary.

                Best,

                BW
                ---------------------------------------------------

                http://www.cnccookbook.com/index.htm
                Try G-Wizard Machinist's Calculator for free:
                http://www.cnccookbook.com/CCGWizard.html

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                • #9
                  First, I think you're putting the cart before the horse. Work from the stepper motor specs. back.

                  Secondly, you don't need a toroid, nice xformer but cheap surplus is fine. Ten to fifteen bucks.

                  Third, Bob, in your circuit the zener has no where to go. To start, zener diodes are low current devices. They won't do zip for the current driving a stepper over , say, 50 ma.

                  A one watt zener at, say, 24 volts means that the maximum current it will carry is 41 ma. I=P/E 1/24= .041 amps. I won't push a device over about 75% max so that brings us down to 30 ma.

                  Your zener has no where to go. If the voltage rises above the breakdown voltage of the zener it will simply blow. You need a resistor in series with the supply output. That means the supply will be severely limited and any load over the 1 watt zener power capability will be dissipated by the series resistor and the voltage will drop on the output significantly. The bottom line is you can’t use a zener for your application.

                  The best thing to do is design your supply to run the voltage you need or run several heat sunk three terminal regulators like the LM1084 in parallel buffered by .1 ohm resistors on each of their outputs.

                  To figure the rectified DC from an AC xformer secondary multiply the transformer secondary rectified and filtered voltage by 1.4 and subtract .7 volts for every diode you go thought to get to your stepper. This will get you close enough to your needed voltage.

                  Also a couple of 1,000 uF electrolytics in parallel is more then adequate for your application.

                  Hope this helps.

                  Rob Dee

                  Comment


                  • #10
                    What has been said about the zener is pretty much true, given the assumptions.

                    With alternate assumptions.....

                    If the zener is meant to be a protection... so the voltage never goes OVER that voltage even if/when there is reverse current back from teh steppers due to over-run, etc... Most drives have a max voltage, above which they will fail catastrophically.

                    Then the zener setup can be useful. If it is supposed to be a "crowbar" protection against input voltages too high, it needs a fuse at least in series with input from capacitors... and probably an SCR or similar added to provide a higher current device that won't require replacement every time. The zener would be used to trigger the SCR on over-voltage.

                    The SCR also will positively clamp fed-back voltage from the drives (possible, depending on design) and as soon as th current drops to zero, it turns off and is ready for the next use.

                    But, no, the zener alone with nothing to "act against" as in your circuit will not do anything but probably fail shorted in the case of an overvoltage.

                    In teh picture of the thing on the capacitor, the "zener" looks like it may be an MOV. They are much tougher than a zener. BUT, they have a very imprecise breakdown voltage, and a "clamping" voltage that may be almost twice the breakdown voltage. So they arent very good at precise protection against overvoltages.
                    Last edited by J Tiers; 07-22-2006, 09:20 AM.
                    1601

                    Keep eye on ball.
                    Hashim Khan

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                    • #11
                      Originally posted by J Tiers
                      What has been said about the zener is pretty much true, given the assumptions.

                      With alternate assumptions.....

                      If the zener is meant to be a protection... so the voltage never goes OVER that voltage even if/when there is reverse current back from teh steppers due to over-run, etc... Most drives have a max voltage, above which they will fail catastrophically.

                      The zener can never be protection in the illustrated circuit. Any power above the device dissipation of the zener will blow it regardless of whether from the stepper or the supply.

                      Then the zener setup can be useful. If it is supposed to be a "crowbar" protection against input voltages too high, it needs a fuse at least in series with input from capacitors... and probably an SCR or similar added to provide a higher current device that won't require replacement every time. The zener would be used to trigger the SCR on over-voltage.

                      A fuse that blows from zener breakdown will still be in the ma. range and any stepper current above that range would blow the fuse regardless whether a zener was in the circuit or not.

                      The issue isn’t whether zener diodes can be useful in power supply applications. Triggering an SCR or TRIAC is a low current application. This is clearly not the case here. The zener is valueless in the circuit shown.

                      Rob Dee

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                      • #12
                        The zener is to protect against back emf from the drives produced by the steppers, at least it should be. The supply should be designed to take into account variation in line voltage using 132 volts AC as the maximum possible input voltage. If protection against higher inputs is required then a zener can be used together with a high capability power device to provide a shunt regulator. Putting any resistance in the output leg of the supply is a bad idea. Even with the resistance of normal wiring if the leads to the stepper drivers from the supply filter cap are more than about six inches long then additional capacitors are usually required close to the driver.

                        [edit] BTW, a zener such as that is recommended in the application notes by various manufacturers of drive modules. The amount of power it handles is very small, it's only meant to sink very short duration overvoltage spikes that would exceed the maximum rated voltage of the driver.
                        Last edited by Evan; 07-22-2006, 09:51 AM.
                        Free software for calculating bolt circles and similar: Click Here

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                        • #13
                          Originally posted by Evan
                          The zener is to protect against back emf from the drives produced by the steppers, at least it should be. The supply should be designed to take into account variation in line voltage using 132 volts AC as the maximum possible input voltage. If protection against higher inputs is required then a zener can be used together with a high capability power device to provide a shunt regulator.
                          Again, what can be done and the circuit shown are two different things entirely. We’re not assuming what can be done with a circuit component we’re evaluating a circuit with a component in it.

                          Putting any resistance in the output leg of the supply is a bad idea.

                          Exactly!
                          And the only way the zener in this circuit has any value is with a series resistor.

                          Even with the resistance of normal wiring if the leads to the stepper drivers from the supply filter cap are more than about six inches long then additional capacitors are usually required close to the driver.

                          Not for regulation, for noise. If you want to regulate a high current driver then you don’t use a zener. It is a low current device.

                          If you want to protect the back EMF then put a standard diode across it like is done for any inductive device.

                          [edit] BTW, a zener such as that is recommended in the application notes by various manufacturers of drive modules. The amount of power it handles is very small, it's only meant to sink very short duration overvoltage spikes that would exceed the maximum rated voltage of the driver.

                          Back EMF, forward EMF, doesn’t matter. The zener has a maximum power dissipation. Anything over that will blow the zener, regardless whether it is a short duration spike or a long one.

                          You might put a zener on the gate of a low power MOSFET circuit but it doesn’t belong anywhere near the B+ supply to the drain of an ‘N’ type MOSFET or the source of a ‘P’ type MOSFET.

                          The bottom line is if you want to protect the driver chip itself then use a three terminal regulator. If you want to protect an inductive device driver from back EMF then use a silicon or schottky diode across the device.
                          Last edited by RobDee; 07-22-2006, 10:28 AM.

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                          • #14
                            Originally posted by RobDee
                            The zener can never be protection in the illustrated circuit. Any power above the device dissipation of the zener will blow it regardless of whether from the stepper or the supply.

                            Then the zener setup can be useful. If it is supposed to be a "crowbar" protection against input voltages too high, it needs a fuse at least in series with input from capacitors... and probably an SCR or similar added to provide a higher current device that won't require replacement every time. The zener would be used to trigger the SCR on over-voltage.

                            A fuse that blows from zener breakdown will still be in the ma. range and any stepper current above that range would blow the fuse regardless whether a zener was in the circuit or not.

                            The issue isn’t whether zener diodes can be useful in power supply applications. Triggering an SCR or TRIAC is a low current application. This is clearly not the case here. The zener is valueless in the circuit shown.

                            Rob Dee

                            as I said.......
                            But, no, the zener alone with nothing to "act against" as in your circuit will not do anything but probably fail shorted in the case of an overvoltage.
                            Although, zeners come in different wattages, and a large wattage (10W +) zener might possibly be able to handle small back emf issues.... but not a good idea.

                            AND, the usual back EMF issue is "pump-up" in a PWM where the DC voltage is raised by returning energy from the load. It may be from braking, reversing, etc.

                            That is a DC condition, not a transient.

                            No regulator on the input can affect it in the slightest way.

                            For that, the best idea is a simple circuit to dump the power into a load resistor and draw down teh over-voltage. The Gecko guy (Mariss) showed an elegantly simple way a year or so back.

                            Note that "pump-up" is inherently energy-limited, as the limit is the mechanical energy in the drive motion. IT IS NOT drawn from the transformer, rectifiers, etc, so a pump-up limiter is NOT in danger from power current.

                            However, for that to be true, the pump-up limiter (a shunt regulator of sorts) must be set ABOVE the maximum DC output of the supply at maximum high-line conditions, which is often taken to be 132V on 120V, but really should be considered to be higher for safety (say 140V). That way it ONLY operates on pump-up.

                            The Mariss circuit operates ONLY on returning energy, and is NOT voltage sensitive.
                            Last edited by J Tiers; 07-22-2006, 11:19 AM.
                            1601

                            Keep eye on ball.
                            Hashim Khan

                            Comment


                            • #15
                              LOL....Level head, did you get all that.

                              You listen to the ramblings of Robdee, J Tiers and Evan long enough you may get to some of the other components figured out too. Then just build your own PS. JRouche
                              My old yahoo group. Bridgeport Mill Group

                              https://groups.yahoo.com/neo/groups/...port_mill/info

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