Relays and Rheostats

SUMMARY: A quick How-To for wiring up water pump relays and fan rheostats. If you are uncomfortable about wiring, GET HELP!

Water Pump Relay


I have taken the cover off the right hand relay for picture clarity; DO NOT take the cover off!

Adding this to your system allows you to automatically turn on your water pump when you power up the PC. The Radio Shack relays pictured above have a 12 volt coil; when the PC is powered up, the coil is energized and contacts are closed.

Wiring Diagram

You ony need one relay – I am showing both wiring diagrams in one picture.

The wiring diagram pictured above shows which contacts to use (wiring diagrams are on the package). You can test which lead does what by using a voltmeter’s continuity tester. With no power to the coil, see which contacts are open and closed. Then power up the coil and do the same. This will confirm what is connected under what state.

Wire up the 12 volt leads to the coil. Then cut the 120 VAC power cord to the water pump. Wire the black and white leads from the plug as shown. Wiring the plug end to the “Normally Open” pins of the relay ensures that there are no other pins “live” when the system is powered down.

Next, wire the leads to the water pump as shown, making sure to match the white and black leads – VERY IMPORTANT!

Insulate all leads! You can use insulated connectors on the leads if you use the larger relay and push them on. Alternatively, use shrink wrap on all leads and then wrap the bottom in electrical tape for good measure.


For water pumps, you will most likely have a green ground wire. Place a male connector on one end and a female on the other and connect. Using connectors on all leads allows you to easily disconnect everything should you need to. Finally, check all connections and test the setup before going “live” to make sure it does what you intend.

Fan Rheostat – 12 volts


A rheostat is basically a variable resistor – used with a fan, you can adjust the fan’s speed. I found the Radio Shack 25 Ohm Rheostat, catalog #271-265B, to be an effective solution with 120 mm 5-6 watt fans. As you can see from the pic above, I mounted it in a spare small serial port on the back of the case – absolutely perfect!

All you do is clip the black wire on the fan, attach one end to the center pole and one end to either of the side poles; insulate with shrink wrap and you’re all set to go. Buy a knob or use it as is.

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