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PDM Installation & Configuration: Types of Electrical Loads

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Types of Electrical Loads


00:00 - The job of a PMU is to safely manage the delivery of power to the other parts of the electrical system in the vehicle.
00:07 To do this, we need to configure the current limits of those output channels so that if the current flowing exceeds these limits, the PMU knows that something is wrong and it can shut that channel down.
00:17 To configure these current limits effectively, we need a bit of knowledge about the other parts of the system and what categories of electrical loads we can classify them as.
00:26 The common categories we deal with are resistive, capacitive and electromotive.
00:31 We'll go through each of these now and discuss the important considerations around each.
00:35 Resistive loads are the simplest to deal with because we can directly measure the resistance of the load with a multi meter and use ohm's law to calculate the current that will flow through that load when the PMU applies voltage to it, turning it on.
00:47 Because Ohm's law is V=IR, voltage equals current times resistance, we can rearrange this into I=V/R and calculate that exact current draw.
01:00 When we do this it's common to assume a system voltage of 13.8 volts as this is what automative alternators commonly output.
01:08 This lets us configure the current limit for the output channel of a PMU that's supplying power to this load very easily.
01:14 Unfortunately, purely resistive loads are quite uncommon and there's usually other effects that we need to take into account when configuring the PMU's current limits.
01:23 The first of these being capacitance.
01:25 To talk about capacitive loads, we first need to talk about what a capacitor is.
01:29 You can think of a capacitor as a very small battery in an electrical circuit.
01:34 They act like dampers in an electrical system, soaking up large voltage spikes which have the effect of charging them up and then releasing this energy over a longer time period, smoothing out that supply voltage for the rest of the connected circuit.
01:48 The flipside to this is when the connected circuit has a large sudden current requirement, the attached power supply might not be able to supply this quickly enough and the capacitor steps in to take up the slack.
02:01 If you've ever switched off an electronic device and the lights on it have stayed on for a few seconds afterwards, this is due to the capacitors continuing to power the circuitry until they're drained of that stored energy.
02:12 Where this meets us in the real world is when we first turn on an electrical module that has large capacitors contained within.
02:19 Those capacitors are sitting there totally discharged and will draw a large current upon initial startup, known as an in rush current while they charge up.
02:27 It's possible this large in rush current could trip a PMU's current monitoring system and cause it to shut down that output.
02:35 There aren't many examples of capacitive loads in automotive applications using solely internal combustion engines but power switching modules like inverters for EV applications have truly massive capacitors which can cause PMUs to mistakenly trip.
02:51 Most electronic devices that have in rush currents high enough and lasting long enough to be a problem, will have specific integrated ways of dealing with this, known as pre charge circuitry.
03:02 But we might still need to configure the PMU to deal with these larger than expected currents.
03:07 Electromotive loads such as DC motors on the likes of fuel pumps and radiator fans are very different to capacitive loads but they give us a very similar problem, being high in rush currents.
03:18 If you measure the resistance across a fuel pump, it's common for this to be as low as 0.5 of an ohm.
03:23 From the rearrangement of Ohm's law we discussed earlier, we could calculate the current draw of this pump as I=V which is 13.8 divided by R which is 0.5, giving us 27.6 amps.
03:37 This is a very high current level and far above what the pump will actually draw when it's in operation.
03:43 This is because as DC motors operate, they produce a voltage opposing the supply voltage which has the effect of reducing the current flowing through them.
03:53 This voltage, called back electromotive force or back EMF is directly related to the speed that the motor is spinning at.
04:01 When we first turn the pump on, it's not spinning at all so there's no back EMF and a large in rush current will flow.
04:07 As the motor speeds up, the back EMF is generated, reducing the current through the motor.
04:13 So even though capacitive loads and electromotive loads are completely different electrically, they present the same problem for us to deal with, the high in rush currents.
04:22 PMU manufacturers are of course aware of this and give us facilities to deal with the problem.
04:27 Mostly this is done by giving us the option to enable a filter on the current monitoring system for particular channels that'll slow down the response and let higher in rush currents flow for a short period of time before tripping and disabling the output.
04:41 This filter can be as simple as an amount of time a current level must be over the upper limit in order for the output to trip, or a more complex mathematical operation.
04:50 Your PMU's documentation will guide you on how this is implemented.
04:54 There are also parts of the system that you'll want to supply power to that aren't as easy to classify such as other electronic modules like ECUs, wideband controllers and dash loggers.
05:05 In the case of electronic devices like these, the documentation they're supplied with will give an expected operating current draw.
05:12 And while they technically can be considered capacitive loads, the in rush currents they draw are relatively small and won't cause the PMU monitoring systems any problems.
05:21 In this module, we've looked at the most common classifications of electrical loads you're likely to encounter when designing and building your automotive electrical systems.
05:29 Being resistive, capacitive and electromotive.
05:32 Resistive loads are relatively straightforward but not commonly seen.
05:36 Capacitive and electromotive loads give us an additional factor to deal with which are in rush currents that can mistakenly trip a PMU's current monitoring system.
05:45 We deal with this problem by enabling filters or delays within the PMU to momentarily allow a higher current level to flow through the connector device when it is initially powered on.

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