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I'm working on a proximity detection project to drive up to 25 IR led on 38Khz with 3.6v mcu.I'm planing to make them parallel with 100 ohm resistor (limit their current to 20ma for each led).MCU pwm pin can not handle 500 ma (may be less because of PWM but for safety reason I assume max), so as you guess I need to use BJT or FET to drive them. Because of some project restriction, I want to use only one mcu pin to drive IR Leds on 38Khz, and other pins for each photo diode sensor.

My questions are,

  1. I can connect different number of led (up to 25) depends on application or if any led in this parallel line burn or not work properly some reason, will it be a problem ? The reason I'm asking that; load of FET's or BJT's will change regarding to count of led in the parallel line.If so, how can I solve this problem ? Shortly , what kind of circuit should I use to drive safely 1 to 25 parallel led without damage any mcu pin, drive electronic or any other leds in the parallel string.

  2. Which BJT or FET should I use ? I have BD135 and IRFZ44N now.I want to use them because to make running prototype.If one of them fit my project, I can change more suitable (current,price etc) BJT or FET later.

Thanks

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    \$\begingroup\$ It's "kHz" not "khz" (those who think me pedantic can have their say of course LOL) \$\endgroup\$ – Andy aka Jul 24 '15 at 12:03
  • \$\begingroup\$ It's "LED's" not "leds". OMG... \$\endgroup\$ – Alexxx Jul 24 '15 at 12:17
  • \$\begingroup\$ @Alex I think its LEDs, I belive LED's would mean "LED is". \$\endgroup\$ – Golaž Jul 24 '15 at 22:17
  • \$\begingroup\$ @Golaz: It should be "it's" (it is) LEDs. The contraction for "LED is" is, as you state,"LED's", and it can also denote possession, as in: "the LED's leads were cut too short." \$\endgroup\$ – EM Fields Jul 24 '15 at 22:49
  • \$\begingroup\$ @EMFields Interesting, I always get confused with these abreviations as english is not my first language. Thanks for clearing it up! :D \$\endgroup\$ – Golaž Jul 24 '15 at 22:55
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  1. If we assume that all 25 LEDs have burned out shorted, then that means that all of the 100 ohm resistors will be in parallel across the 3.6 volt supply.

Since for equal-valued resistors in parallel, the total resistance will be the value of any resistor divided by the number of resistors, the 25 resitors in parallel will have a value of:

$$ Rt = \frac{R}{n} = \frac{100\Omega}{25} = \text{ 4 ohms} $$

Then, from Ohm's law, the current the supply must furnish into that resistance is:

$$ I =\frac{E}{R} = \frac{3.6V}{4\Omega} = \text{0.9 ampere} $$

Of course, if all the LEDs don't burn out it'll be less, but 900 milliamperes is worst case.

Your circuit should be common source, with the LED array connected between Vcc and the transistor's drain, and its gate connected to the MCU's PWM pin.

  1. I'd use a logic-level MOSFET with an Rds(on) of 40 milliohms or less with an ampere or so of drain current at a Vgs lower than the lowest Vhigh out of your MCU.
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The IRFZ44N needs 4.5V to conduct properly. The BD135 needs at least 10 mA to drive 500 mA (via a 300 Ohm resistor from the MCU pin) Can your MCU pin supply this much? Check the datasheet. As a test with just 1-5 LEDs your could lower the pin current. The datasheet says something about hfe 40-100 minimum (current amplification) Make sure each LED gets its own resistor in series. Only then can you parallel them.

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