How do you design a power supply for an LED strip if you do not have any specifications?

Question

I have a monochrome LED strip of six meter length installed in some indoor furniture.

The manufacturer used a very under-dimensioned power supply that "died" after half an hour of use.

Unfortunately I have no documentation of the LED strip.

Below is a picture of the LED strip.

The manufacturer foresaw to connect the LED strip with the power supply from two ends of the LED strip forming a loop - see the second picture (possibly to have enough brightness over the whole length of the strip). I would like to keep this design.

I would like to buy a suitable power supply but am unsure about the power consumption of the LED strip. Additionally I would like to be able to dim the LED strip.

I have the following questions:

1. How can I design the power supply without knowing the LED strips consumption?

2. Additionally I want to dim the LED strip. Is this possible with every LED strip?

Background information:

I could imagine that the manufacturer thought to create a very intimate light atmosphere by using an under-dimensioned power supply. After complaining to the seller, I was informed that the manufacturer stopped equipping this kind of furniture with LED strips.

Answer 1

Judging by the pictures, your LED strip is composed of blocks of three LEDs in series with a resistor. The blocks are in parallel to make up the length of the strip.

White LEDs need about 3V to light up. With three in series, that's 9V. That leaves 3V to be dropped across the resistor.

The resistors in your photo are 150 ohm parts.

Using Ohm's law (\$ I = \frac {V}{R}\$,) we find that the current through the resistor will be \$ I = \frac {3}{150} = 0.02 amperes\$.

Since the resistor and the three LEDs are in series, the resistor current is also the current for the block.

Multiply 0.02 amperes by the number of LED blocks on your strip. That's how much current your power supply must deliver.

Find a 12V power supply rated for that current or higher. More won't hurt the LEDs, it'll just be more expensive.

Your LED strip is dimmable. There's nothing in it but LEDs, resistors, and wire.

• You can vary the current to dim it.
• You can vary the supply voltage to dim it.
• You can use pulse width modulation (PWM) to dim it.

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For a more accurate estimate of the current, measure the forward voltage of one of the LEDs. White LEDs usually use a blue LED with a phosphor coating to make white. Blue LEDs usually have about 3V as a forward voltage, but some are lower. I have a bunch here with 2.5V forward voltage. That would change the calculated current from 0.02 to 0.03 amperes.

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I find 12V DC constant voltage dimmable power supplies available from various places with various power (current) ratings and for various prices - up to eye-wateringly expensive.

Look around, see what you can find.

Answer 2

If the power supply died after half an hour, the seller should replace it; in many jurisdictions, he must.

That aside: Most white LED strips consume between 5W/m and 15W/m, so for 6m, that would be between 30W and 90W or between 2.5A and 7.5A at 12V; quite a wide range, but without specs or measurements that is as close a guess as you are going to get.

You could check what it says on the original power supply to see if it was indeed underpowered, or is simply broken and could perhaps be fixed.

You could go for (almost) safe and get a, say, 150W LED strip power supply; try googling for "12V 150W dimmable LED power supply" if you want to buy one.

Answer 3

If you have another (relatively large) power supply available, use a multimeter to measure the current through and voltage across the strip terminals while powered.

If the voltage is nominal, then the measured current is the required capacity of your new, dimmable, power supply.

If below nominal, the supply cannot provide enough current. You can use Ohms Law to work out the proportional additional current required for the equivalent resistance, though beware this is just an approximation since equivalent resistance will likely not change linearly with current.

Or, if you don't mind temporarily cutting the LED strip, remove a shorter section a small power supply will definitely be able to drive, measure the current draw of that, then scale it up as ocrdu suggests. Re-solder when done.