This switch design was born out of a desire to not allow the circuitry to melt if the forward and reverse switches are put on together which is inevitable with children involved. The simple method to avoid a fire and have minimal wiring could be to just fuse the battery leads and have to replace the fuse if it is shorted. A relay is best used to reduce the losses that accompany a system that has long leads run from the battery (in the back) to the switch (up front) then back to the motor. Also without a relay the switches would have to be rated quite high which are bulky and hard to press on for a child. I chose against this setup as I don't trust the lag time for a fuse to blow as it would need to be rated high enough to not blow when the vehicle is in use, especially when under load and this could put excessive stress on the relay contacts and wiring in a short.
With this circuit the forward switch is pressed to go forward, with both switches pressed (forward & reverse) the vehicle will go backwards and there are no shorting problems. It would be best to add in the line & close to the battery, a fuse anyway and I also added a key switch to be able to control the vehicles use. I also added a 15 amp thermal cut-out just for added safety but this really isn't necessary. This could be set up with a speed controller instead but I didn't find the need as it doesn't really go fast enough. If you're interested email me as I do have several circuits you could use. The disadvantages with a speed controller could be loss in the circuitry and harder to set up reverse. It may be able to be set up so that at full throttle a switch is thrown that isolates the circuitry for less power loss. My first post of the circuits below were drawn wrong (and were potentially dangerous) so my apologies to those who were affected.
This is how the circuit works. The forward relay throws with the forward switch pressed. When the reverse is pressed the forward and the backward relay both throw on. If both switches are (mistakenly) pressed on, the 2 relays switch on just as they do when only in reverse so there is no problem. (except a savage reversal of the drivetrain if the initial direction is forward) Try to keep the high current wiring from the battery to the motor (through the relays) the shortest distance for minimal loss. This part of the system is the most critical so make all the connections strong and tight. If you like, consider it the same as you would the starter-motor wiring on your car although it doesn't draw quite that amount.
Forward relay. SPST (Single pole single throw) normally open. Needs to be rated at the current your motor will draw so about 15 amps. The relay coil is the same as the battery voltage. I used a car spotlight type.
Reverse relay DPDT (double pole 2 position). Needs to be rated at the current your motor will draw so about 15 amps with the coil also the same as the battery voltage.
Battery. 12 volt 35 amp hour Deep Cycle lead acid. Mine was designed for a Golf Buggy and lasted well above what we needed in endurance. Always keep these topped up after use as they will last longer. The best chargers to use fade out as the voltage rises (as opposed to a constant current type) Ask the battery supplier for the max charge rate and max voltage cutout.
Wire. The connections from the battery to relay switch to motor need to be rated for the current draw of the motor. (at least 15 amps) The thinner it is the more resistance and therefore power loss.The rest can be light gauge hook up wire.
Key Switch. This doesn't have to handle much current. Home alarm types will be fine.
In case it's hard to understand, the 2 drawings here refer to the same 2 relays. The first drawing shows the wiring for the coil part of the relay. When the coils (curley wire bit) are given power they act as an electro magnet and pull the switches (arrow's) along side them on. The forward switch is shown with 2 sets of contacts as it needs to be double pole (2 switches built into one). If you can't get this you may be able to get away with mounting 2 switches side-by-side but then you will need to be sure they are both pressed at the same time, maybe by pedal. Another option is to use a single pole switch turning on a relay that has 2 poles. I actually did this as I couldn't find the double pole momentary action switch needed and had a spare relay on hand. This adds an extra level of complexity but was worth it in my case.The second drawing therefore refer to the switching part of the relay. Notice that one of the relays (reverse) has 2 switches built in as well that come on at the same time and both sides of these switches are used. Be sure to get all of these connections right first time or you may very well see smoke. The internal diagram of the relays are often printed/scribed etc on the outside of the relay case or are given on a separate sheet. You need to know this so make sure you get it when you purchase them. Consider in your design having an outlet for extras that are controlled by the keyswitch. We also installed a horn and window washer (that squirts passes by) and I'd love to have the room for a cassette/radio player.
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last Updated Sunday, January 20, 2002