10

The question is in title. Why there is no ground pin on the 12V RGB header? The 5V ARG header which uses only 3 pins has a ground.

How does it work without a ground?

enter image description here

JRE
  • 67,678
  • 8
  • 104
  • 179
DEKKER
  • 755
  • 1
  • 12
  • 23

4 Answers4

27

This is how it looks on the driver end. Whatever channel you want to drive, the R, G and/or B becomes ground with the help of a transistor, completing the circuit and allowing current to flow.

schematic

simulate this circuit – Schematic created using CircuitLab

EDIT: Added colors for clarity.

winny
  • 13,064
  • 6
  • 46
  • 63
  • 3
    Good answer. Note that this configuration is an example of low-side switching, where the switching device (transistors in this case) are in the return, or low-side path of the power flow. – SteveSh Apr 14 '23 at 13:29
  • 1
    @SteveSh Unless someone invents a new type of transistor with majority carrier yet controlled by negative gate voltage (depletion MOSFET comes close if they would be off with 0 V Vgs), we will have low side switching for the foreseeable future. – winny Apr 14 '23 at 13:46
  • @winny Isn't that just a P-channel FET? – Hearth Apr 14 '23 at 21:36
  • @Hearth P-channel is minority carrier. Hence 3 times as expensive for the same Vds and RdsON. – winny Apr 14 '23 at 22:19
  • 2
    @winny No, p-channel devices are still majority carrier devices. The fact that holes have lower mobility in silicon doesn't make them minority carriers--it's the doping that determines that, and the doping in p-channel devices makes holes the majority carrier. Just like the doping in n-channel devices makes electrons the majority carrier there. – Hearth Apr 15 '23 at 02:45
  • @Hearth All those years at a university… Thanks for the correction. So it’s all about the mobility, not if the holes and electrons are in majority or minority. – winny Apr 15 '23 at 07:46
  • 2
    @winny Yes. Minority carriers are only involved in conduction in any significant way in BJTs, diodes, and thyristors; MOSFETs, JFETs, and HEMTs are majority-carrier devices. – Hearth Apr 15 '23 at 11:36
  • @Hearth You learn something every day. – winny Apr 15 '23 at 11:57
15

But there is a ground. Or actually, three grounds.

The three RGB wires are the grounds for the three LED colours.

The three colours just share a single +12V supply wire.

Justme
  • 127,425
  • 3
  • 97
  • 261
  • The RGB wires are the cathodes. They're grounded through the controller. (I know you know. You just didn't explain it well.) – Transistor Apr 16 '23 at 20:03
6

The anodes of all LEDs are connected to the 5V/12V. The cathodes of the LEDs go to the R, G, and B pins of the connector. The controlling device then connects the pins to ground to make the respective color light up.

11011100
  • 331
  • 1
  • 5
5

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. A manually switched controller for an RGB LED strip.

Closing the R, G or B switches will ground the last cathode of each string of R, G or B LEDs and cause them to light up. The switches can be replaced by transistors and these can be controlled by a microcontroller.

How does a 4 pin RGB connector work without ground?

The ground is provided by the control circuit.

Transistor
  • 168,990
  • 12
  • 186
  • 385