by see3d » Thu May 01, 2014 6:31 pm
Several comments:
@jasonharper: A large cap across the relay coils is not a good idea with this diode circuit, as it would suck all the power away from the stepper and perhaps even destroy the driver circuits.
If pulling the plug to the stepper motors on the controller will kill the controller drivers, something is wrong with the design.
I would be concerned with the sparking on the relay contacts reducing their life if it were not for the fact that this should be a more rare event.
@Nicholas: I would experiment with something simple first, like shorting one or two unconnected motor coils and letting the bed drop to see what happens in each case.
Relays usually have a pull-in current and a release current. There is a built-in hysteresis to disconnecting. That makes them more tolerant of dropping out due to driving voltage variations. Low average voltages into the relay coil due to low voltage motors makes it more difficult.
The driver may output up to the power supply voltage during initial steps (it is actually a current regulator, not a voltage regulator). This could create too large a variation for the relay coil. A small resistor in series with the coil and a small cap in parallel with the coil would smooth out the voltage/current variations it sees. The coil voltage/resistance/current specs have to be derived from what the minimum average voltage seen at the coil from this diode circuit. It is best done with an experiment and an oscilloscope. You can substitute a larger resistor for the relay coil for the initial experiment. For instance motor coils to 4 diodes to a 100 Ohm resistor to a 1K Resistor with a 1 uF 50V cap across it, to 4 diodes to the motor coils. Measure the differential voltage across the 1K resistor with the steppers micro-stepping slowly, and at high speed moves. That should give a good idea of what the relay coil would see. The look for what might be available as a coil resistance/current in a relay.
If the voltage variations are too high, and the average voltage is too low for a practical relay coil, then I would look at bringing out a power supply and ground wire to the relay board. Then the relay coil can be driven from a transistor that can be gated by the low voltage signal derived from the motor coils.