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First of all I apologise if this is a silly question; I fully admit to being an enthusiastic amateur and although I've done a search for the answer already, it's possible that this is super simple and I've just missed it.

Anyway, I'm trying to mod my 3D printer with some quieter fans. As part of the process I tried to figure out what pin on each of the fan JST connectors go to ground.

I can see that that one of the connectors for each are shorted to each other, and that those same pins cause my multimeter to show continuity to other ground connections, but only for about 3-4 seconds. If I swap my mutlimeter prongs over, it will need for another 3-4 second.

The only conclusion I can come to is that they go to ground through a capacitor, but I can't figure out why that would be?

It doesn't really block my modding, but I'd like to understand why this might be.

EDIT: The circuit board is the main board for a Flashforge Adventurer 3 3D printer.

Here are some pictures which should help somewhat:

Top view of board Bottom View of Board

Better description of what I'm seeing:

The fan ports for fan A and fan B connect to chassis fans in the printer with fan A running all the time that the printer is on. Fan B only runs whilst printing.

I'm in the middle of creating a small, very simple board that will take the 24v, convert it into 12v, and use that to power better, noctua fans. The conversion is necessary because the 40mm fans don't seem to come in 24v. I'm more or less happy to struggle along with that aspect myself.

With the fans removed, if I stick my multimeter (I don't have an oscilloscope) across A1 and B1 I get a short, which I don't think is conclusive of them being ground, as the logic to turn them on and off could be past the load/fans?

If I use my multimeter across either A1 or B1 and one of those ground pads with the red arrows, I get a beep for about 3-4 seconds as previously described. Swap the probes, and the beep comes back again for the same amount of time.

An annoying wrinkle just discovered, is that the same pins (A1, B1) do a similar intermittent beep when checking for continuity against VCC, but for about 1 second per probe swap.

I would really like to have an understanding of what might be going on here, but more than anything, I would like to have some clue as to the best way to discern ground from these fan pins so I can wire my little board up properly!

Really appreciate everyone's help.

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    \$\begingroup\$ Unlikely that they go to ground through a capacitor. Is it possible you are instead measuring continuity from V+ to GND? Maybe power the board up and measure the voltages? \$\endgroup\$
    – Wesley Lee
    May 18, 2021 at 14:04
  • \$\begingroup\$ Fan part number / web page / photo - ? \$\endgroup\$
    – AnalogKid
    May 18, 2021 at 14:05
  • \$\begingroup\$ Will sort out pictures etc when I get home. Interesting thing of note, I've seen another post of someone saying they suspect their 3D printer is driving a 12V fan from 24v via some form of PWM. I wonder if that's part of what I'm seeing? \$\endgroup\$
    – Glitcher
    May 18, 2021 at 14:18

2 Answers 2

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It is as you have discovered and described. The two coresponding pins on each connector are connected together and then connected to ground through a capacitor. This accurately describes their connection to ground, but that is not all they are connected to. See the figure below. 4 pin connector to motor with bypass capacitor

Now you can see that you are likely measuring continuity between the Vcc pins (+5V or whatever) and GND. Each connector of course shares the same source, which explains why they have continuity, and Vcc is connected to GND through what is called a bypass capacitor. This capacitor is in parallel with the load and provides instantaneous current for short but large current draw from the load (ie. on motor startup).

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  • \$\begingroup\$ If your intention is to show a 4-wire PC fan pinout, the diagram you show doesn't follow the Intel standard. May want to revisit that. \$\endgroup\$
    – hacktastical
    May 18, 2021 at 19:05
  • \$\begingroup\$ I have updated my question with more information \$\endgroup\$
    – Glitcher
    May 18, 2021 at 20:00
  • \$\begingroup\$ Regarding your answer. If I say that pin 4 was analogues to my pin A1 (for example) wouldn't I get a continuous beep from my MM when probing to ground? Although I can see that if A1 was VCC then It would act in the way you're describing... \$\endgroup\$
    – Glitcher
    May 18, 2021 at 20:11
  • \$\begingroup\$ Based off this I've checked and I can see that the 24v connection on the board that is fed directly from the PSU is indeed directly connected to my A1 and B1 pins. I'm going to mark this as correct as I think this is what got me the closest. Very much appreciated! \$\endgroup\$
    – Glitcher
    May 18, 2021 at 20:21
  • \$\begingroup\$ @hacktastical Its just representing a generic 4 pin connector to visualize the concept; I made it before Glitcher updated his question with more details. Thanks for the heads up though! \$\endgroup\$
    – Ben Watson
    May 18, 2021 at 22:15
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Assuming the fan is a brushless DC type, there is a controller chip and circuitry inside of it that will have a local bypass capacitor to stabilize their voltage.. Your meter is charging or discharging this cap when you measure ohms.

A brushed motor won’t have that. It will appear as a resistance.

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  • \$\begingroup\$ I should have explicitly said: the fans are not connected at this point; I am/was just trying to figure out what pins were for ground \$\endgroup\$
    – Glitcher
    May 18, 2021 at 18:25
  • \$\begingroup\$ Can you measure the voltage on the board's fan connector? At any rate this pinout generally follows an industry standard set by Intel for PC fans. Pin 1 is GND, pin 2 is +12, pin 3 is tach, pin 4 is PWM. More here: landing.coolermaster.com/faq/3-pin-and-4-pin-fan-wire-diagrams \$\endgroup\$
    – hacktastical
    May 18, 2021 at 19:00
  • \$\begingroup\$ I need to install this new mainboard into the printer, then I could measure the voltage across the pins. I know that it was 24 volts, but I did have problems discerning which direction it was going in. If I remember correctly (it was a couple of weeks ago) it seemed to say +24 regardless of the position of the probes, although I would need to double check to be sure. I've added more detail to the question also. Appreciate your time. \$\endgroup\$
    – Glitcher
    May 18, 2021 at 20:03

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