by LROBBINS » 18 Apr 2023, 22:29
Some further information:
The Roboteq Dout pins can handle a maximum of 1 Amp (and no more than 4.5 Amps for all Dout combined). I don't have its data sheet handy, but my recollection is that the latching relay John used draws much more than 1 Amp - hence using solid state relays between the Dout and a latching relay as he does is ESSENTIAL. Overloading a Dout will destroy at least that gate, and may even destroy the whole Dout chip. If just one gate goes bad, you can switch to using a different one, but if the chip is destroyed so is the Roboteq (unless you pay dearly to have it repaired).
Non-latching relays draw much less current; the one I'm using draws 0.17 Amps so can be powered directly from a Dout, but, of course, it is on the whole time the motors are powered. To improve its service life, the script has a CONTACTOR subroutine that opens and closes Dout for the contactor only when negligible current is flowing: a few msec before sending power to motors, or when motor output has fallen to near 0. This means that in normal operation the relay coil is not active all the time, but only when the motors are running and by doing the switching when little or no current is flowing means that there's no contact arcing to damage the relay itself. (This way of doing things was vetted for me by TE application engineering.)
Having the relay Dout switch on and off, however, means it can't be directly used for Roboteq's fault detection, so I also added a CK_RUNAWAY subroutine that actually does two things. One is that it reads the MOSFET fail flag in the Roboteq firmware and uses that to open the contactor Dout. The other is that it will detect a runaway condition and open the contactor in that situation as well; a runaway state is defined as anytime that motor command and motor power are ZERO but there's even minimal current flowing for 500 msec. These two subroutines essentially mimic Roboteq's fault detection, but still allow the CONTACTOR subroutine to work the relay without contact arcing, and not have its coil powered even when not moving.
The stud of a stud mount diode doesn't get directly soldered. Solder a ring terminal in your wiring harness and then use a nut to secure that to the diode.
Paralleling two diodes may not get you double the current rating. No two diodes will be exactly matched and the forward voltage drop can be substantially different for the two in at least some conditions - especially as they are warmed by current flowing through them. The result is that
one will pass more current than the other and may fail at not much more that the rated current of just one diode. What can happen is that first one diode fails, then all the current goes through the other one, and it fails too. Its better to use a single large capacity diode for the contactor's (reverse voltage) bypass.