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I bought these type of LEDs for my car interior: https://www.amazon.com/gp/product/B01J0ZO0V0/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1).

  1. LED Type: 31mm 5050 6-SMD.
  2. Width: 15mm
  3. Length: 31mm
  4. Cross Reference: DE3175 DE3022 DE3175 3022 3021 3175 6428 6430 6461

It was suppose to be a plug and play but after a while I noticed a minute pulse of intensity and it is quite irritating.

I know a fix would be to mount a resistor but for SMDs I have no clue where to solder it. I had no luck on the web or my car forums.

May I get some advice on how to: 1- Methods to stop the slow gentle pulsation? 2- Where to attach resistors and or volt regulator to this SMD? (The two methods that I now know of to approach this problem) 3- Will this ruin my car's electrical system if I just put up with the SMD LEDs being fed with more power than they need?

I am a total greenhorn at this but I am willing to learn. Thanks for your time!

Front Side

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    \$\begingroup\$ Do you have a schematic for the circuit? Are there any components on the back of the PCB or inside the contacts on the side? Why do you think a resistor is the fix? \$\endgroup\$ – Dampmaskin Sep 1 '16 at 12:31
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    \$\begingroup\$ If you mean you see some flicker, I'm not sure why mounting a resistor would fix it. \$\endgroup\$ – Scott Seidman Sep 1 '16 at 12:35
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    \$\begingroup\$ If there were enough room to mount a capacitor such as this one on the back side, this might help with "flicker". But without knowing how "bad" this "flicker" is, we're really just guessing. \$\endgroup\$ – rdtsc Sep 1 '16 at 13:08
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    \$\begingroup\$ @ScottSeidman: Based on my desperate search for more knowledge through google and my car forums, my old bulbs were incandescents so they are not energy efficient as the LEDs. My power is the same but my new LEDs requires less power through out the car. I believe I am overfeeding the LEDs with my 14v car battery so I thought a resistor could help? Is my logic incorrect? \$\endgroup\$ – ScottSkyes Sep 1 '16 at 19:30
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    \$\begingroup\$ If you're staring at it then it might be that your pupils are oscillating in size while trying to adjust to the brightness. Does anyone else see the effect? If you increase the engine speed, does the brightness change? \$\endgroup\$ – Andrew Morton Sep 1 '16 at 19:45
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Unlike incandescent light bulbs, LED do not have as much thermal mass. They respond instantly to changes in voltage. Adding a resistor might reduce the voltage across the LED, but it will not prevent voltage fluctuations from reaching the LED. To prevent voltage fluctuations from reaching the LED, a voltage regulator is necessary.

Consider the possibility that the car's voltage regulation system is not function as expected.

High powered LEDs do generate heat. But this is unwanted heat which can damage the LED. In such cases, measures are taken to quickly remove heat from high powered LEDs.

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  • \$\begingroup\$ Thank you for the information! This was very informative compared to my other forums T_T. I shall read more into it. \$\endgroup\$ – ScottSkyes Sep 1 '16 at 19:56
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It's probably two strings of ~3.2V white LEDs, with resistors for each. If we assume 60mA at 14V, the resistors would be in the ballpark of 75 ohms.

The very nature of this cheap construction is that a 10% variation in your car bus voltage will change the LED current by ~30%, so minor voltage variations caused by loads switching from your A/C compressor, turn signals, heater fan etc. may be causing a visible flicker.

On the plus side, it's simple and not much can go wrong. There no easy way to improve it (well, you could rewire the LEDs to waste more energy in more resistors or add constant current circuits, but neither of those is worth it to improve a $3 device).

It won't damage your car - the car electrical system should be fused to protect itself from shorts etc. It's very simple and fairly unlikely to catch fire, though with some offshore products one never knows.

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  • \$\begingroup\$ Thank you, you are correct. Closer inspections indicates that there are resistors but I don't know it's ratings (or what 151 would indicate). I have reedit my post with an image. Please have another look. For the sake on knowledge, if I were to add resistors where can I place them? On each individual SMD? I can't find an anode or cathode. \$\endgroup\$ – ScottSkyes Sep 1 '16 at 19:55
  • \$\begingroup\$ 151 is 150 ohms. Doesn't look easy to work with, and I have a good stereo microscope etc. \$\endgroup\$ – Spehro Pefhany Sep 1 '16 at 21:17
  • \$\begingroup\$ Gotcha. Once I have more time, I shall purchase more and mess around with them. Thank you for your input. I'll just suck it up and deal with them. \$\endgroup\$ – ScottSkyes Sep 1 '16 at 21:42

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