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I took apart my LED light fixture to clean its diffuser, when I noticed something odd:

enter image description here

As you can see, it has a DC power source connected to 4 daisy chained circuits. I expected that these 4 circuit boards to be connected in parallel, with the daisy chaining forming a shared + and - bus.

However, there are these P- contacts connected to P+, and I cant figure out what they're for.

Any ideas?

Edit:

I've laid out the traces of the first two boards. It appears that the first two boards form a pair, with 2 current paths.

  • The green current path goes between the + and - connections of the first board, passing 11 LEDs along the way.

  • The magenta current path passes from - through 6 (only 5 are numbered on the picture, I missed one) LEDs, to P-, then from P- through all 16 LEDs of the second board.

  • The cyan indicates the etched portions of the board.

  • The red and black are the power rails, chained straight through the boards.

enter image description here

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    \$\begingroup\$ That's an interesting question. Can you get an angled shot or change the exposure to reveal the copper track layout on one of the boards? It might also be useful to clip your meter negative probe onto the black and mark up the photo with the voltages read at each connection node. \$\endgroup\$
    – Transistor
    Sep 4, 2017 at 18:23
  • \$\begingroup\$ @Transistor I've posted the trace layout of the first two boards \$\endgroup\$
    – Alexander
    Sep 4, 2017 at 19:56

3 Answers 3

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You missed the common point on the right board.

enter image description here

Figure 1. Second board has two strings of LEDs. I've marked up the 11-LED string in orange.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 2. Layout of first two boards.

It's a weird layout alright. Each board has eleven LEDs in series between + and - but spread between the two boards are another ten LEDs also wired in series between + and -.

Whoever designed the board mustn't be a regular reader here!

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  • \$\begingroup\$ Nicely done, I totally missed that! Is that "jump" between D1 and D3 supposed to be a junction with the + rail? \$\endgroup\$
    – Alexander
    Sep 4, 2017 at 20:46
  • \$\begingroup\$ This is such a strange design. It seems like this could much more simply done using 4 identical, independent boards, each with one series circuit of LEDs, with the 4 boards being in parallel, with a shared + and - rails. This would save on tooling (only one board type) and assembly (only 4 pairs of +/- wires, totalling 8, rather than the 10 in this design) \$\endgroup\$
    – Alexander
    Sep 4, 2017 at 20:50
  • \$\begingroup\$ [Comment 1] No, I think you missed that D10 and D11 both connect to black. [Comment 2] Agreed. It is most strange. Lay one or two sheets of printer paper over it when powered up and you might see a difference in intensity between the 10 and 11s. (The paper will tone down the light level.) \$\endgroup\$
    – Transistor
    Sep 4, 2017 at 20:52
  • \$\begingroup\$ I can't really perceive a difference, but theoretically it should be there. \$\endgroup\$
    – Alexander
    Sep 4, 2017 at 20:57
  • \$\begingroup\$ What about voltage drop across each LED? The there should be a 10% difference. \$\endgroup\$
    – Transistor
    Sep 4, 2017 at 20:59
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P- and P+ (if they represent power supplies terminals for each module) are probably routed in and out on each module and hence all modules are in parallel. Try connecting a DVM and measuring resistance between P+ nodes and do the same for P- nodes. My guess is that they all connect due to internal copper on each module.

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  • \$\begingroup\$ If that's what P- and P+ are for (simply daisy chaining along the positive voltage rail), then that raises the question of what the + connections are. If you look closely, you'll notice that these P- to P+ connections only exist between modules 1/2, and 3/4, but not 2/3, and not between the power source and the first module. \$\endgroup\$
    – Alexander
    Sep 4, 2017 at 18:32
  • \$\begingroup\$ I just measured: both + terminals of a module are connected to each other, and likewise with both -. Neither the + nor - are connected to the P* terminal \$\endgroup\$
    – Alexander
    Sep 4, 2017 at 18:34
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    \$\begingroup\$ Now try measuring dc voltages and see what you get. \$\endgroup\$
    – Andy aka
    Sep 4, 2017 at 18:36
  • \$\begingroup\$ I'll have to wait for someone to become available to come help me, I don't have a way of both probing and viewing the measurement at the same time D: \$\endgroup\$
    – Alexander
    Sep 4, 2017 at 19:56
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Here's the final schematic I ended up with, based on @Transistor's answer.

schematic

simulate this circuit – Schematic created using CircuitLab

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