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  1. #11
    Just before I push the button on a transformer could I just have a quick sanity check please?

    Here's my thinking:

    I have 4 qty 3A max steppers and I've ordered 4 qty 2DM860H drivers. I reckon that it's highly unlikely to have all four motors chugging away pulling 3A each at any given time so I've assumed 10A as the max draw. The drivers will take AC so I won't have voltage drops and capacitance calcs to take into consideration. I am looking at a toroidal transformer with 2x35V AC secondary windings in series ie 70V AC (the drivers are rated up to 80V AC). Therefor 10A * 70V = 700VA (750VA is the closest standard size).

    Am I missing anything?

  2. #12
    Neale's Avatar
    Lives in Plymouth, United Kingdom. Last Activity: 6 Hours Ago Has been a member for 5-6 years. Has a total post count of 1,178. Received thanks 215 times, giving thanks to others 5 times.
    Can't argue with your arithmetic, but the usual choice is 650VA. In fact, I ran mine with a 500VA toroidal for some time, until it failed. I don't believe that the failure was due to overloading - just one of those things - as it never became particularly warm. I went for one size up because it cost very little more and fitted the space available. There are a few factors here which mean that you tend to get more volts than you would expect. One of these factors is that the transformers will often deliver 5% or so over the nominal value on the basis that it will drop to nominal at full load. 2X35V in series will give you more than you expect, and input mains voltage is also often above nominal so the output volts go up a bit there. Keep this in mind when you are looking at the max voltage input of your drivers.

  3. #13
    Plus one.
    ..Clive
    The more you know, The better you know, How little you know

  4. #14
    Quote Originally Posted by Neale View Post
    Can't argue with your arithmetic, but the usual choice is 650VA. In fact, I ran mine with a 500VA toroidal for some time, until it failed. I don't believe that the failure was due to overloading - just one of those things - as it never became particularly warm. I went for one size up because it cost very little more and fitted the space available. There are a few factors here which mean that you tend to get more volts than you would expect. One of these factors is that the transformers will often deliver 5% or so over the nominal value on the basis that it will drop to nominal at full load. 2X35V in series will give you more than you expect, and input mains voltage is also often above nominal so the output volts go up a bit there. Keep this in mind when you are looking at the max voltage input of your drivers.
    Thanks for that Neale. Good point about the output voltage. I have the option of 2x35V, 2x30V or 2x33V (from different suppliers), I think I'll plump for a slightly lower voltage just to be on the safe side.

    Is there anything to be gained from using 4 qty 160VA transformers over a single 625VA? My thinking is that I could assign a transformer to each driver. I can get the 4 160's for about 15 more than a single 625VA so the price difference is neither here nor there. I suppose it would be cheaper to replace a 160VA in the even of a transformer failure but I wondered if there was any performance benefit to the system?

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  6. #15
    Neale's Avatar
    Lives in Plymouth, United Kingdom. Last Activity: 6 Hours Ago Has been a member for 5-6 years. Has a total post count of 1,178. Received thanks 215 times, giving thanks to others 5 times.
    Can't think of any particular advantage of 4 individual transformers, except that it enforces good wiring practice - separate connections from PSU/transformer to drivers, with no daisy-chaining from one to another. Yes, could replace one if it fails, but you need more space to mount them. Swings and roundabouts!

  7. #16
    Quote Originally Posted by Neale View Post
    Can't think of any particular advantage of 4 individual transformers, except that it enforces good wiring practice - separate connections from PSU/transformer to drivers, with no daisy-chaining from one to another. Yes, could replace one if it fails, but you need more space to mount them. Swings and roundabouts!
    Yeah, I like the idea that there are four completely separate channels - I imagine it will help with fault finding too. Space isn't too much of an issue, I'll probably design an enclosure to suit the kit that I have rather than try and shoehorn everything into a given space.

    I've been making some (albeit slow!) progress on my design over the weekend. I am getting frustrated with Fusion so I decided to go back to the beginning and watch some tutorial videos to try and understand the basic concepts, I am still thinking in Sketchup! I guess you can teach an old dog new tricks, it just takes a bit longer

    Inspired by Joe Harris' YouTube series I thought it might be fun to try and document my build in the same way so I have started recording my build in the hope that something I have to say may help others. I am not sure I can add anything to Joe's impressive videos but I reckon the more information that's out there, the better. And if nothing else it should be entertaining to watch me bumble my way through the whole project!

  8. #17
    AndyUK's Avatar
    Lives in Southampton, United Kingdom. Last Activity: 2 Hours Ago Has been a member for 1-2 years. Has a total post count of 9.
    Quote Originally Posted by CaptainBarnacles View Post
    And if nothing else it should be entertaining to watch me bumble my way through the whole project!
    Can't wait! :D

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