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Determining Channel Current Limits

PDM Installation & Configuration

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In that lecture, is said that using 2 20AWG wires (8Amps rating at 80degrees each) are enough to supply power to the cooling fan. But setting the current limit of the channel at 150% of 12Amps which is 18Amps exceeds the current rating of both wires. Do I miss something? shouldn't the current limit be bellow (8+8) Amps??

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With high quality wire, a limit of 18amps can be reasonable, but everyone's combination of installation, materials, use case, and comfort level is different.

You are correct, a lower limit such as 16 amps, would offer greater margin of safety, however in practice may result in a circuit shutdown which isn't necessarily totally required for safety. That's the compromise we each make when deciding safety limits.

I think the point here is that if the current reaches 18 amps and triggers the PDM to shut down, it's going to be for a very brief period which isn't likely to damage the wire. As Mike said above, there's quite a bit of flexibility in how you configure a PDM based on your personal preference and risk profile. In this example with the fan drawing 12 amps and the wire able to support a continuous 16 amps (give or take), you could choose to set the current limit at 16 amps and call it good. You can also clamp the current limit tighter to the actual current draw if the PDM has separate control of 'fusing' the in rush current.

On that note the inrush current is a great example of exceeding the theoretical current handling capability of the wire but only briefly. With a thermo fan drawing a steady state 12 amps, it's quite likely that it will momentarily pull 20+ amps when the fan first starts up. The reason this is acceptable is that the inrush current will only last for maybe 200-500 ms.

Chris I hope my following comparison helps the general idea come through, and explains a bit more about why the max value selected wasn't making sense. I use the same comparison when talking about EGTs or combustion temperatures actually, and even though it's very non intellectual, I've seen it help this concept click for people over the years. It goes something like this:

If you wave your hand across a flame quickly, you may not even feel it.

If you move your hand across a bit slower, you feel the heat.

If you hold your hand in the flame, then you burn your hand.

Most people know what that feels like, so I think that experience helps relate to other scenarios where the real danger is the impact of heat increasing over time and building up to the point of damage in the item you're trying to protect.

In terms of current monitoring, we're most concerned with situations where there's extreme over current which can cause extreme heat relatively quickly, or there's mild over current, but for significant enough duration that heat does built up to an unacceptable level over time, or a combination somewhere in between that would still result in temperature of the circuit rising beyond safe levels.

In rush current of 2-3x normal current is not uncommon in some devices, but it's so brief that there just isn't enough time for it to cause a concerning increase in wire temperature. For example, 40 amps for 200 ms won't heat a wire anywhere near as much as the usual 12 amps will in 20 minutes. I hope this helps.

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