Even Deeper into Rabbit Hole - 4 Pin PWM with External Power Precautions

As from my previous post, I discussed the dangers of a wrong PWM connector used on D5 pumps. Let's dig a bit deeper shall we?

For a refreshment, here's the papers (again) that I'll keep mentioning throughout this post:

- Noctua PWM Specifications White Paper:
https://noctua.at/media/wysiwyg/Noctua_PWM_specifications_white_paper.pdf

- Intel 4-Wire Pulse Width Modulation (PWM) Controlled Fans:
https://docplayer.net/23812689-4-wire-pulse-width-modulation-pwm-controlled-fans.html

What can possibly go wrong?

Now, as far as I can remember, not only pumps that were externally powered from molex or sata, but also a number of PWM fan hubs.

"Wait a sec, are you trying to say that fan hubs are also unsafe?!"

They are generally safe, but not without any cautions. Basically there are 2 types of hubs that can be plugged into MB's PWM fan header:

• Simple fan splitter cables are safe, but you also should understand the current capability for your motherboard's fan header to avoid damage.
• Powered fan hubs are also safe, because you're not taking power from the MB. But you also has to be more careful when plugging the input to the motherboard.

For now we will be focused down to the powered hubs. They usually took power externally by plugging in to molex or sata connector. And most of them works by simply splitting the PWM/Control signal from the MB, take a TACH/Sense reading from one of the fans to the MB, and give power from the external connector. They usually have only 2 wires on 4 pin PWM connector to the MB. Some of them also features a big electrolytic capacitor to smooth out the power to the fans.

The implementation for splitting the PWM signal this way is already different from the specification, intel suggests adding buffers if intended to drive multiple fans with a single PWM output (Intel specification, page 11: "Considerations for Motherboard Designers"). As the intel specification, PWM control output from the MB is an open-drain or open-collector, and the pull-up is located in the fan. Noctua, on the other hand, stated that there are no special requirement for driving the PWM signal, they shows that either push-pull and open-collector outputs are fine (Noctua PWM specifications, page 8: "Driving circuit"). So I guess it's okay to say that although different, this splitting approach is safe enough. As long as the pull-up circuits are identical between all connected fans (in another words, using the same fans for a single hub).

My trusty homemade portable PWM fan tester,

driving the PWM signal straight from the MCU output pin.

Moving on.

Now let's bring up the connector precaution again. What will happens if the 4 pin connector is plugged wrongly onto the MB header? Let's refer to my previous post again for the diagram and schematic of plugging the connector on an offset. This time, the current will flow from the +12v pin from the MB header, into the TACH input on the fan hub, out to the TACH input on the connected fan (usually the first header or the different colored one on the hub), into the transistor/mosfet inside the fan, and out to the ground via external supply (molex/sata). Poof goes the transistor.

Wrong current path

Granted, this have a high possibility only when the manufacturer is using a wrong part for the 4 pin PWM connector like in the case of my D5 pump, and I've never see a hub with wrong connector, yet. But! It doesn't mean that it can't happen on a proper connector as well.

Whomp!

Alright, alright, this is obviously a worst case scenario. You had to jam it pretty hard to get into this position. That plastic is pretty hard to bend and surely you'll feel it if you plugged it wrongly. I can only imagine this situation might happen if someone (presumably not really experienced in building a computer) is in a hurry.

Then again, this is all depending on how the fan hub manufacturers implement the driving circuit. Most of the time, especially with cheaper ones, they tend to use this direct PWM signal splitting method. There are few exceptions though, like Phanteks' Universal Fan Controller that can take both PWM and DC signal from the MB to control the fan speed, in which obviously they have some active circuitry to split the control signals.

Summary

So the conclusion is always pay attention to the fan connector. Not only for the pumps, but for anything that needs to be plugged into PWM fan header and be externally powered. And as I said on my previous post, do be aware with devices with improper connector.

On the very least if something goes wrong, there will be just a puff of magic smoke released from the transistor/mosfet, and your RPM reading will be gone. But at the worst case, it could be a fire hazard (if the transistor fails by keep shorting the collector and emitter, and if the motherboard didn't have a current limiting capability on the fan headers).