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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.

Circuit Diagram

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  • \$\begingroup\$ If the voltage is measuring less than rated, then the power supply is being unable to keep up with the current demand of the LEDs (or you are experiencing huge losses in an overly long run of undersized wire). Generally speaking questions about lashups of undocumented discount consumer products don't really belong here. It's not clear that your design is sound - where in the system is current regulation supposed to be accomplished??? \$\endgroup\$ – Chris Stratton Dec 17 '18 at 1:18
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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:

enter image description here

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: enter image description here

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:

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

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  • \$\begingroup\$ Why do you think these are for automotive use? \$\endgroup\$ – pipe Dec 17 '18 at 13:14
  • \$\begingroup\$ @pipe I DO NOT think they are for automotive use, I didn't think I could be any clearer. They have a simple resistive current limiter completely unsuitable for operation over a range of voltages. To use these in an automotive setting would require PWM that never reaches 100%, CC drivers and potentially thermal feedback to cope with the temperature extremes. \$\endgroup\$ – Jack Creasey Dec 17 '18 at 17:07
  • \$\begingroup\$ So why did you have to write it out explicitly? They are also not for use in space, or for submarines, or any other crazy environment. OP never talks about automotive use. \$\endgroup\$ – pipe Dec 17 '18 at 18:02
  • \$\begingroup\$ @pipe There were a couple of answers that suggested they were these WERE suitable and in fact designed for automotive use. I was simply responding to those answers which were clearly wrong. FYI ….your first comment says I was suggesting they were good for automotive use ...I was not. \$\endgroup\$ – Jack Creasey Dec 17 '18 at 18:43
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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.

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  • \$\begingroup\$ This. Also, these strips are actually fully powered at 14V not 12V, so even if wire resistance isn't an issue, they will not be as close to the rated current draw when only powered by 12V. \$\endgroup\$ – Passerby Dec 17 '18 at 2:35
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    \$\begingroup\$ @Passerby You are wrong. These units ARE NOT suitable or designed for 14-15V DC applications they get afar too hot and the adhesive (which is not thermally viable) lets go. \$\endgroup\$ – Jack Creasey Dec 17 '18 at 18:45
  • \$\begingroup\$ You mean the counterfeit "3m" adhesive that doesn't hold at any temperature? \$\endgroup\$ – Passerby Dec 17 '18 at 22:29
  • \$\begingroup\$ @jack you are clearly wrong. Otherwise these strips would display their stated power usage at 12V I stead of 14V. Instead they draw significantly less then their datasheets state. JUST LIKE HOW OP IS not getting the stated 18W draw when he powers it at 12V. \$\endgroup\$ – Passerby Dec 17 '18 at 22:36
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    \$\begingroup\$ @Passerby: The OP stated that when he did get 12 volts to the panel, the current was 2.5 amps, for a power of 28 watts - pretty close to the predicted 28.8 watts for 1.6 metres. Looks to me like they are rated for 12 volt operation. \$\endgroup\$ – Peter Bennett Dec 17 '18 at 23:57

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