Thread: PSU question

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  1. #1
    I have four 42BYGHM809 http://dlnmh9ip6v2uc.cloudfront.net/...42BYGHM809.PDF stepper motors all driven with four A4988's.

    How do you calculate what PSU i should get? the A4988 can take 2A and 35V but i have not a clue how this translates to the 1.7A/phase and 3.06v for the spec of the steppers?

    based on the driver i would have comeup with a 35V 8A PSU? but im sure its not that easy?

    Thanks for looking

  2. #2
    24 volts max and 5 to 6 amps
    John S -

  3. #3
    Quote Originally Posted by John S View Post
    24 volts max and 5 to 6 amps
    Thanks John, is there a formula on how you reached that figure?

  4. #4
    Think you'll find the formula is 20yrs experience and several dozen blown Allegro(A4988) chips.?

    The Amp's is simple multiply the number of steppers Amp's plus a bit spare capacity.

    The lower voltage is safety margin due to the allegro chip being notoriously bad at letting out the Magic smoke any higher. Coupled with the fact the next standard PSU will be 36V.

  5. #5
    There is a wonderful gizmo called the "crowbar circuit". You put a fat Diac across the PSU output and a zenner diode to trigger it should the volts rise above your max threshold.

    When you trigger a Diac it stays triggered, hard on, until the Volts go away. A cheap Diac can handle lots of Amps.

    In this case the Volts go away because the fuse blows. You then replace a 10p fuse rather than expensive hardware.
    It's called a crowbar because the effect is like dropping a crowbar across the power input terminals

  6. #6
    Quote Originally Posted by JAZZCNC View Post
    The Amp's is simple multiply the number of steppers Amp's plus a bit spare capacity.
    in that case as i have four steppers i should look for a 7-8A ( 1.7x4 + a bit ) ideally?

    Thanks

  7. #7
    No you want about 60% of the total because not all 4 motors are working under max load at any one time and even though your motorsare 1.7 amps they will never pull that current.
    You can go larger but it will not do anything.

    Voltage as Jazz says is based on what the chips will take before they catch fire, stated voltages is always max and no room for any errors.
    John S -

  8. #8
    Thanks for the pointers, what do you need to look for in a PSU, some are 20 quid and others 100 quid does the wattage come in to play?

  9. #9
    Wattage is amerage, it's Watts Law, so say 24v 200W = 200 / 24 = 8.3333 amp
    John S -

  10. #10
    Quote Originally Posted by John S View Post
    Wattage is amerage, it's Watts Law, so say 24v 200W = 200 / 24 = 8.3333 amp
    Two simple formulae for DC electricity in resistive circuits -

    1. Ohms Law is Volts = Current x Resistance (V=I x R) and there are two transpositions of that:- V/I = R, V/R = I.
    2. Power (Watts) Law is W = V x I and you can transpose that in the same way. W/V = I, W/I = V


    But when you get to AC or with capacitors or inductors in the circuit then it gets complicated, and I never did that at school.

    Cheers,

    Rob-T

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