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PDM Installation & Configuration: Overcurrent, Undercurrent & Under-Voltage Faults

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Overcurrent, Undercurrent & Under-Voltage Faults


00:00 - Now that we understand that one of the main purposes of a PMU is to replace the fuses and circuit breakers in our electrical system, we can have a talk about how they actually do this and why it's beneficial.
00:11 We use fuses and circuit breakers in our electrical systems to give us a layer of protection in the event that something goes wrong with the wiring harness or a device that we're providing power to.
00:21 It's important to understand that we use fuses not to protect the devices in our electrical system but to protect the wiring itself.
00:30 They do this by preventing a device that has failed from causing current draw too large for a particular wire size which could easily result in a serious fire.
00:39 When you pass more than the rated current through a fuse, it physically melts, opening that circuit and stopping the flow of current.
00:47 Circuit breakers perform a similar operation but trip a spring loaded system that physically breaks the circuit that's running through them.
00:54 This happens quickly enough that the connected wiring doesn't have enough time to heat up to an unsafe temperature.
01:01 PMUs provide the same feature but instead of it being a physical interruption to the circuit they use solid state electronics to switch the outputs on or off.
01:09 PMUs monitor the current flowing through each of these output channels through internal analog hall effect or resistive current shunt sensors.
01:18 And if they see a current level that rises too high, they simply switch off that output.
01:23 This gives us the same result as with the fuse because that output will switch off before an abnormally high current level can heat the connected wire up to unsafe temperatures.
01:34 This electronically monitored system gives us a heap of flexibility in our system because we can program the maximum current level to whatever we like, not having to work with fixed value fuses or circuit breakers.
01:45 We can also program these outputs to allow a higher current level for a short period of time and only switch off if that high level persists for an amount of time which we can also set.
01:57 This lets us cater for capacitive and electromotive loads like DC motors that can have an initial switch on current much higher than their steady state running current.
02:07 Trying to do this with fuses can be troublesome as the fuse might be perfectly fine for the steady state current level of the connected device but will blow when that device is initially turned on due to that higher in rush current.
02:19 Radiator cooling fans are particularly notorious for this and the solution has previously been to run a large gauge wire to the fan than it might actually need and also run a larger fuse to match that larger gauge wire.
02:33 While the solution works, PMUs certainly handle the problem in a more elegant way.
02:39 When a fuse blows or a circuit breaker trips, it causes a physical break in the circuit and that requires a physical action to reconnect the circuit.
02:47 This can either be replacing the fuse or resetting the circuit breaker.
02:51 PMUs give us another great option here in that they can be programmed to automatically retry a circuit a number of times.
02:57 If the situation that caused the initial fault has cleared, the device will power back up and the vehicle can continue.
03:04 To give an example of this, we'll pick on radiator cooling fans again.
03:07 Debris on a racetrack surface is a pretty common thing and it's possible for this to get flicked up into cooling fans, stalling them.
03:15 Stalling the motor in a radiator cooling fan will cause the current through that fan to increase dramatically.
03:22 The PMU will detect this, it will log an over current fault and it will shut that output down.
03:27 If the debris then falls out again and the PMU retries the output, the fan will turn back on.
03:34 The circuit breaker would have tripped and need to be manually reset and while this is possible for a driver to do mid lap, it's not ideal and a fuse would have blown likely requiring a trip the pits to replace.
03:46 PMUs can also help us detect other faults that we can't see when we're using fuses and relays.
03:51 One of these is an under current failure.
03:54 While all PMUs have maximum current settings on their outputs, many also have a minimum current setting.
04:00 When that output is turned on, the PMU will throw an under current error if the current flowing doesn't rise above this lower level.
04:08 This can help us detect open circuit breaks in the wiring harness, devices that have failed, blown lightbulbs or even something as simple as a connector that's been left unplugged.
04:18 Another issue many PMUs detect is an under voltage fault.
04:22 When an output on a PMU is turned on, the voltage at the output pin should rise to the connected supply voltage as PMUs are primarily high side switching devices.
04:32 If a PMU turns on its output and the voltage at that pin doesn't rise, it means that pin is most likely short circuited to ground.
04:41 The PMU will immediately trigger a fault and switch the output off again.
04:45 This situation would also result in an over current fault but because the current monitoring system has settings and filters built into it to deal with in rush currents, the under voltage fault system will typically trigger first.
04:57 This is great for detecting a dead short on an output, giving us crucial information to follow to find and repair that fault.
05:04 In this module, we've looked at how a PMU provides one of its key functions, that being replacing the fuses in our electrical system.
05:13 A PMU does this by monitoring the current through each of its outputs, making sure it stays within programmed limits.
05:20 It can detect several different fault types.
05:22 Over current, under current and under voltage.
05:25 Each of these will cause the PMU to switch the output off, protecting the connected wiring and the PMU can be programmed to automatically retry the output, allowing a vehicle to continue, if the cause of that fault has been cleared.

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