The PSU on my milling machine blew up again last night. This is the second time it has happened. I haven't pulled the box out of the machine yet, but I expect to see this again: https://picasaweb.google.com/lh/photo/xxfP_tT7Ae0op6GxUhSDvtMTjNZETYmyPJy0liipFm0?feat=directlink Which is a power resistor that has got so hot that it has exploded. (quite an odd failure mode for a resistor).
The PSU is home-made and consists mainly of a bunch of capacitors and rectifier. Also in the box are two relays, two power resistors and a timer circuit. One resistor is a soft-start on the AC input side. When power is first applied the power passes through one of the resistors to limit the surge current, and then the timer closes a relay that short-out that resistor. I am fairly happy that that part of the circuit is reliable. A second relay is used in change-over mode. This has a 240V coil connected to the incoming power. When the PSU is powered up the relay closes, and supplies power to everything else inside, when the power goes off the relay opens and the NC contacts connect a crowbar resistor across the capacitors to discharge them. I think that the failure mode is that, in the case of a power glitch, the crowbar relay switches and is discharging 300V DC at a fair current, then the power returns and the NC contact tries to break 300V 10A and welds the NC contacts closed. Then the NO contacts close and we now have the crowbar resistor connected directly across the rectifier output. The crowbar resistor is not sized for continuous operation. I think that there are two problems here. If the crowbar contacts on the relay are welded shut then the power contacts should not be able to close. I think I need a more explicitly interlocked relay. Currently I am using: JG64U here: http://www.maplin.co.uk/round-base-10a-relays-2567 I think that the term I need to search for is "force guided" but would like some confirmation that that means what I think it means. Are "contactors" more dependable in this sense? The real problem is that the relay is not capable of breaking 300VDC. However, DC rated contactors are pretty expensive, and seem to jump for 220 to 44 and then 690V. I don't think I have room in the box for the big ones. I wonder if http://uk.rs-online.com/web/p/contactors/4111687/ would be OK? It really should never try to break with the capacitors charged (interlocked in HAL, which can see the DC bus voltage, and with the crowbar in circuit the voltage will drop pretty quickly anyway) My inclination is to look for a solid-state solution, but solid-state isn't very good at doing anything at all on the basis of the power going off.... -- atp If you can't fix it, you don't own it. http://www.ifixit.com/Manifesto ------------------------------------------------------------------------------ October Webinars: Code for Performance Free Intel webinars can help you accelerate application performance. Explore tips for MPI, OpenMP, advanced profiling, and more. Get the most from the latest Intel processors and coprocessors. See abstracts and register > http://pubads.g.doubleclick.net/gampad/clk?id=60134791&iu=/4140/ostg.clktrk _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
