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I have a Seasonic FOCUS-SPX-650. Since sleeved cables are often times out of stock and too long for my use case, as I use the PSU in an ITX system, I decided to build the CPU, PCI-E and ATX cables by myself.

The CPU cable was no problem, I just copied the original cable. However, the ATX cable is not that trivial. The reason for that are the sense wires for the PSU which result in 10 + 18 pins on PSU side (= 28 in total) while only using 24 on mainboard side (ATX plug).

Seasonic resolved this issue by just crimping two cables into one Mini-Fit Jr terminal. They use the same gauge (= 2 x 16 AWG) for both the current and the sense cables. While I wonder how they did that, I know that I can't do that, because the terminal will only take one or multiple cables totally not exceeding 16 AWG. Even if I use 17 or 18 AWG it will be too thick.

What I thought about is just crimping 17 AWG for current and 23 AWG for sense together. That works, the crimp is fully cosed. However, what I do different is that I don't make it directional, I just strap it by using the cable as a jumper. Please see the photo above so that you can see what I mean:

enter image description here

My questions are:

  1. Is the jumper approach a good idea? Does it have any drawbacks other than it may not look that good (however, as it is on PSU side, I do not care at all).

  2. Is 23 AWG enough for the sense wires? I assume that the sense must have a high resistance, so there can't be much current on it.

  3. Is it fine to crimp two cables together as long they together do not exceed 16 AWG?

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  • \$\begingroup\$ Wait,what are we looking at in that photo? Where does the free end of that black wire get plugged into? A sense wire surely has to go all the way back to the PSU,or how would the PSU sense anything? \$\endgroup\$
    – TooTea
    Feb 4 at 18:42
  • \$\begingroup\$ The thin black wire is a jumper for the sense pin. Seasonic uses two pins with two wires (1 pin = 1 wire) on PSU side which are then inserted together into into one pin on motherboard side (1 pin = 2 wires). My approach is to use a jumper cable instead of a full length sense wire from PSU to MB, which is then inserted into the sense pin on the PSU plug. \$\endgroup\$
    – Tintenfisch
    Feb 4 at 19:05
  • \$\begingroup\$ It should be OK for the voltage sense wires to be something like 24 AWG because they do not carry enough current for the voltage drop across them to be significant in this context. However, I am not an electrical engineer so there might be some other criteria that I am not aware of. \$\endgroup\$
    – Andrew Morton
    Feb 4 at 20:44

1 Answer 1

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The point of sense wire is for the power supply to sense the voltage at the load.

If you skip that and connect the sense wire to measure voltage at the power supply, then it senses voltage at the wrong end of the wire, and it's lower on the motherboard.

So that's not the intention how to do it. It may still work, or lead to problems.

  1. No, not a good idea. Because it should monitor motherboard voltage.

  2. Almost any wire should do. Very little current flows in the wire, but the less resistance there is in the wire, the better the measurement.

  3. Hard to say what's needed due to current in main wire or sense wire, and hard to say what is OK for your crimp terminals, if there are no specs for that.

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  • \$\begingroup\$ @Justme Thank you! I don't understand point 1 fully though. How come that this makes a difference? I know that in series - other than current (A) - voltage is not the same at all points. Over the distance it will drop due to resistance. The original sense wire ends with the pin of the ATX plug. If you think about the two cables (current + sense) one can say that it virtually is only a thick cable split into two, which is a parallel connection from PSU plug to ATX plug. I don't understand why the jumper would be so much different from that (plus due to its shortness resistance is very low). \$\endgroup\$
    – Tintenfisch
    Feb 4 at 20:49
  • \$\begingroup\$ @Tintenfisch The thick main wire carries a significant current, so its resistance creates some voltage drop along that wire. The thin sense wire carries next to no current, so voltage drop on it is negligible. The PSU can thus use it to measure the voltage drop on the main wire and compensate for it. Say, when the PSU is putting out 12V via its output pin and 0.5V gets lost along the way, the motherboard end is at 11.5V. The sense wire carries that information back without any loss, so the PSU sees 11.5V on its sense pin. The PSU then knows it has to crank its output up to 12.5V to compensate. \$\endgroup\$
    – TooTea
    Feb 4 at 21:08
  • \$\begingroup\$ @TooTea Wait, so the reason for the sense wires is primarily (or only?) to measure the voltage drop on the cables? Then this really makes sense for me now! I didn't know before that high current increases the voltage drop, I wrongly assumed it is the resistance alone. I thought the voltage drop on the cables would therefore be negligible because they are thick (18 to 16 AWG) and not so long. \$\endgroup\$
    – Tintenfisch
    Feb 4 at 21:14

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