Power Supply

[elogicca]

I was thinking of building the power supply found here

http://pminmo.com/simpleps.htm

for my mill, has anyone made this one? would it be a problem replacing the 40,000uf cap with a 39,000uf or 43,000uf ?

Any better designs out there? it's only to drive a 3 axis mill

Thanks

[daedalus]

this is a very standard unregulated psu design. You dont need d1,d2, just size the transformer secondary correctly for your target dc voltage. The transformer secondary voltage is NOT the dc voltage you get. To find the DC voltage of the psu multiply the secondary voltage by 1.414.

I dont fuse the drives individually in my psu (f2 and d3), but it might be a good idea.

When sizing fuse f1 you want to use a slow-blow, in my psu ive just got a 6A trip switch instead, which does the same job, as i had it laying around.

note R1 should be a high power rating resistor, not a thin-film one, check the power dissipation through it, and size accordingly. Personally i like to have a led in series with R1, (and size R1 so that current through led < led max current). This is so that when i turn the psu off the red light is on as the cap discharges, lets me know when its safe to mess with the insides.

As for the caps, more capacitance = lower ripple on psu, it doesnt matter if you use a bit more (or less).

This type of psu is perfect for what you want to do.

[elogicca]

Ok so here's my plan
a 6a breaker followd by a Toggle Switch which will power a 25v 10a transformer
to a 15a bridge rectifier, then a 39,000uf 50v cap with a 25w 470r resistor

now my question is this is d3 required? what should it be and should i fuse d3 also? at 6amps also fast/slow burn?

if i do fuse the motor drives what fuses should i use?

Thanks

[daedalus]

d3 is there so that back emf from motors doesnt travel through the fuse, dont add another fuse to this, its meant to bypass. Back emf can be a problem when a moving load is attached to the motor, as it acts like a generator, for example a z axis freewheeling after power off.

Check your motor drives, most have an overcurrent circuit, which shuts down the drive if the motor draws too much current, make sure its set to the correct current, should be set to above max running current, but below stall current draw. If they have this then imo theres no point fusing the drive.

if you do end up fusing the drives individually, then you want to rate the fuses for above the max running current draw of your motors, plus a margin to cover the drives current requirements. Again you probably want to have slow-blow fuses here, as drives may draw high current momentarily on startup. Note fuse rating should be less then stall current draw of motors, that way if an axis binds a fuse burns and not a servo.

with those parts you will have 35vdc output from the power supply. if your buying parts i would choose a higher voltage rating cap, just to be on the safe side, back emf wise (may not be necessary, not sure on this).

as for the resistor, 25W is overkill, if you have one then it wont do any harm, but if your buying a cheaper part will do.
power dissipated = v^2 * r = 1.65 W

to find the time it will take for cap to discharge after turning off psu:
t=1/rc
t is time constant of circuit, measured in seconds, r in ohms, c in farads

you want to leave the lid on the psu for at least 5 x t seconds before opening it up, this should ensure that cap is pretty much discharged before you go putting your hands near it.