Re: Ben's 2.x Laser Build
Posted: Tue Sep 13, 2011 9:52 pm
bdring wrote:The main board cap is on the 5VDC line. The 5VDC power supply has short circuit protection, so it is not supposed to take down the 24VDC power supply.
Ah, I didn't realize where that cap was. I had the reverse problem: 24V short took out 5V (hence no fan, one of the obvious visible signals)
bdring wrote:The 5VDC current is quite low in the drivers and the cap is so close to the driver that I don't see it inducing ripple in caps on the driver. I think the cap that failed on your board was on the VMM (motor) line.
That's correct. I just assumed the cap was on the 24V line. It probably needs a cap on 24V (if not per driver, at least somewhere on the board). A 24V cap on the board will reduce the ripple in the cap that blew on mine and reduce its power dissipation. In fact it's possible that the 4.7u caps on the pololu modules are "sharing" between the modules and the weakest one is dissipating more power than the rest and blowing up. Survival of the fittest!
In fact, although it's not an ideal location, you could easily clamp an electrolyic cap in the 24V fan screw terminals. I'll put a scope on it tonight and see what it looks like.
(speaking of those fan terminals: Those blue screw terminals are easily the worst screw terminals in the system. My fan ground lead kept falling out even though it appeared to be clamped. The removable green terminals are nicer and the DIN rail terminals are nicer still)