Very often you can find that there are two signal pins (+) and (-) for the LED in the motherboard. 1) Why not use one signal (+), and minus replace the ground? 2) What signal can be considered a common (+) or (-)?
\$\begingroup\$
\$\endgroup\$
4
-
7\$\begingroup\$ There's no guarantee that either is ground or VCC, nor that there is a common pin in there anywhere. \$\endgroup\$– Tom CarpenterCommented Sep 14, 2017 at 17:00
-
4\$\begingroup\$ In addition to the other good answers, there's the simple explanation that it makes it easy for case manufacturers to provide a header plug for the corresponding LED. \$\endgroup\$– peterGCommented Sep 14, 2017 at 20:20
-
1\$\begingroup\$ So you're asking why they don't have the exact same pins but label one of them GND? \$\endgroup\$– Stack Exchange Supports IsraelCommented Sep 15, 2017 at 12:00
-
\$\begingroup\$ Note that on the motherboards I've seen there isn't a ground pin on PWRSW or RESET either, just two pins both marked PWRSW or RESET. \$\endgroup\$– Stack Exchange Supports IsraelCommented Sep 15, 2017 at 12:01
Add a comment
|
4 Answers
\$\begingroup\$
\$\endgroup\$
1
If the LEDs are driven using the most common method, as shown below.... you have no commons, no Vcc and no Ground.
If you have more than one LED you can not join any of those wires together either.
simulate this circuit – Schematic created using CircuitLab
Note: you could swap the LED and resistor to generate a common plus side. However it is usually better to have the resistor on the high side to prevent inadvertent shorting of the Vcc to ground, or elsewhere, in a bad or faulty cable connection. Also, if the cables are any significant length, attaching a long antenna directly to the power line adds EMI, EMC, and static discharge issues.
However, either way you would label the pins LED+ and LED- so whomever is creating the cable or attaching the LED knows which way round it goes.
-
2\$\begingroup\$ And for completeness, you could also put the transistor above the LED and have a common ground, but then you'd have to use a slightly more expensive transistor. \$\endgroup\$– Jack BCommented Sep 14, 2017 at 17:53
\$\begingroup\$
\$\endgroup\$
In addition to the other answers, note that the format for connecting LEDs to computer motherboards became standardised quite a long time ago, long before the currently popular integrated case front panels. When these connections became the de facto standard at some stage in the early 90s, the LEDs would literally be separate LEDs, mounted directly in the case with no connection between them. In this case, using a common connection for multiple LEDs to the motherboard would be counterproductive: additional circuitry would be required in the case to connect the LEDs together in order to be able to use the common connection for all of them. This would probably be more expensive than just providing an extra couple of wires to support the 3 LEDs that most cases had back then.
\$\begingroup\$
\$\endgroup\$
Pin 1 is probably 5V and -ve is low with a series R to direct connect a LED. just a SWAG, that you can easily test.
simulate this circuit – Schematic created using CircuitLab
\$\begingroup\$
\$\endgroup\$
1
Because the "D" in LED stands for diode and diodes only pass current in one direction. (which is why they're used for bridges to convert AC to DC)
-
2\$\begingroup\$ If that was the reason they would use A and K. Diodes are frequently intentionally reverse biased so +/- are meaningless. \$\endgroup\$ Commented Sep 15, 2017 at 19:40