Strange led serial connection?? - Page 1
Forum section: electronics / beginners

author: sdancer75
author_rank: Frequent Contributor
timestamp: September 01, 2021, 02:12:01 pm
content: At the photo attached (pioneer car radio CD) the push buttons are having two LEDs to choose the preferred illumination. Using the diode mode I can confirm the anode + and the cathode - for orange light (left side) and green light (right side).The strange thing to me, is that anode from the 1st led is connected directly with the cathode of the second led (green line) from the back side of the pcb. Why is that? What am I missing here?

author: jmelson
author_rank: Super Contributor
timestamp: September 01, 2021, 02:45:20 pm
content: It sounds like the LEDs are wired in series. Since 12 V is available in the car, that makes sense.Jon

author: sdancer75
author_rank: Frequent Contributor
timestamp: September 01, 2021, 03:09:00 pm
content: That's my question. It doesn't connect in series. It seems like the connection below--- (+) led >| (-) ------||---- (-) |< led (+) ------------

author: Zero999
author_rank: Super Contributor
timestamp: September 01, 2021, 03:31:55 pm
content: Quote from: sdancer75 on September 01, 2021, 03:09:00 pmThat looks like a series connection to me.Where's the resistor?Are the other sides of the LEDs commoned too? If they're in reverese parallel, then changing the polarity, would determin which LED lights.

author: sdancer75
author_rank: Frequent Contributor
timestamp: September 01, 2021, 05:28:58 pm
content: Quote from: Zero999 on September 01, 2021, 03:31:55 pmThere is no resistor. Check the continuity photos from the both sides of the pcb.For some of the buttons in the end of the line doesn't apply this pattern.For series connection it should be like this below, am I correct?--- (+) led >| (-) -----------(+) led >| (-) --------

author: Zero999
author_rank: Super Contributor
timestamp: September 01, 2021, 05:43:30 pm
content: Quote from: sdancer75 on September 01, 2021, 05:28:58 pmThat's also a series connection, just drawn differently.

author: sdancer75
author_rank: Frequent Contributor
timestamp: September 01, 2021, 05:51:55 pm
content: Quote from: Zero999 on September 01, 2021, 05:43:30 pmThe LEDs below are connected in reverse mode, does they light both? I far as i understand only one it lights every time when we reverse the polarity, am I right? So here I have two questions1) does the reverse polarity destroys the led?2) how the heck they light both when I power on the device?--- (+) led >| (-) ------||---- (-) |< led (+) ------------

author: Benta
author_rank: Super Contributor
timestamp: September 01, 2021, 05:56:21 pm
content: They're in anti-parallel. Reversing polarity light one or the other. AC modulating them (probably PWM) light both.

author: sdancer75
author_rank: Frequent Contributor
timestamp: September 01, 2021, 06:14:36 pm
content: Quote from: Benta on September 01, 2021, 05:56:21 pmAre you sure that it works with AC instead of DC? Is there anyway to verify this?Regards

author: Zero999
author_rank: Super Contributor
timestamp: September 01, 2021, 06:18:19 pm
content: The ASCII schematics are ambiguous. They don't show how the LEDs are connected to the power supply. I've drawn my interpretation using real symbols, which shows them both in series.If it's wrong, then provide a clear schematic.

author: sdancer75
author_rank: Frequent Contributor
timestamp: September 01, 2021, 07:07:26 pm
content: Quote from: Zero999 on September 01, 2021, 06:18:19 pmThat's correct

author: Jwillis
author_rank: Super Contributor
timestamp: September 01, 2021, 07:09:19 pm
content: It would be my guess that R1912 is the mystery current limiting resistor to the LED series . LED series goes all the way back from button 3 to the smaller buttons beside Button one . Can't make out traces to buttons 4 and 5 or the purpose of R1916 .

author: langwadt
author_rank: Super Contributor
timestamp: September 01, 2021, 07:24:27 pm
content: also possible that they are in series from Vcc to ground, driving the center with a three-state out can light either one or if toggling both

author: james_s
author_rank: Super Contributor
timestamp: September 01, 2021, 10:01:26 pm
content: Quote from: sdancer75 on September 01, 2021, 07:07:26 pmThose are exactly the same circuit, one is just drawn wrapped around, what are you seeing that I'm not? They're electrically identical.It's very common for bi-color LEDs to internally have two chips connected anti-parallel across two pins. If you connect + on one side and - on the other it lights one color, if you reverse the connections it lights the other color. If you feed it with AC both will appear lighted at the same time (persistence of vision) and you can vary the apparent color by varying the duty cycle of the polarity. They cannot be damaged by the reverse voltage because the forward drop of one LED clamps the reverse voltage of the other.

author: Zero999
author_rank: Super Contributor
timestamp: September 02, 2021, 09:33:14 am
content: Quote from: james_s on September 01, 2021, 10:01:26 pmI said that in aprevious post. The electrons don't care how it's physically laid out. The two circuits are the same, just drawn differently.

author: sdancer75
author_rank: Frequent Contributor
timestamp: September 02, 2021, 10:36:38 am
content: Quote from: Zero999 on September 02, 2021, 09:33:14 amYes, it seems that I was a little bit confused with this, cos there is a direct rail from the backside of the PCB which is going from (+) of the 1st LED to the cathode of the 2nd LED of thesame colorwithout passing through the 2nd LED. So right now I am thinking of it, it still seems wrong to me. They can not possibly lit both (orange color pin are all from the left side of the push button).

author: sdancer75
author_rank: Frequent Contributor
timestamp: September 02, 2021, 04:53:48 pm
content: I attach the service manual. Page 31 is the schematics of the front panel keyboard if it helps.

author: james_s
author_rank: Super Contributor
timestamp: September 02, 2021, 07:07:03 pm
content: There are no schematics on page 31, that's just the PCB layout. The schematic is on page 19 and shows a standard series/parallel arrangement of four LEDs. They are shown as single color LEDs but I suppose it's possible that they are actually bicolor with the color determined by the polarity.

author: sdancer75
author_rank: Frequent Contributor
timestamp: September 03, 2021, 05:35:59 am
content: Quote from: james_s on September 02, 2021, 07:07:03 pmYes, that's correct, I was using my phone to download and check the service manual pages and as the screen was too small to view the details I have mixed up a little bit the pages:-) Thanks!