On Tuesday, February 10, 2015 10:15:22 AM John Kasunich wrote: > On Tue, Feb 10, 2015, at 10:01 AM, Dean Posekany wrote: > > On 2/10/2015 3:00 AM, Gene Heskett wrote: > > > > snip > > > > > The diode checker of the modern meter reads the voltage drop, fwd > > > direction on this type of diode s/b under .15 volts, while the > > > reverse of the probes should show a much higher voltage, maybe > > > even off-scale for reverse. Right where they sit in circuit. A > > > reading in both directions that is similar and probably under .1 > > > volts is a shorted one. > > > > I checked the three diodes on the bottom near output port and they > > all read generally the same: forward bias ~0.48V and reverse bias > > ~1.47V. I checked a few of the topside diodes of the same value and > > got: forward bias ~0.48V and reverse bias .OL. It appears that the > > bottom diodes are bad. > > Not neccessarily. You are testing them in-circuit, and the 1.47V might > be a completely different path through some other part(s) on the > circuit board. > > The 0.48V forward voltage seems reasonable. Usually a shorted diode > would read lower than that, and the same both directions.
Not to me John. Thats low by about .1 volts for a fast Si diode, and too high for a schottky, which should be under .15 volts. Even Germanium is lower, but has a poorer reverse ability, and much easier destroyed by its own heat. The main problem with the schottky's is their limited reverse breakdown limits them to uses in the sub 60 volt range. Dean said they are 75 volt drives? They probably are Si's then, which would raise the heat dissipated about 3x what it would be with schottky's. And since speed is of the real essence at that position in the circuit, and IIRC Dean said they are 1997 vintage, I would sure check the books for a more recent device that could be 10x faster than those ever were, and that cannot be anything but good for the circuits overall performance and longevity. The ideal diode there would be a mechanical switch that turns on in 5 picoseconds and turns off just as fast when the current starts to reverse. But "we don't hardly have some more of those" as one ex truck driver turned tech we had at KRCR in Redding CA was fond of saying. The industry keeps giving us better stuff, but that trick has only been accomplished by much more complex synchronous switching ciruits where an actual FET is used in the diode position with its gate drive usually a few picoseconds late on and early off just to remove the spector of them both being on at the same time as that would crowbar the supply for those overlap times. I have seen the circuits, but for a design we must be able to afford, it would run the price of the driver up by 10 bucks a motor lead. Or more. > If they look scorched, it probably isn't a bad idea to replace them > anyway. I'm sure they're less than $1 each, the main issue might be > unsoldering and re-soldering if you're not familiar with surface mount > stuff. > > If you do unsolder one, test it with the meter again, and see if the > reverse readong goes up to .OL. I wouldn't be at all surprized. Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> US V Castleman, SCOTUS, Mar 2014 is grounds for Impeaching SCOTUS ------------------------------------------------------------------------------ Dive into the World of Parallel Programming. The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
