Thanks for the advice and clarification! I'd gone ahead and attempted this idea whilst I was waiting for an answer and changed the C code so I used PWM outputs to the enable pins. Still slightly concerned though. I tried 12V (since my 5V regulator doesn't like going up to 24V). I set analogWrite(5, 38) = 15% duty cycle, which should therefore give me approx. 2V into 6.6ohms = 300mA, rather than going for 76 (of 255) for the 30% duty cycle.
Sod all happened!
Tried reversing a pair of pins and still nothing (it wasn't twitching even). I could still easily rotate it with my fingers. I just expected at least some movement or added stiffness to rotate it at half max rating?
I considered going up to the full 30%/600mA but then I noticed the ATmega328 seems to be getting very warm. I disconnected it as soon as I noticed, which was pretty quick. Definitely no shorts I can find, but I have another Arduino here and that runs cool running the same code but nothing connected. Disconnected the "hot" Arduino from all the drive circuits and it still gets hot. Any guesses? I don't want to connect this other one if I've fried the hot one. I've already blown up one Arduino board already this week (an accident involving 12V the wrong side of the 5V regulator; so I've been stupidly careful all of today
). Stepper's fine as I connected it to a PSU with current limiting on, and it happily did individual steps as I touched the wires. Circuit seemed fine as I checked it all with a multimeter.
The only other thing I can think of is my diodes from the H-bridge aren't fast types, just unknown regular silicon ones, and the current rating is only 1A, not the 2A suggested. I figure if I only want 600mA from the L298, rather than it's stated max of 2A they should be ok though? Does the diode switching speed make any difference? I've ordered some 2A Schottky ones which are also higher voltage, but they're not here yet.
Thanks for the link to the wiring diagram too. Hadn't found that on the NMB website!