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USB is 5V, and the voltage drop across the 5mm IR LEDs I'm gonna use is around 1.2V, if I connect 4 IR LEDs in series and perhaps more in parallel, do I still need a series resistor?

Will cooling fans help?

My Circuit

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    \$\begingroup\$ Don't ask "Can I..." because sure you can. The LEDs might be damaged, the USB port might be damaged (although that is unlikely), the LEDs might just not light up. It might work at first and then stop working. Avoid all problems above by calculating a proper series resistor value and using that. The fact that something can be done doesn't means it's a good idea. We don't care about your LEDs, if they break it is your problem. So that's why every sane person uses a current limiting resistor or a current source to drive LEDs. But it's a free world, if you think different then go ahead. \$\endgroup\$ Jan 28, 2020 at 9:51
  • \$\begingroup\$ If the question were Can I operate these devices from USB with guaranteed operation, then the answer is no. \$\endgroup\$ Jan 28, 2020 at 10:27
  • \$\begingroup\$ Why don't you want to use resistors? \$\endgroup\$
    – HandyHowie
    Jan 28, 2020 at 11:14

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LEDs are current controlled devices. You will surely either fry them sooner or later unless you control the current OR the LEDs won't lit due to insufficient voltage.

  1. Have a series resistor based on the required current
  2. Or choose a LED driver IC or a constant current circuit for efficiency if you are powering multiple LEDs.
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