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PDM Installation & Configuration: Determining Current Draw

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Determining Current Draw


00:00 - One of the main reasons for installing a PMU is to remove the need for discrete fuses in our electrical system.
00:06 To do this, we need to know how much current the connected devices are going to draw so we can configure the PMU to allow that level of current to flow but shut the circuit down if a larger flow is seen.
00:17 This means we might need to research, measure or calculate some expected current draw values for the connected devices.
00:24 Which is actually no different to designing a conventional automotive electrical power distribution setup as we need to undertake the same work to size our wires and fuses.
00:33 Calculating current draw for a purely resistive load is relatively easy as we can simply measure the load resistance, assume a supply voltage of 13.8 volts, rearrange ohm's law and calculate that current draw.
00:44 We've discussed this in the wiring fundamentals course so we won't go through the procedure again here but unfortunately as previously mentioned, purely resistive loads are quite uncommon in the automotive arena.
00:55 Things start to get a little more complicated when we're working with our electromotive and capacitive loads as the way these devices operate mean that we can't directly measure their resistance and use it to calculate the current draw.
01:07 There are two options in this situation, the first and usually the best is to rely on the factory documentation for the device you're working with.
01:14 In the case of devices like thermofans, fuel pumps or xenon headlight ballast, this is usually very easy to find if you've purchased the item new.
01:22 They'll typically have two current values listed, one for the steady state current draw under normal operation and the other being the in rush or stall current.
01:32 And hopefully how long that in rush current can be expected to last for.
01:37 If only the steady state operating current is listed, most electromotive loads will have an in rush current on start up of 3-5 times this steady state value.
01:46 As an example, a common 14 inch thermofan might draw around 12 amps during normal operation so we'd assume an in rush current of around 50 amps while that fan is accelerating up to speed.
01:57 Thinking about how quickly a thermofan accelerates up to speed, we'd expect this in rush current to be present for around half a second.
02:05 If the factory documentation for the device isn't available in this case that you'll be purchasing OEM components, or have purchased something second hand, you'll need to measure the current draw of those items.
02:15 The best tool to use for this is an oscilloscope and an inductive current clamp probe.
02:20 Using these you'll be able to capture the current level over the entire start up period and get values for the in rush current, how long it lasts in the steady state operating current.
02:30 The common approach here is to directly power the device from either a high current benchtop power supply or a fully charged automotive battery and capture the current flow in a scope trace.
02:39 Now there's a little bit of a chicken and egg scenario here as you'll need to select some wire for this that will handle the required current, which is also the main parameter that we're trying to measure.
02:49 In this instance, I keep a couple of lengths of 10 AWG wire handy because they're capable of handling the current most devices that we're going to power from a PMU will draw and then I just err on the side of caution.
03:00 Getting this current level information now will also be crucial for sizing the wire in the wiring harness design process.
03:08 If a scope and current clamp probe aren't available, you can use a common multimeter for this as well.
03:13 However you'll be limited to measuring only the steady state current.
03:17 To measure current with a multimeter, make sure that the probes are in the correct ports and wire the multimeter in series with the device, not in parallel like we would for a voltage measurement.
03:27 Keep in mind that many common multimeters can only measure up to 10 amps and if they pass any more current than this through them, they will blow their internal protection fuse.
03:37 It's important to note that these documented or measured current values are not the current limit values we configure within the PMU.
03:45 They obviously relate to the configured limits but we'll outline this relationship later in the practical discussion section of the course.
03:52 In this module, we've talked about why we need to know the expected current draw of the devices in our system.
03:58 We've touched on two ways of determining this, the best being to refer to the factory documentation of the device and the second being to power the device temporarily directly measuring the current draw, either with an oscilloscope and current clamp or with a multimeter.

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