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Author Topic: Characterizing LED Strip Tape ...  (Read 29 times)

Offline Spencer

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Characterizing LED Strip Tape ...
« on: December 11, 2017, 09:59:25 pm »
I thought I would put up some info on how to characterize LED strip tape, seeing as how a lot of people are using it these days.

I'll be starting to use it myself soon, so I think it's important to understand some of the basics. There will be some math and simple circuit theory (Ohms's Law) here.

Here's some double density natural white color tape, 5 meters, or 200 1-inch sements I bought from e-bay. The tape itself says 12V, but there is no other information about anything else. So, time to break out the multimeter and power supply.



I set the supply to 9V DC and got a total current measurement of about 340 mA. These were both confirmed with my multimeter.



Strip tape is manufactured so that each segment has 3 LEDs in series with a resistor, and each segement is connected in parallel with all other segments. That's why you can cut whatever inch-length segments you want. If an LED or resitor fails, then only that segment will go down. If, however, the main power rails that run along the edges of the tape fail somewhere, all segments further down the line will go dark as well.

After counting, my strip does indeed have 200 segments. Each 20 are actually soldred together.

The problem is that as you go further down the strip, the voltage will drop as the the power rails are not perfect conductors, a.k.a you lose energy.
So I assume between a 15-20% percent loss for the total current over 5 meters, which would put the ideal draw up to around 400mA. This loss would be negligible if only using a few segments at a time.

So, the ideal current draw of each segment at 9V is ~400mA / 200 = 2mA.
That's not a lot, which is why the LEDs are dimmer. This value was later confirmed by measuring the current only on a single isolated segment.

Now, we can also measure the voltage drop across the LEDs themselves, in this case about 2.9V averaged over ten. That's how much voltage these need to turn on and it will typically stay around this value. How bright the LEDs are depends mainly on the current.



That means we have 9V - 3*2.9V = 0.3V left across the resistor. Ohm's law tells us that the resistance is thus R = V / I = 0.3V / 2mA = 150 Ohms.
Of course, I could have checked that by looking at the resistors themselves.



They say 151 which means 15 * 10^1 = 15 * 10 = 150 Ohms.
So all my measurements and assumptions are in order, a good check.

So we know there are 200 segments, each with 3 LEDs that operate at 2.9V each, in series with a 150 Ohm resistor.
At 9V, each 1-inch segment draws ~2mA, and ideally 400mA for a 200 segment strip.


If we were to run at 12V, then there would be more current through the strip. For each segment, we would have:

I = (Vsupply - 3*Vled) / R = (12V - 3*2.9V) / 150 Ohm = 3.3V / 150 Ohm = 22mA, over ten times higher.

These LEDs likely have a 30mA limit, so that's okay. But for all 200 segments, that would be a bit less than 200 * 22mA = 4.4 Amps!
I'm not sure I trust my cheap supply to try for that much.

The power dissapation for each segment at 9V would be P = I * V = 2mA * 9V = 18 mW.
The resistor itself will dissapate 2mA * 0.3V = 0.6mW as heat.

At 12 Volts, the resistor generates 3.3V * 22mA = 73mW of heat. Not terrible, but still, over 120 times more!

You can repeat the math and measurements for almost any LED strip.
Running the LED strips at lower than stated voltages can be a good idea, because the current and heat dissapation are lower.
This will prolong the life of the LEDs and generally keep the brightness "in scale" with smaller kits.
Of course, running brighter means you can sometimes use less strip overall, especially for bigger models.
It's really a judgement call for the situation.

Anyway, my two cents on the whole thing. Hope someone finds this useful ...

Cheers,
Spencer
« Last Edit: December 11, 2017, 10:22:55 pm by Spencer »

Offline simi

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Re: Characterizing LED Strip Tape ...
« Reply #1 on: December 12, 2017, 11:08:32 am »
Nice tech details!  Sorry, but I can't resist this link;

"I was my understanding that there would be no math"

https://vimeo.com/65921206

Cheers!

Simi
As a software architect, I'm pretty darn good.  As someone with knowledge of building things in the real world, well, I'm a software architect.

Offline RossW

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Re: Characterizing LED Strip Tape ...
« Reply #2 on: December 12, 2017, 11:43:58 am »
I thought I would put up some info on how to characterize LED strip tape, seeing as how a lot of people are using it these days.

I'll be starting to use it myself soon, so I think it's important to understand some of the basics. There will be some math and simple circuit theory (Ohms's Law) here.

Here's some double density natural white color tape, 5 meters, or 200 1-inch sements I bought from e-bay. The tape itself says 12V, but there is no other information about anything else. So, time to break out the multimeter and power supply.



I set the supply to 9V DC and got a total current measurement of about 340 mA. These were both confirmed with my multimeter.



Strip tape is manufactured so that each segment has 3 LEDs in series with a resistor, and each segement is connected in parallel with all other segments. That's why you can cut whatever inch-length segments you want. If an LED or resitor fails, then only that segment will go down. If, however, the main power rails that run along the edges of the tape fail somewhere, all segments further down the line will go dark as well.

After counting, my strip does indeed have 200 segments. Each 20 are actually soldred together.

The problem is that as you go further down the strip, the voltage will drop as the the power rails are not perfect conductors, a.k.a you lose energy.
So I assume between a 15-20% percent loss for the total current over 5 meters, which would put the ideal draw up to around 400mA. This loss would be negligible if only using a few segments at a time.

So, the ideal current draw of each segment at 9V is ~400mA / 200 = 2mA.
That's not a lot, which is why the LEDs are dimmer. This value was later confirmed by measuring the current only on a single isolated segment.

Now, we can also measure the voltage drop across the LEDs themselves, in this case about 2.9V averaged over ten. That's how much voltage these need to turn on and it will typically stay around this value. How bright the LEDs are depends mainly on the current.



That means we have 9V - 3*2.9V = 0.3V left across the resistor. Ohm's law tells us that the resistance is thus R = V / I = 0.3V / 2mA = 150 Ohms.
Of course, I could have checked that by looking at the resistors themselves.



They say 151 which means 15 * 10^1 = 15 * 10 = 150 Ohms.
So all my measurements and assumptions are in order, a good check.

So we know there are 200 segments, each with 3 LEDs that operate at 2.9V each, in series with a 150 Ohm resistor.
At 9V, each 1-inch segment draws ~2mA, and ideally 400mA for a 200 segment strip.


If we were to run at 12V, then there would be more current through the strip. For each segment, we would have:

I = (Vsupply - 3*Vled) / R = (12V - 3*2.9V) / 150 Ohm = 3.3V / 150 Ohm = 22mA, over ten times higher.

These LEDs likely have a 30mA limit, so that's okay. But for all 200 segments, that would be a bit less than 200 * 22mA = 4.4 Amps!
I'm not sure I trust my cheap supply to try for that much.

The power dissapation for each segment at 9V would be P = I * V = 2mA * 9V = 18 mW.
The resistor itself will dissapate 2mA * 0.3V = 0.6mW as heat.

At 12 Volts, the resistor generates 3.3V * 22mA = 73mW of heat. Not terrible, but still, over 120 times more!

You can repeat the math and measurements for almost any LED strip.
Running the LED strips at lower than stated voltages can be a good idea, because the current and heat dissapation are lower.
This will prolong the life of the LEDs and generally keep the brightness "in scale" with smaller kits.
Of course, running brighter means you can sometimes use less strip overall, especially for bigger models.
It's really a judgement call for the situation.

Anyway, my two cents on the whole thing. Hope someone finds this useful ...

Cheers,
Spencer

Excellent. Very happy to see this level of detail here.

 




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