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I bought a cheap LED strip and a 12V 5A power supply unit on Aliexpress. After testing it I realised that the LEDs closer to the PSU end get quite hot (around 55°C) while the ones at the other end of the strip don't get hot at all. I'm a bit worried that continuous exposure to this kind of temperatures might change the colour of the paint on the wall over time.

I measured the open circut voltage output of the PSU and it turned out to be slightly higher than it should be (12.2V), so I thought I could lower the voltage a bit by adding a couple of diodes in series with the strip.

I went to the local electronic parts shop and they sold me FR302 diodes which are rated at 100V 3A. I added 2 in series and indeed all the LEDs now are now cold to touch without noticeably losing in brightness. I measured the voltage drop and it turned out to be 0.8V across each diode, and the current drawn by the strip dropped from 1.1A to 0.9A, but here comes a new problem: the diodes get really hot (79°C according to my multimeter).

I can't find much in the diode's datasheet about how hot it should get in these conditions. I can see that it can operate in the -65..+150 range, but my question is: is it normal for the diodes to get this hot in these conditions? Can I avoid it by using a different type of diode? Should I use a different method of lowering the voltage altogether?

I'm also worried that some of my measurements might be wrong, because according to the description on Aliexpress:

Power Consumption (W/m): 7.36W/m

Which I thought should mean that the strip draws 7.36 W/m / 12V * 5m = 3A, whereas I can only see 1.1A on my mulitmeter. Am I wrong in my calculations?

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    \$\begingroup\$ That kind of LED strips should be used with heatsink. Aluminum enclosure or just strip. And power distribution is complicated, internal conductor can not hold big current. Run wires in parallel and connect in th middle, or, at least two sides. \$\endgroup\$
    – user263983
    Commented Mar 13, 2021 at 13:08

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The "wires" on the strip are just thin flat printed circuit board strips, so they lack the plumpness that you need. You can make all the LEDs heat up equally by hooking up the positive at one end and the negative at the other end of the strip, or you can reduce the currents on the super thin built in wires by tapping power in at several places over the length of the strip. The latter is generally the better option although if you wanted to be a snob you could split the strip into multiple sections and do both.

As far as the voltage rating of your LEDs, you have voltage rated LEDS, which waste a bit of power by using a resistor to regulate current per series string. 12.2V isn't far enough over 12V that you'd be putting the LEDs at risk(especially if you add wiring to feed them properly as that lets them age the same), but underpowering them will actually increase their operational life as long as brightness is still adequate. As you've been finding with your diodes, dropping the voltage by wasting the extra power is a non ideal plan. The power your diodes waste is equal to the voltage drop over them multiplied by the current, and with 0.8V over each diode and a significant current, the diode is heated considerably. You need to either cool the diode or divide the wasted power between multiple diodes so each doesn't heat up too much. A better way would be to decrease output voltage, perhaps use a dimmer driver. Your diode can survive the thermal abuse for some period of time, but it's lifetime will be shortened the hotter it gets.

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  • \$\begingroup\$ Thank you for a helpful reply! I will try your suggestions. Any comments on theoretical 3A vs the real 1.1A I see? \$\endgroup\$
    – torvin
    Commented Mar 13, 2021 at 8:29
  • \$\begingroup\$ If you're really overloading those teensy "wire" strips, that would account for at least some volt drop to the LEDs. Volt drop decreases current. El cheapo electronics are often batches that didn't meet some quality standard to be sold as more expensive product, so once you have them wired properly they should be closer to their ratings, but you should expect some variance. \$\endgroup\$
    – K H
    Commented Mar 13, 2021 at 8:40
  • \$\begingroup\$ Note that if you know your rough operating current, you can easily look up resistance per meter for different wire sizes and calculate your resistance based on length. Multiply resistance by current to find voltage drop. \$\endgroup\$
    – K H
    Commented Mar 13, 2021 at 8:49
  • \$\begingroup\$ I tried connecting both ends and it worked like magic. No LEDs are hot to touch anymore. Thank you! \$\endgroup\$
    – torvin
    Commented Mar 13, 2021 at 23:55
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    \$\begingroup\$ No problem. All you've done is share the voltage drop evenly between the Leds though. You'll get further improvement if you separate it into multiple strips connected the same way. You spend a bit extra in wire but your Leds will run cooler and last longer. \$\endgroup\$
    – K H
    Commented Mar 13, 2021 at 23:58

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