Simple LED strip design choice query.

Thread Starter

jjbrown84

Joined Jul 19, 2019
3
Hi all,

Just joined so hope this is done correctly.

I think this is the place that could answer my query.

I came across a simple piece of equipment that seems to have a redundancy. It's a backlight for an eye chart.
As pictured, it's a very simple design that consists of two 12V LED strips each of 8 segments and powered by a standard 12V PSU.
The PSU had failed but as I was inspecting the unit as a whole I noticed the links on both ends of the strip.

According to the technical file, "links are added top and bottom to stabalise voltage" .

I can understand preferring to link the bottom of the strips rather than the top so that the power source is effectively connected to 8 sections either side but to me, the top link looks completely redundant and unnecessary (I doubt it would be much of an issue if the only link was at the top to be honest as these are designed to run several metres).

My argument is that if the strip were one long section of 16 segments and you connected the power to the centre of the strip, it would look silly if you then ran a link from one end to the other and this is what is happening here.

Is anyone able to add their thoughts to this?

Always willing to learn.
 

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ebeowulf17

Joined Aug 12, 2014
3,046
I wouldn't imagine that there's much voltage stabilizing effect, but I do see a different potential benefit.

The extra connections provide redundancy in case one of the traces in the flexible strip material fails. Without the extra wires, any failure in the strip knocks out all the LEDs downstream from the point of failure. With the extra wires, there are two paths taking power to each LED, so any one failure would only knock out one LED, or none at all.

Having said that, I would expect the rigid, fixed enclosure to provide pretty good protection against mechanical failure, so the redundancy does feel like overkill.
 

Thread Starter

jjbrown84

Joined Jul 19, 2019
3
Thanks for the quick reply.
Yeah there's almost zero chance of mechanical failure beyond someone getting in there and pulling the wires. It sits in that metal tray, mounted to a wall and has a translucent eye chart over the front.
Out of interest would you (or anyone wishing to contribute) have an idea about what kind of voltage drop these strips have usually (assuming they're copper), I've seen several metres of these light up off one power source and they seem pretty stable judging by their brightness from one end to the other (and that's not even connecting the power to a middle section)
 

ebeowulf17

Joined Aug 12, 2014
3,046
I'll make some wild guesses, but be aware that they really are guesses.

Let's say the copper traces have a cross section comparable to 24 gauge wire. That would put their resistance at 26 milli ohms per foot. You say you've seen "several meters" before - to be lazy I'll round that to roughly three meters and call it 10 feet. You have to double that to get the round trip figure, so now we've got:

20ft x 0.026 = 0.52 ohms

Now in order to get voltages from that, we need to know current through the wire, which brings me back to more guessing. I really have no idea, but let's say these things need to be really bright and its:

100mA per LED x 16 LEDs = 1.6A

Now to find the voltage drop across the wires, you multiply current times resistance:

1.6A x 0.52 ohms = 0.832V

In reality, the LEDs are running in parallel in such a way that some portions of the strip carry more current than others. To get a more accurate number, you'd need to analyze the current through each length separately and add the voltage drops... and if course you'd want better resistance and current guesses to base your calculations on.

This post is really just meant to illustrate a simple way to get a rough estimate of possible voltage drop, not come up with an accurate figure.
 

TeeKay6

Joined Apr 20, 2019
560
Hi all,

Just joined so hope this is done correctly.

I think this is the place that could answer my query.

I came across a simple piece of equipment that seems to have a redundancy. It's a backlight for an eye chart.
As pictured, it's a very simple design that consists of two 12V LED strips each of 8 segments and powered by a standard 12V PSU.
The PSU had failed but as I was inspecting the unit as a whole I noticed the links on both ends of the strip.

According to the technical file, "links are added top and bottom to stabalise voltage" .

I can understand preferring to link the bottom of the strips rather than the top so that the power source is effectively connected to 8 sections either side but to me, the top link looks completely redundant and unnecessary (I doubt it would be much of an issue if the only link was at the top to be honest as these are designed to run several metres).

My argument is that if the strip were one long section of 16 segments and you connected the power to the centre of the strip, it would look silly if you then ran a link from one end to the other and this is what is happening here.

Is anyone able to add their thoughts to this?

Always willing to learn.
@jjbrown84

Here's an informative site: https://www.instructables.com/id/Intro-to-LED-Strips/
 

oz93666

Joined Sep 7, 2010
730
I work with these strips all the time , and you are correct the top link serves no purpose , just the bottom one is needed...

And what does that mean "stabilize voltage " it makes no sense
 

Thread Starter

jjbrown84

Joined Jul 19, 2019
3
I work with these strips all the time , and you are correct the top link serves no purpose , just the bottom one is needed...

And what does that mean "stabilize voltage " it makes no sense
Hi Oz,
I think the manufacturer (these tend to be relatively small businesses knocking out a few simple modular products) had someone in their business draw up a tech document and perhaps they didn't know exactly how they work or maybe mistranslated a Chinese document discussing voltage drop on longer runs.
If you had to choose to keep one link, I can understand the bottom link being a (marginally) better design than just the top link as the voltage drop should be less.
The top link would serve as increasing the cross-section of the + and - tracks and further lowering any voltage drop but it's over such a short distance (2 x 40 cm) I think they'd have more voltage variation in the cheap power supply they used! From what I can see it is an unregulated 3-12V adjustable power supply with a hot glue seal over the adjustment slot and the correct barrel connector heatshrunk onto the cable.
Which brings up another question actually. Are these PSU's suitable for such an application?
They're not exactly purpose built led drivers are they?

BTW thanks for the link TeeKay
 

oz93666

Joined Sep 7, 2010
730
Hi Oz,
I think the manufacturer (these tend to be relatively small businesses knocking out a few simple modular products) had someone in their business draw up a tech document and perhaps they didn't know exactly how they work or maybe mistranslated a Chinese document discussing voltage drop on longer runs.
If you had to choose to keep one link, I can understand the bottom link being a (marginally) better design than just the top link as the voltage drop should be less.
The top link would serve as increasing the cross-section of the + and - tracks and further lowering any voltage drop but it's over such a short distance (2 x 40 cm) I think they'd have more voltage variation in the cheap power supply they used! From what I can see it is an unregulated 3-12V adjustable power supply with a hot glue seal over the adjustment slot and the correct barrel connector heatshrunk onto the cable.
Which brings up another question actually. Are these PSU's suitable for such an application?
They're not exactly purpose built led drivers are they?

BTW thanks for the link TeeKay
The issue is , that with long runs 5 or 10 meters long , the voltage seen by leds far from the power input is slightly less... the leds are very slightly dimmer , just noticeable ...

But the point is , putting in the cross connections at the top as in your photo will do nothing , the voltage at the top will still drop by the same amount and those leds will be imperceptibly dimmer ,,,

The way to resolve this and get leds at precisely the same voltage , is to have the 4 cross links as shown but connect one power input wire to the top , the other to the bottom ( opposite ends of the strips) ...

As for the suitability of the power supply the current demand is so low anything will be fine , as long as the leds look OK ( don't flicker)
 
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