Stall current is the answer, since the full motor current goes through the diodes in the off phase The voltage is supply voltage with a margin. And make sure they are high speed diodes since with a small deadband the rails will short in the reverse conduction spike of the diodes.
Jan. On Sun, May 9, 2010 at 9:11 PM, Greg Bentzinger <skullwo...@yahoo.com> wrote: > I'm putting a small H-bridge together using TIP121/126 to replace a > SN754410NE. The schematic I'm following didn't list any values for the > flyback diodes. In fact some examples left them out. I know that when this > circuit is in use it switches from full forward to full reverse with no > ramping. This tends to overheat the SN754410NE with small motors and blow it > with motors slightly larger. > > The H-Bridge is driven by a LM339 and about the only adjustment I have is the > amount of deadband between directions. > > My question is how to determine values for the flyback diodes. In the past I > used the motor stall current spec to be on the safe side. In other circuits I > often see very small diodes like 1A on a 7A drive. Also voltage rating, does > the back EMF rise much greater than the supply voltage? If so how much higher > % should one use? > > Since someone will ask - this is the ram servo control for a small tabletop > sinker EDM - the abrupt movements really aid in flushing as well as breaking > momentary shorts sometimes caused by debris in the work contact area. > > Thanks much team. > > Greg > > ------------------------------------------------------------------------------ > > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users > ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
