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  • BobWarfield
    replied
    I hate to resuscitate this rather exciting exchange, but I did note that Mariss has decided to incorporate a TVS directly into his updated driver design:

    http://finance.groups.yahoo.com/grou...e/message/9048

    His products are extremely well made and he will test the thing to destruction to make sure it works properly. At any rate, it won't be long before the debate about whether to zener the back-emf is taken care of for us by the drivers, at least if you are using Geckos.

    Best,

    BW

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  • Sophiedoc
    replied
    Coupling motors to machines

    I have a Bridgeport clone to which I would like to add at least an x axis drive or Cnc control.No one recently at least has discussed the coupling mechanism.Are the motors coupled with cnc control connected the same was as the x axis electical feed with large brackets etc.Sorry for being naieve about this but I would at least like to know how this works.Used to have electronics as a hobby and understand that part of the equation pretty well.If I convert to Cnc what size motors for x and y are needed for this machine with an 8x 29" table?

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  • John Stevenson
    replied
    Originally posted by wmgeorge
    I'm using the same steppers amp draw as yours, I did an over kill. My PS is 18 amps at 36 volts DC, running Gecko 202 drivers, works fine.

    my motors are 2.3 volts at 5.5 amps .I read earlier that my power supply voltage has to betweeen 5 and 20 times the motor voltage .So to get this straight the 2.3v times 20 is 22.3v then i found a wiring diagram for my gecko 202 at cnc zone and it says the p.s should be between 24 - 80 volt at 5 amps since my motors are 5.5 will that make a difference .And is this wirring diagram a accurate diagram for my motors. http://www.cnczone.com/forums/attach...chmentid=19401
    [/quote]


    Check your maths, 2.3v times 20 is 46 volts, not 22.3v and your amperage needs to be around 11 to 12 amps if you are running 3 motors or 15 if you are running a 4th axis.

    Sorry can't see the wiring diagram as although I'm logged on CNCZone says I'm not.

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  • Evan
    replied
    I like the supply to be as stiff as possible so will use more capacitance than recommended. By using a soft start resistor and a time relay the inrush current to the caps can be limited so it isn't a problem. Likewise for a bleeder resistor and relay for fast bleed down. I also like to put a latching relay on the AC input side so if the power kicks out it doesn't turn back on when the power does.

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  • JRouche
    replied
    Originally posted by John Stevenson
    The rule of thumb is amps multiplied by 80,000 divided by the output voltage, result in Microfarads and working voltage needs to be 25% plus of the output voltage..
    Pretty much the standard. I have researched many schematics and formulas and they all revolve around this basic concept. JRouche

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  • wmgeorge
    replied
    Stepper Power supply

    I'm using the same steppers amp draw as yours, I did an over kill. My PS is 18 amps at 36 volts DC, running Gecko 202 drivers, works fine. I went a little larger in case I wanted to add a 4 axis. I purchased a donut type transformer off eBay, Radio Shack 25 amp bridge rectifier for $5 and I had to buy 3 30,000 MFD caps to get one, so I you need one... I'll make you a deal!!



    easy to build a p.s from a micrwave than you can give me those links .



    my motors are 2.3 volts at 5.5 amps .I read earlier that my power supply voltage has to betweeen 5 and 20 times the motor voltage .So to get this straight the 2.3v times 20 is 22.3v then i found a wiring diagram for my gecko 202 at cnc zone and it says the p.s should be between 24 - 80 volt at 5 amps since my motors are 5.5 will that make a difference .And is this wirring diagram a accurate diagram for my motors. http://www.cnczone.com/forums/attach...chmentid=19401[/QUOTE]

    Leave a comment:


  • John Stevenson
    replied
    Originally posted by Evan
    I as well. It's just a matter of deciding how much.

    The formula that Mariss has on page 12 of his support page gives a very good idea of what size to use given the total amp load and voltage.

    The rule of thumb is amps multiplied by 80,000 divided by the output voltage, result in Microfarads and working voltage needs to be 25% plus of the output voltage.

    .

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  • RobDee
    replied
    Originally posted by J Tiers


    Yes the Pac Sci "chip" (actually it is probably a hybrid circuit) is not the newest... in fact I don't think it is made any more.

    But, the principles of stepper operation are the same with any controller. The regeneration voltage issue is one that really can't be avoided with any design, it can only be dealt with.
    Thisis exactly my point about spliting the control circuit from the mosfets. You don't have the regen problem to contend with. Mosfets can run pretty high voltage or you can use IGBT's.

    I'll never see it as anything less then a poor design.

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  • Evan
    replied
    I agree with J.S. .....I also am highly in favor of the large capacitor solution.
    I as well. It's just a matter of deciding how much.

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  • J Tiers
    replied
    If you look at the 1.5KE75, by 10A peak they are pushing 10V high in voltage. That could be an issue, and is a comparable voltage change to that of the regeneration energy itself in a typical circuit.

    You don't even need it, maybe, as it won't hold pump-up any closer than a larger capacitor will.


    No problem for many applications, typically transient spike limiting, but if you need a closely held max voltage for surges, you are far better off with a different approach, or some way to avoid the need.

    I agree with J.S. .....I also am highly in favor of the large capacitor solution. It works, it actually saves energy, and in general, another motor will pull the energy that the first one dumps.

    Improperly applied zeners are worse than no limiter, in many ways. They don't work, and yet they tend to make a person THINK they are safe from regen surges when they are not.

    Stay 20 - 25% under the max voltage of the device, and use a larger than required capacitor. Should usually keep problems away except in really odd cases.

    Is it optimumized? No, but that isn't required. What IS required is a working system that does not give trouble.
    Last edited by J Tiers; 07-23-2006, 02:40 PM.

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  • Evan
    replied
    Yes it is the diagram I first quoted. I posted only the part relevant to the discussion at hand and it should be obvious looking at the part I posted of the schematic that it isn't the entire schematic. The people here, new or not, aren't idiots and I also made it clear the information was to be found at the link I posted to the data sheet where the entire schematic is found.

    According to application notes from various manufacturers for various driver boards and modules the filter cap(s) should be no more than six inches away. If the main cap is further than that then smaller caps should be placed as close as possible to the driver board.

    By placing individual caps at the boards they also serve as decoupling caps which helps prevent ripple from the load of one driver affecting the others.

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  • John Stevenson
    replied
    Originally posted by Evan
    John, they are shown in the schematic.



    Placing individual caps at the modules is more effective (and required) as it reduces effects from resistance and inductance in longer wiring to a single cap located further away.
    This isn't the diagram you first quoted.
    Remember we have plenty of newbies on this board who take everything the more senior members say literally and leaving much needed caps out is a problem.

    You only need caps at the drivers IF your run of cable is over 12" from caps to driver [ recommended ]

    By using one large central cap you can handle any back EMF from the other drives better.
    This way with a safety margin built in on max voltage the whole thread on zenners is not needed as you won't have to dump any excess voltage.

    A lot of what is advised is overkill for safety reasons or CYA.
    I have a big mill, size of a Bridgy Series II running at 72 volts into Gecko 210's driving those big horrible type 42 motors and the main transformer is 4 feet away from the cabinet, two of the drives have 470 uf caps direct on the motor leads, two don't.
    Been in production now about a year and a half from conversion and no problems other than silly ones like keyboards packing in.

    If you ever want to try anything on the limit then those 42's as fitted to the Bridgy's will guarantee to loose steps on acceleration and pump out loads of back EMF on braking.

    .

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  • Evan
    replied
    Their one problem is that they have rather loose specs for breakdown voltage, and usually have a much higher clamp voltage than breakdown voltage, relative to a Zener (avalanche) diode.
    Not according to the data sheets.



    I've used these for many years and many types clamp very precisely with values within millivolts of spec and within less than a volt or two of the avalanche voltage in the current ranges of interest. In this application they aren't going to see currents anywhere near Ippmax.

    [edit]
    As for looseness in the breakdown voltage they are usually very consistent within a batch and if on the low side can be adjusted with a regular diode in series to raise it.
    Last edited by Evan; 07-23-2006, 01:05 PM.

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  • Evan
    replied
    John, they are shown in the schematic.



    Placing individual caps at the modules is more effective (and required) as it reduces effects from resistance and inductance in longer wiring to a single cap located further away.

    Leave a comment:


  • J Tiers
    replied
    Originally posted by Evan
    Use a close relative of the zener, a TVS diode. They have response times in the picoseconds and will withstand peak powers in the kilowatts with clamping curves that are cliff shaped, with maximum currents in the hundreds of amps. The total dissipation is a product of time and power with steady state values of 5 to hundreds of watts depending on type. Typical rating for a 5 killowatt peak unit is a 1 kilowatt for 10 milliseconds and 100 watts for .1 seconds. They are also cheap.
    A good plan.

    Their one problem is that they have rather loose specs for breakdown voltage, and usually have a much higher clamp voltage than breakdown voltage, relative to a Zener (avalanche) diode.

    So they are not very good for a situation needing a closely limited voltage.

    But they are MADE for transient limiting, and within their limitations they work very well.

    However, they basically ARE a zener, optimized for power dissipation and clamp speed. The zener speed is comparable, and I use one or the other depending on the requirements of precision limiting traded against raw pulse power handling.

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