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veedub1955
13-05-2012, 06:08 PM
Hi I have in my possession a capacitor and diodes that I think are wired up for a centre tap transformer.
Ive found it in a box of bits from my grandad who unfortunately is non compos mentis now so unable
to ask, he used to build PA systems as a job/hobby so I suspect it is from a PS off one. Could anybody
confirm this at all? I will be after a transformer for it to drive my Myford lathe project im looking to get a
system4 control board from diy cnc, Ive had a look for a suitable transformer to go with this, but not knowing
full details of the cap Im not sure, can anybody recommend one? The system4 takes 30v 3A max , It would
be a shame not to reuse this Cap

Cheers, Nick

http://i154.photobucket.com/albums/s248/veedub1955/20120513_164711.jpg

JAZZCNC
13-05-2012, 07:48 PM
From the pic it looks like it's 24,000Uf and 100V so although on the large side it could be used for 30Vdc supply.

Regards system4 board it's not 3A Max power supply required, it means the drives can supply 3A to the motors. I'd also be careful about supplying it 30Vdc if thats the Max it can handle, your always better leaving a safety margin, check it out with Roy first before giving 30Vdc.!
The Min PSU Amps required will be approx 67% of the total ampage of the number of motors used IE: 4x3A motors = 12A/67%= 8A Min PSU needed. More or less Amps wont hurt anything but less and the drives/motors will under perform when Max'd out, more and your just wasting electricty and money on more expensive transformer.
With the above example a 160Va 2x20Vac transformer wired parallel would work giving 30Vdc @8A when rectified.

Working on 4 axis system with 4x3A motors giving 30Vdc @8A then approx 16000uF would be ideal, so 24000uF is a touch on the large side thou think it will just give more smoothing and not hurt anything. Thou should always test output voltage before connecting to drives.! Dont want to let the magic smoke out.!!!

Jonathan
13-05-2012, 08:15 PM
More or less Amps wont hurt anything but less and the drives/motors will under perform when Max'd out, more and your just wasting electricty and money on more expensive transformer.
Too much less and it wont work clearly. A bigger transformer will (up to a point) help provide a more stable/constant voltage output and since the motors only only draw how much current the difference in power drawn from the mains is absolutely negligible so you wont notice a difference in for electricity bill. So get whatever you can find cheap (on eBay) greater than or equal to the required power rating.


With the above example a 160Va 2x20Vac transformer wired parallel would work giving 30Vdc @8A when rectified.
20*2^0.5-1.1=28.2V, so actually a bit less than 30V which is good since as you said don't want to be pushing the limit of the driver's ratings. Particularly bearing in mind the number of posts on here about those drivers going pop!


Working on 4 axis system with 4x3A motors giving 30Vdc @8A then approx 16000uF would be ideal, so 24000uF is a touch on the large side thou think it will just give more smoothing and not hurt anything. Thou should always test output voltage before connecting to drives.!

Yep, nothing wrong with using a bigger capacitor than calculated as it will improve the regulation of the PSU slightly. Wouldn't be worth buying a bigger one specially as the difference is so slight, but that doesn't apply here.

Also since this is for the lathe it's surely 2 motors not 4... either way the capacitor is fine. You'd be much better off with an m547 or similar 50V driver.

JAZZCNC
13-05-2012, 09:02 PM
You'd be much better off with an m547 or similar 50V driver.

Not sure the point to the rest of your reply was to agree or disagree.???? Either way not botherd has the figures were approx not exact and the point being it will be ok!! . . But on the above we both Agree.

Jonathan
13-05-2012, 09:29 PM
Not sure the point to the rest of your reply was to agree or disagree.????

Neither, the point was to clarify.

veedub1955
16-05-2012, 12:10 AM
Thanks a lot for the advice, much appreciated