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Practical Wiring - Club Level: Power Supply Design

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Power Supply Design

08.58

00:00 - As detailed in the electrical wiring fundamentals course, the design of our power supply scheme for our EFI systems consists of determining which of the elements of the EFI system require power from the vehicle's 12 volt charging system and when that power should then be supplied.
00:15 With our EFI component listing document completed, we can pull from this, all the elements of the system that will require 12 volt power and then determine how and when this power should be supplied.
00:25 This will naturally tend to group our powered items and help us to determine the number of relays the system will require.
00:32 From our previous component listing document we can see that for our FD3S RX-7 example we are going to need to supply power to the ECU, four fuel injectors, four ignition coils, the idle air control solenoid, boost control solenoid, secondary throttle butterfly control solenoid, oil metering pump, fuel pump, and the O2 sensor heater circuit.
00:53 As this car is primarily for street use and not being used for motorsport, it would be possible to reuse the original power supply system to accomplish this.
01:01 But careful consideration and examination of the factory electrical diagrams would be needed to ensure that the original system is capable of supplying the required current to the new EFI system.
01:12 In our example here we have chosen to retain the original power supply relays and wiring to the radiator cooling fans, as the factory fitted cooling fans are a good multi speed design that function well.
01:23 But we will be installing our own power supply scheme for the rest of the EFI system as much of the factory wiring has been removed by a previous owner.
01:31 This will also allow us to design the system with future upgrades in mind, such as a higher flowing fuel pump.
01:36 When designing the power supply system, we first group our EFI system components into three main sections.
01:43 Those that need to be powered regardless of whether the engine is running or not, which we call main power.
01:49 Those that need to be powered only while the engine is running, which we call enable power.
01:53 And those that are switched on or off as required by the ECU.
01:58 In our FD3S RX-7 example, we put the ECU and oil metering pump into the main power group.
02:05 We put the oil metering pump in this group as the ECU documentation stipulates that a stepper motor like this should be powered from the same relay as the ECU to allow the ECU to correctly control the inductive voltage spikes the switching of the stepper motor creates.
02:20 The injectors, ignition coils, control solenoids, and O2 sensor heater circuit, we put in the enable power group.
02:26 The fuel pump needs to be independently controlled by the ECU and will therefore get its own relay.
02:32 This is a pretty typical setup and results in us needing three relays.
02:36 One each for the main power and enable power groups, and one for the fuel pump.
02:40 The main power and enable power relays will be controlled by the driver via the key barrel with the fuel pump controlled by the ECU.
02:48 If this was a power supply system for a dedicated race car, we would most likely have a relay for the starter motor solenoid which was controlled by the driver via a push button switch.
02:58 In this instance, that will be taken care of by the factory wiring from the key barrel.
03:03 As is often the case when working with a street car that is retaining the original key barrel, there are four key positions.
03:08 Off, accessory, on and a spring loaded start position.
03:12 At first though it would seem appropriate to trigger our main relay from the accessory key position and our enable relay from the on key position.
03:21 However most key barrels will actually disconnect the accessory key position, when the barrel is turned to the spring loaded start position.
03:29 This would then remove power from all the EFI system components powered by the main relay while the starter motor is cranking.
03:36 This includes the ECU and having the ECU power off whenever the starter motor is engaged is obviously not a situation we want as, the vehicle will never be able to start.
03:45 This is the case with the FD3S RX-7 key barrel and to get around this, we will have both the main power and enable power relays triggered by the on key barrel position which remains connected while the starter motor is cranked.
03:58 This results in the main power and enable power relays being triggered simultaneously but it's not a problem as long as we remember that we will not be able to stop the engine while keeping the rest of the electronics active.
04:09 In practice this is not something there is much cause for on a street car, so it's not a large sacrifice.
04:14 An important detail to note is that we've powered the positive side of our enable and fuel pump relays from the output of our main relay.
04:23 This ensures that they can never be part of the EFI system which could be receiving power while the ECU is not, eliminating the possibility of back feeding.
04:33 With the number of relays we're going to use determined, we now need to size the wires and fuses connected to the relays that supply power to the EFI components.
04:42 To determine this, we need to know how much current each of the components is likely to draw.
04:47 This can be an estimated value but always estimate on the high side to ensure an adequate safety factor.
04:53 As our ECU in this example acts as a low side switch to control all of the system actuators, and doesn't supply any current to devices, it will not need particularly large power supply wiring itself.
05:04 Check your ECU documentation to see if it has an approximate current draw for the unit, but in this application, we can expect our ECU to draw one amp or less.
05:14 We can measure the resistance of our fuel injectors and control solenoids and then use ohms law to determine their peak current draw.
05:21 Our injectors measure 13.7 ohms and we can assume a 14 volt supply to them while the engine is running and the alternator is supplying the system.
05:30 14 volts divided by 13.7 ohms gives us a peak current flow of 1.02 amps.
05:37 This procedure is then repeated for the control solenoids with our aftermarket boost control solenoid having a peak current draw of 400 milliamps, our idle air control solenoid drawing 1.1 amps, and our secondary throttle butterfly control solenoid drawing 412 milliamps.
05:54 Our wideband O2 sensor heater has a rated power of 7.5 watts.
05:58 Using the power equation and an assumed supply voltage of 14 volts, our heater current can be calculated as 7.5 watts divided by that 14 volts, which is 535 milliamps.
06:11 The factory service documentation for this year of FD3S specifies the oil metering pump stepper motor coils to have a resistance of between 16 and 31 ohms.
06:21 We can determine from this the peak current through the stepper motor windings will be less than one amp.
06:26 These current draws are all relatively low meaning we can safely use 22 AWG wire to supply power to these devices.
06:34 The documentation for our IGN-1A ignition coil specifies their peak current draw at 19 amps.
06:41 In practise we will not dwell them long enough to reach this value, and we can safely assume a peak current draw of 10 amps.
06:47 This 10 amps will actually only be flowing through the coil for a short time, right before the spark is initiated.
06:54 And because the coils are an inductive load, will have had to have built up from zero amps over our dwell period.
06:59 Meaning we can average the current draw of the coil down to five amps.
07:03 This assumes the coil is being fired continuously with no wait between the spark event occurring and the next dwell period during which the coil is charging.
07:12 This is a worst case scenario and the one that we will use to size our wiring as it will give us the required safety factor.
07:18 This current level dictates that we will supply power for our ignition coils using 18 AWG wire.
07:24 We are going to be upgrading the original fuel pump in the car to a Walbro 255 litre per hour pump using the original fuel pump hanger and level sender.
07:33 These pumps draw approximately 10 amps when working hard, so we would choose to supply power to one with 14 AWG wire.
07:40 With our wire sizing determined we now need to determine the size and number of fuses we're going to use to protect our harness wiring.
07:47 To do this, we divide our power supply system up further into smaller groupings, each of which is protected by a single fuse.
07:54 The groupings we make are usually fairly logical by this point, and for our FD3S they will be the ECU, which will be protected by a five amp fuse, the boost control, idle control, and secondary butterfly control solenoids, also protected by a five amp fuse.
08:08 The wideband sensor heater, again with a five amp fuse.
08:11 And the oil metering pump, which also gets a five amp fuse.
08:15 The injectors will get a 10 amp fuse, the ignition coils will get a 15 amp fuse, and the fuel pump which we will protect with a 20 amp fuse.
08:23 This means our system requires seven fuses in total.
08:27 Now that our power supply system is designed, we need to document it, as visualising it this way can be very helpful in determining any issues that we may strike.
08:36 There are electronic design software packages that can be used for this, but I find a tidily hand drawn document to be the quickest and easiest way to go, at the street car and club day track car level, it's more than adequate.