14V/20Ohms = 0.7A capability with flyback
If the two coils have independent magnetic paths (which I expect), low voltage flyback is the best. Either:
- 2 free FET (TO220 or DPAK) with flyback. The injector FETs have flyback applied already: on this 6 cyl machine that would use the two remaining high power FETs if you use the injector drivers, none to spare for other functions.
- or 2 stepper channels (free anyway, seems like a good choice) - finally it has been choosen
- or IGBTs. the IGBT-s have internal 400V clamping, but IMHO it's a bit rude to dump the coil energy in the IGBT. So I recommend to apply flyback diodes onboard if you choose this. (there are throughole pads for one end of the flyback diode at the IGBT)
If the 2 solenoids have the same magnetic path (very unlikely, only unipolar steppers have that), than high-voltage flyback (16V or above, 18..20V recommended) needed instead of low-voltage flyback:
- FET is good, but with high voltage flyback. Throughole 18V transient suppression diodes are easy to mount for either the DPAK FETs or a free TO220 FET.
- if using the injector FETs with already applied low-voltage flyback, an SMB diode must be replaced with an SMB transient protection to get high-voltage flyback
- SN754410 stepper chip is not sufficient, it has internal low-voltage flyback. L293 would work with external diodes, but we don't use it since v2
- IGBT is good, but high voltage flyback recommended.