What Toroidal parts do i need

[irving2008]

Quick tutorial using simple rule of thumb calcs:

V(DC) =1.4 x V(AC) so for 42v DC you need 42/1.4 = 30v AC transformer. Ideally you should add 2v for the drop in the rectifier but 32v transformer is non-standard size, so you'll actually get 30 x 1.4 - 2v = 40v out.

Now calculate the current needed as 60% of the total motor current, e.g. for 3 axis @ 4A each you need 3 x 4 x 60% = 7A.

Now calculate the size of the transformer, simply multiply V x A. 30 x 7 = 210 so a 250VA transformer will do the job, going bigger won't hurt, just costs more but gives you flexibility for the future. This could have either one 30v @ 8A secondary winding, two 15v @ 8A secondary windings (wire in series to give 30v) or two 30v @ 4A secondary windings (wire in parallel).

The capacitors you need can be roughly calculated using C (in uF) = 100000 x current/voltage = 100000 x 7/42 = 16666uF. For this application the exact value isn't critical so within 20% is good enough. 3 x 4700uF wired in parallel would do, rated at 1.5 or 2 times your working voltage, in this case 45 - 60v or higher (don't go too high as price get silly).

Below 50v a bleed resistor across the capacitors isn't necessary but it's good practice to have one, R (ohm) = 5000000/C(uF). Here 5000000/(3x4700) = 350 so use 330ohm rated at 2 x v^2/R watts or in this case 2 x 30 x 30/330 = 5.5w so use 5w or higher. These can get expensive so either use multiple parallel resistors e.g. 8 x 2700ohm 1w (equivalent to 2700/8 = 338ohm 8w) or it's common practice to use a relay to switch them out of circuit when power is applied, in which case a v^2/(2 x R) rating is ok (in this case a 330ohm 2w or two 680ohm 1w in parallel)

Hope that helps.