[DaveBoettcher]

Hi Lapponia,

Thanks for the link to the manuals, it worked this time.

The first suggestion my guys came up with was tackling the back emf at source by fitting a reverse biased diode across the pump, but when I explained that I really wanted to fit it inside the relay they suggested the Varistor. I think both of these and your electronics shop suggestion of a freewheeling diode are all variations or different names for breakdown diodes like zeners and tranzorbs, and that they will all do the same job as far as we are concerned: after all we are not trying to do anything sophisticated, just kill a couple of hundred DC volts to avoid the arc starting.

Now the next job is to try it out. I think the only way to really test whether our modification is working will be to measure the transient voltage across the relay contacts, both without and then with the protection fitted. This seems to me to need an oscilloscope, which is not something I have access to. Any ideas?