4-pin RGB Hub Design - Page 1
Forum section: electronics / projects

author: asder
author_rank: Newbie
timestamp: September 24, 2018, 06:38:20 am
content: Hello, I want to make an 4-pin RGB (analog) hub that is powered by an external power supply (sata power connector).It takes up to 2 sources. I have an DG9454E for switch between 1:8 and 2:4 mode. I would like to know it this thing will work.I based my design of some arduino schematics I found.Parts:DG9454Ehttps://gr.mouser.com/datasheet/2/427/dg9454e-1019484.pdf3 * SI5902DChttps://gr.mouser.com/datasheet/2/427/si5902dc-241117.pdfHere's what I have designed:Spoiler: A picture is worth a thousand words

author: dzseki
author_rank: Frequent Contributor
timestamp: September 24, 2018, 07:00:25 am
content: Not entirely sure what you intended, but I don't think it will work.See the schematic below, I'd try something like that, use proper input/output termination via 75 Ohm, also use active buffering on outputs. What about the sync signals BTW exactly what type of RGB are you going to use?

author: asder
author_rank: Newbie
timestamp: September 24, 2018, 07:05:34 am
content: Hi,thanks for the reply. I use 12V strips + 3 ground pins for R G B. I basicaly followed this wiring. On your design I was a bit confused what are A B supposed to be?? you are referring to the switching part on the 5-8 outputs ??

author: dzseki
author_rank: Frequent Contributor
timestamp: September 24, 2018, 07:21:34 am
content: A and B supposed to be the two inputs. The picture you posted has no relevance to RGB switching IMHO. What that does is shorting the RGB lines to ground according to the Wi-Fi module.

author: asder
author_rank: Newbie
timestamp: September 24, 2018, 07:47:25 am
content: That's what I want to do. I want to take a signal from a source that can power only one strip and use an external power supply for powering that 4 / 8 headers. The switch ic i have is for switching between 1 source to 8 pins or 2*(1 source to 4 pins)

author: ajb
author_rank: Super Contributor
timestamp: September 24, 2018, 05:27:08 pm
content: The switches in the DG9454E have a typical on resistance of 85and are only rated for 30mA on any pin. You don't want to use them to switch LED strips! You should do the switching on the low current input side.Your inputs aren't going to work. The R,G,B inputs will be pulled high by the load when off, and will be pulled to ground when on, but your transistors will be off when the input is low and on when the input is high. You need a second layer of transistors to invert the input signal. You also need to add pullups or pulldowns to the the gates of the transistors, otherwise they can float into the linear region (transistor is not fully on or fully off) and get cooked.I am skeptical of your transistor choice. An RDS(on)of 0.085is pretty good, but it's such a tiny package that it won't handle any amount of steady-state power dissipation and you don't have much headroom to tolerate accidental overloads.

author: asder
author_rank: Newbie
timestamp: September 24, 2018, 09:05:46 pm
content: Ok, so 1 I change the switch is. I will need to dig deeper for thisCould you provide a small schematic for the circuit you mean??You also say that the transistors might not tolerate long periods of high currentDo you have any replacements to suggest?Thank you