the power supplies are 19.1V-24.1V and 20V-30V. So maybe no need to open them up?
I thought 20.6V was the lowest it (not sure which one) would go? No
need to open up the supplies. I was only thinking 'wouldn't it be nice if....'
I'm still dithering as to whether it would be advisable to go for 18V. On balance I think (though there's no guarantee) you will get away with 20.6V, based on:-
1) You've given the pumps a soak test at 20.6V and they've survived; whereas the pump that died was being run at 24V.
2) The Melexis application notes show a 24V system with a circuit virtually identical (apart from the uncertainty of the actual Hall IC) to the one KISS found in the pump.
3) Melexis seem to have cornered the market on 3-pin ICs for 2-phase motor control. Your pump IC is 3-pin so the odds are it's a Melexis one and hence suitable for a 24V system (albeit that 24V gets dropped and held to max 18V within the IC).
4) At 20.6V the pump power dissipation will be a good bit less than at 24V.
5) If you build the Synchronous Pulse Delay (SPD) speed module and it works as it says on the tin (i.e. as it did for the two fan motors I tried) then the speed setting can be adjusted to give a pump power dissipation similar to using an 18V supply for the long term and without wasting power in an external volt-reducer circuit.
BTW, the alarm problem was apparently due to my not wrapping up the IC. Pins 4, 8 and 9 were not connected together and pins 12 & 13 were not grounded.
Glad you've resolved that.
Anyway, I'm going to remove C4 from the other PDM and add the 4.7K to Q1. Final version?
I'd like to think it's the final version
While you're at it, R5 and R6 shown in the Mk14 mod are then redundant. If you remove R5 the D5 cathode (right-hand end) should connect to Q1 collector.