We bought a used Milli-q 7005 ultrapure water purifier from an auction for a great price. It came with most of what we need, but was missing the external feed solenoid valve. This solenoid stops the feed water flow if a leak is detected, but we weren’t going to use it in our setup anyway. The Milli-q is far too smart for its own good and won’t dispense any water if this part is missing. It’s about $500 for a new part. Can we hack it and fix it for much much less?

The computer in the Milli-q is likely just looking for the right resistance of the solenoid when it’s energized. For our purposes, the solenoid is just a resistor. Through product pictures, I found the solenoid is 24V and draws 6.9W full-time. Back to high school physics, Ohm’s law gives:

p = i * v
6.9w = i * 24v
i = 6.9/24 = .2875A

v = i * r
24V = .2875A * r
r = 24/.2875 = 83.47Ω

I found a 85Ω, 50W wire-wound resistor on Mouser Electronics for a few bucks. The extra wattage capacity should allow the resistor to discharge heat effectively. After the shipment arrived and the resistor was wired up to the existing solenoid wires, we were in business! The Milli-q no longer complained about a missing solenoid and dispensed water as normal.