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PDM Installation & Configuration: Determining Channel Current Limits

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Determining Channel Current Limits


00:00 - We talked about it in an earlier module but I think it's best to mention it here again for clarity.
00:05 Fuses or current limiting in the case of a PMU are used to protect our wiring harnesses in the event of a failed component or short circuit.
00:15 They're not and in fact cannot be used to prevent a device from failing or a short circuit from occurring.
00:22 For this reason, when we're configuring the current limit of a PMU output channel, we set it well clear of the expected operational current of the connected device.
00:31 But still within the continuous current draw specification of the attached wiring.
00:37 By setting the current limit above the expected operational current draw we ensure the current limiting will not interfere with the normal operation of the device.
00:46 To do this, we need to know the expected steady state current draw of the connected device which we've talked about in the earlier determining current draw section of the course.
00:55 Once this is known, the wire size required to supply the device is decided upon and between this wire size information and the expected current draw of the device, we now have the information to set the current limit of the output channel.
01:10 As an example of this, if I had a thermofan that I know will draw 12 amps of current under normal operation, I'd most likely wire this to the PMU output channel using two 20 AWG M22759 wires in parallel as this is likely to make the harness construction and connector selection more straightforward.
01:29 I then set the PMU current limit to trigger at a level around 150% of this which is 18 amps.
01:36 At this level, there's no chance of current limit being triggered during normal operation but in the unlikely case that there is a sustained draw of just less than 18 amps on those wires, they won't get hot enough to be dangerous.
01:50 If the fan jams and stalls or there's a wiring harness fault that shorts these power wires to ground the current will spike above this 18 amp limit immediately and the PMU will shut the circuit down before it can cause a fire.
02:03 Setting the current limit of the channel to 150% of the expected current draw is a guide to get you to a good starting point but there are other factors that we need to consider here.
02:14 If the maximum current handling capacity of the wire used to make the connection is less than this 150% level, you should set the channel limit to the lower wire capacity level instead.
02:26 The current handling capacity of a wire is directly related to how hot that wire is.
02:31 And what we need to do is keep the temperature of that wire below a point where the wire insulation will begin to melt or burn.
02:38 Passing current through a wire will cause it to heat up but there are also other heat sources in vehicles.
02:45 If the wire passes close to another heat source like a hot exhaust system, its current handling capacity will decrease.
02:52 This problem is really best dealt with at the harness design stage by specifically routing the harness away from any external heat sources.
02:59 But in the situation where we have a wire passing current close to its rated limit and that wire is going to be subject to external heat sources as well, it could be worth bumping up the size of that wire.
03:12 This will ensure the wire doesn't overheat and the PMU current limit can still be set well clear of the expected operating current.
03:19 In this module, we've looked at determining the current limit setting for an output channel.
03:23 This level needs to be above the expected current draw of the device when it is operating normally to ensure that the limit is only hit if there's a real problem.
03:33 A guideline for setting this limit is 150% of the expected current draw of the device or the current rating of the wire, whichever is lower.
03:43 We've included a table of relevant wire sizes and current limits as a guide below this module.

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