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I am making an LED lamp using an LED strip. The LED strip I am using is a 240 LED/m SMD2835 strip, which operates at DC12V, and is supposed to use 18W of power per meter.

My lamp is constructed as seen on the diagram: I cut it into 20cm strips, installed them on an aluminum heatsink and wired in parallel. I have tested each of the 20cm strips separately, and they draw around 0.3A of current at 12V, which is very close to the specification.

However when I wire everything together, the completed circuit draws only around 1.4A of current, which is 16.8W, while it's supposed to be around 28.8W (18W/m * 1.6m). I also noticed that each individual 20cm strip is not as bright, compared to when powered individually. I also measured voltage in the beginning and in the end of the strip - and it is much lower than 12V!

First I thought the problem is with my power supply - and I have tested it with different ones, ranging from cheap 12V 3A wall plugs, to bigger 60W bricks, but unfortunately - same result. Finally I tested it with an adjustable voltage power supply, and I noticed that when I set output voltage to around 14V, then voltage across the strip becomes around 12V and it draws around 2.5A of current, which makes it much closer to how much power it is supposed to draw according to my design (and it is much brighter, too - like when I powered 20cm strips individually).

I am very confused... Can you explain this behavior? Should I buy a power supply with a slightly higher output voltage, so that it drops down to 12V when connected to the strip? If so, how do I calculate how much output I need? Thank you.

From your mention of a test with a 14 volt supply, it appears that most voltage is being lost in the wires between the supply and light panel - if so, you need significantly larger wire for that connection.

All wires have some resistance, and you must consider this resistance when determining the required wire size for a given current and cable length.

Your connecting wires are too thin, you are dropping voltage in your leads.

You need to select the right size of wire and potentially use a star connection to drive the strings.

If your strip is using SMD2835 LEDs these will be FULLY bright at 12V DC. The typical LED characteristics (when warmed up) will be as below:

Notice that the Vf is around 3.3V maximum and you will find you strip has three LEDs in series with each current limiting resistor. The LEDs visibly turn on at about 9-9.1V (low brightness). If there were 4 in series you would have to use above 12V DC to get them to turn on, and this is NOT the case.

To select your wire size. there are plenty of charts and in the one shown below you would not use less than 16AWG wire for you application:

The leads from your power supply MUST be able to carry the full current rating. The leads from LED string to LED string could be much smaller if you used a star connection for your wiring.

Most LED strings actually have the voltage marked clearly on the strip, here is one image:

NOTE: The vast majority of LED strings will NOT work in automotive use as they get too hot.

It sounds like 12 volts is actually the 'turn-on' voltage of the LED's, so you need at least 12 volts just to get them to have dim light. By going up to 14 volts you have gone far past the point to where the LED's would be at maximum brightness. Use at least 22 gauge wire.

LED's are current driven devices, so above 12 VDC they will consume more current in a non-linear fashion such that at 14 volts you should have current limiting to protect the LED's from burning up. If they are too HOT to touch then they have too much current flowing through them.

Current limiting can be done with a power supply that sources 14 to 15 VDC and also has an adjustable current limit. Maximum brightness actually may be too bright. Inserting a 1 ohm 10 watt resistor in series with each strip will help lower the current as the resistors will dissipate the over current as heat.

With the resistors in place you can measure the DC voltage across each strip and get a better idea of the voltage the LED's will clamp hard at. Measure the voltage across the resistors to get a reading on the current consumed by each strip. They should be close. Do not force the LED's to run at a higher voltage or current then what seems like a 'comfortable' maximum brightness.

They can be warm to the touch and that means the current is at a safe long-life setting. Properly driven LED's should last 20 to 50 years. If they are 'cool' or slightly warm to touch then lifespan could be close to 50 years. Heat is their only enemy when they are in a circuit, and heat is created by current.

The 12V stripleds intended for automotive use ( in the description) are rated at 14.2V with engine running.

They will be dim at 9V and full brightness at 14.2 with 3 LEDs in series 1 or 2 resistors for the current limiting at rated drop voltage.

This explains your results.

If however, they include a 12V supply or do not mention Auto or motorcycle use, the heat rise may not tolerate 14.2V and the power rating may be optimized for 12V 2% tolerance.

The lack of specs and standards is pervasive issue.