Near Intersection of I-35 & I-90 Southern Mn. | To run a polarity reversing valve with a single trigger wire requires using a DPDT relay with the outputs tied together in a criss cross fashion.
Lets talk about running a polarity reversing valve. The valve has three wires on it. One is constant ground - that's simple. The other two need opposite polarities for the valve to work properly.
If wire A is 12V and wire B is ground the valve should open a 1/4 turn and stop. When wire A is ground and wire B is 12V then the valve should turn 1/4 turn in the opposite direction to close the valve and stop.
One way to run these valves is with a DPDT switch. DPDT means double pole double throw. These switches always have 6 terminals. They are really two double throw switches mounted side by side in one housing.
If you think of this switch as two knife switches mounted side by side it will help the understanding. Both of the knife switches could be UP so each connects to its own upper terminal or both of these knife switches could both be down so each connects to its own lower terminal. Thats why there are six terminals. Since both knife switches are controlled by a common handle they are either both UP or both DOWN.
If you hooked 12V as the source of one of the knife switches and ground as the source for the other then when the switches are both up, the left upper terminal would be 12 V and the right upper terminal would be ground. If both knife switches are down, then the left lower switch is 12v and the lower right terminal is ground. Draw this out on a piece of paper.
Now draw in a piece of wire and connect it to the upper left terminal and the lower right terminal and then extend it out. Draw in another piece of wire and connect it to the upper right terminal and lower left terminal and extend it out also.
When the knife switches are up, one of these wires will be 12V and the other ground. When the knife switches are down, the polarity on the two wires is reversed. Remember the switches are both either completely UP or both completely DOWN.
This is what is referred to as the "crisscross" pattern.
Now to the real question. By using a DPDT relay the same reversing can be accomplished. The DPDT relay is like the switches described above in that it has two sets of points that each have their own set of upper and lower contacts.
DPDT relays always have 8 terminals. Six of them are like the terminals on the DPDT switch. The other two are for a 12V trigger and ground to potentially activate the coil. Because of a spring inside the relay each input terminal will be contacting one of its own set of points.
When electricity is applied to the coil of the relay, the magnetic force of the coil causes both sets of points to contact their other terminals.
In other words a DPDT relay behaves just like a DPDT switch so it can be used for controlling the polarity reversing valves. The wiring is done so that when no electircity is present on the coil, the polarity going to the valves causes them to close.
When a boom switch is turned ON, it really just opens the points causing the output to reverse and the valve should open. If the action is backwards of what you want then just swap the two wires going to the valve.
One of these DPDT relays would be needed for each boom section.
There is another more straight forward solution for many of you. Don't fight it, remove the polarity reversing valves and replace them with Banjo, Teejet, KZ or other brands of valves that do not require a reversing switch.
These valves actually have the polarity reversing scheme built inside them so you need not worry about it. The typically have three wires, constant 12V, constant Ground and a 12V trigger. When the trigger is hot, the internal circuity runs the valve open and stops. When the trigger is off, the valve turns in the opposite direction to close the valve and stops.
These valves are easily controlled by a simple toggle switch or controller.
Edited by tedbear 4/19/2008 20:48
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spra-coupe with ez-boom
