A Homebrew High Voltage Knife Switch
by W5JGV
Just the thing for your high power antenna tuner!
The knife switch is installed in the WC2XSR/13 antenna tuner. It is working at 7,500 volts RF.
The switch is quite easy to fabricate. All of the copper parts are made from flattened 3/8" diameter copper refrigeration tube. The switch handle is a short length of 1/2" ID PVC pipe. Using a hacksaw, a cut is made about half way through the PVC and then the end of the copper blade is pressed into the saw cut.
WARNING!! This handle arrangement is NOT SAFE to use with power applied to the switch! When you operate the switch, you will automatically wrap your fingers around the handle, and this will place the insides of two of your finders up against the sides of the switch blade. Bzzzt!! Ouch!
A safer arrangement is to wedge the end of the switch blade into the end of a length of PVC pipe, about 6 to 6 inches in length. This will keep your hand at a safe distance from the metal contact.
After assembly, a drop of lubricating oil on the pivot end of the switch blade, and a light coating of Vasoline on the stationary contact points makes operation smooth and easy. If the switch contacts grind or "hang up" when they are opened or closed, the lubrication is insufficient or there are burrs on one or more of the contact surfaces.
The Business end of the switch.
The stationary contact is fabricated from three pieces of the flattened copper tube. The outer contact blades are bent slightly apart at the ends to allow the moving contact arm to easily fit between the stationary contacts. To maintain proper pressure on the switch blade, the two stationary contacts are squeezed together just a little bit after the stationary contact is bolted together. This may be seen by careful inspection of the picture. To maintain the proper spacing between the two stationary contacts, a short section of the flattened copper pipe is placed between the two stationary contacts.
A 1/4" diameter hole is drilled through all three contact pieces, and they are bolted tightly together against a PVC pipe cap with a 1/4" X 1" galvanized bolt which goes through a hole that is drilled through the pipe cap. A flat washer is used between the contact assembly and the PVC pipe cap, and between the contact assembly and the clamping nut.
This picture pretty much explains how the moving contact works at the end of the knife blade. The inner bolt is tightened up enough so that the blade movement is slightly stiff, but not enough to squeeze the lock washer completely flat. The bolt through the end of the blade tends to rotate when you operate the switch, so make allowances for this to avoid breaking the connecting wire attached to the bolt.
The standoff insulator for the switch is made from 3/4" PVC pipe and a couple of PVC pipe caps. A 1/4" bolt goes through the right hand pipe cap to attach the switch to the cabinet. Don't use pipe smaller than 3/4" or the switch structure will be too flexible, and you will have problems operating it. If the length of the insulator is more than about 4 inches, 1" PVC pipe would be a better choice.
73, Ralph W5JGV
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