Building my power supply

[quamit]

I have been working on a 4 axis controller using 201's with a campbell combo board. My supply is 30vdc with 2 capacitors and fed to the 201's from a fuse block.
1. When I setup my first power up test for one drive I was surprised at how long it took the red light to go off on the drive indicating the caps had lost their bite. Do I need to worry about creating an arc if I unplug a motor while this light is on?
2. I have a 3 prong plug on my AC side of the transformer which also powers the Campbell board. The green ground is attached to the case chassi. Should the neg side of the capacitor for the DC side be attached as well or does this negate isolation on the 30 VDC curcuit?
Thanks in advance.
Regards
Tim

[project5k]

if you tie any part of the dc side of the powersupply to ground you kill your isolation..

i wouldnt connect or disconnect a motor while the system has any power in it, the potential for problems is not a good gamble, and if my memory serves me correctly, i think i remember reading something about geckos and it specifically saying do not make/break connections while powered...

if you are concerned about the bleed down time, you can add a resistor across the caps to decrease the time it takes to drain the caps.. at 30v i would suggest something like a 1K 1W... if you go with something smaller in R value the W rating will need to go up, and it will be extra drain on your system. using too small of a W rating on the bleed resistor will cause it to burn up the resistor.. you can go as big as you want but not smaller. and with 1k your "parasitic" current is only 30mA @ 30V

going to a larger resistance value would allow you to use a smaller resistor (W) but would increase the bleed down time.

i dont know what value your caps are, or how thier wired, (series or parallel) but there is a formula to calculate the bleed down time.

you can pickup 5W and 25W resistors on ebay for just a couple bucks... just pay attention to the R value.. you definately dont need a precision part here, theres no need to spend the extra money for it...

i read in another post about someone that wanted to bleed down thier caps faster, but didnt want the parasitic current of having the bleed resistor connected at all times, so what was described was to use a single pole double thro switch with no center position on the powersupply output so that in one position the power supply was connected to the drives, and in the other position it turned off power to the drives and connected the bleed resistor.. a good idea, that i plan to implement on my system, but one thing to look out for is to make sure that the switch that you use can cary the total current of the system plus a saftey margin...

one other thought that i had was to use the parasitic current of a bleed down system to do you some good, something like a small light or even better, a fan.. this will decrease the bleed down time and do you some good... just be sure to set it up to be able to handle the V of your powersupply.. just a thought would be to use something like a 7812 voltage regulator(available at radioshack for a couple bucks), which has a max input of 35V and 1A to drive a fan... and this way your fan allways gets a good 12v and runs any time the powersupply is on, and will bleed off the caps. some of the little fans i have around here pull around 20mA and then there will be some small drain current (in the mA if not smaller) through the regulator.

just my thoughts...