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254 Build Your Own Electric VehicleIf you wish to utilize a more modern digital voltmeter readout in place of the analogmeters, you need to adjust the DVM’s sample-and-hold circuit (it memorizes the valueat any instant) either to display the average of the last few moments’ sample-and-holdvalues, or to give a steady readout when a read button is pressed. Otherwise, the rapidlychanging voltage or current will be hard to interpret.The subject of safety interlocks is an important one. Jim’s design uses three—allwired in series on the low-voltage 12-volt key-switch line: a fuel injection impact switch,a main safety cutoff switch, and a charger cutoff switch (to be covered in the “ChargerSystem” section). The fuel injection impact switch’s normal role is to shut off the fuelsystem in the event of a crash impact. Jim points to its location under the passenger’sside of the 1987 Ranger’s dashboard in Figure 10-25 (bottom). The main safety cutoffswitch is a highly accessible, dashboard-mounted switch wired in series with the keyswitch. Punching it immediately removes energizing voltage from the main highcurrentcontactor. A few EV converters also use a seat interlock switch that latchesclosed when the driver’s presence in the seat is detected. You might wish to considerthis as an option.Jim opted to use a battery as the source of the 12-volt accessory system power. Youcan do the same or utilize the DC-to-DC converter shown in Chapter 9 that’s drivenfrom the main battery pack voltage. If you opt for the DC-to-DC converter, now is thetime to install it and wire it in place; its input side goes directly across the main batterypack plus and minus terminals. Its output side provides +12 volts at its positive terminal,and its negative terminal is wired to the chassis. If you elect to use a 12-volt deep-cycleaccessory battery, do the wiring for it now but wait until the battery phase to purchase,install, and connect it up.Try to use AWG 12 (20-amp rating) or AWG 14 (15-amp rating) stranded insulatedcopper wire for the low-voltage system. The instrumentation gauges can be wired withAWG 16 or even AWG 18 wire.Safety fuses of the 1-amp variety should be wired across the potentiometer and alldelicate instrumentation meters. The key-switch circuit can utilize the original fusepanel but don’t use the original wiring for any loads greater than 20 amps. The mainfuse should be of the 10-, 15-, or 20-amp variety.Unlike the high-current system, the low-voltage system is grounded to the frame;the negative terminal of the 12-volt battery (or DC-to-DC converter) is wired directly tothe frame or body. Most internal combustion engine chassis come this way. You eliminaterewiring, extra wiring, and potential ground loops by using the existing negativeground-to-the-frameconvention.Junction BoxA good junction box design cleans up the hodgepodge of instrumentation wiringrunning every which way inside the engine compartment, enables you (on anyone else)to later retrace your wiring, and provides convenient mounting and tie-off points forvarious components. However, not all junction boxes are created equal. Jim’s and Paul’s“magic boxes” are more equal than most—they combine simple design and layout withhigh utility (see Figures 10-26 and 10-27). The high-current safety fuse (in the center,behind the power cable), the ammeter shunts (large one on center of back wall, smallone in front of main contactor), and the main contactor (on left side of box) form, alongwith the terminal strip, the “backbone” from which all interconnections are made.Notice all the safety fuses are located in one convenient area at the right rear of the box.