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I need your advice whether to use a Tip122 Darlington transistor or a BD137 medium-power transistor Or a IRL3705 MOSFET to drive 10 Parallel infrared LEDs.

Each LED has a forward voltage of almost 1.2-1.5 V and takes 40mA of current. This makes the total current required to be 400mA. I am using a 5 V power supply which can give 2 amperes continuous current that I will apply to source/collector.

I will be using an ESP32 to generate 38 kHz pulses to the base/gate of the transistor. Keep in mind that ESP32 can provide 3.3V and very low GPIO source current (some say 12mA and others 40mA max.)

Can you tell me which one will be optimal from my usage? I will be connecting these LEDs to the source/collector.

As from my research Tip122 may not work properly at 38Khz switching frequency, the BD137 only has a gain of 10 that means that if I am giving 40mA or 12mA of base current I may either get 400mA or 120mA (extra doesn't hurt but having less definitely does).

IRL3705 is expensive and also has Vgs(th) of 2V maximum, meaning that either it may not work correctly or it would have a high Rds(on).

enter image description here

P.S. You can see the picture attached for 4 of these LEDs and you can imagine the rest of 10 of these. The picture would be same if I am using a n-channel enhancement MOSFET.

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  • \$\begingroup\$ I could be wrong with my research thats why i need your help, kindly point out where did i analyze wrong. \$\endgroup\$ – Informal Limits Lad Jul 14 '20 at 17:54
  • \$\begingroup\$ You can reduce the overall current by 50% using pairs of LEDs in an array of 5P2S \$\endgroup\$ – Tony Stewart EE75 Jul 14 '20 at 18:16
  • \$\begingroup\$ I understand where you are coming from. But that means a single fault will affect both the LEDs. I am making a permanent setup so i want something that needs minimum maintenance. \$\endgroup\$ – Informal Limits Lad Jul 14 '20 at 19:13
  • \$\begingroup\$ 40% of 40mA is well below max rated current. What maintenance? \$\endgroup\$ – Tony Stewart EE75 Jul 14 '20 at 19:24
  • \$\begingroup\$ If an led gets broken, that means that two leds will not work at the same time because they are in series. \$\endgroup\$ – Informal Limits Lad Jul 14 '20 at 19:25
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You can use a small logic-level MOSFET to switch 400mA, eg. AO3400A. The IRL3705 will probably work too, but it is not characterized for 3.3V drive and has a lot more gate charge (more than 10x) so it will switch more slowly.

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  • \$\begingroup\$ it can dissipate a maximum of 1.4 Watts. 1.4 Watts divided by 5V gives 280 mA which is too low for my application. \$\endgroup\$ – Informal Limits Lad Jul 14 '20 at 19:15
  • \$\begingroup\$ Rds(on) with 2.5V drive is less than 0.048 ohms so it will dissipate less than 0.4 ^2* 0.048 = 8mW. Typically more like 4mW with 3.3V. So it will run cold. BTW, 1.4W is not something you should ever consider trying with an SOT-23, it will unsolder itself lickety-split and fall on the floor. \$\endgroup\$ – Spehro Pefhany Jul 14 '20 at 19:23
  • \$\begingroup\$ I am a beginner in MOSFETs, i confused its working with that of a bjt. \$\endgroup\$ – Informal Limits Lad Jul 14 '20 at 19:32
  • \$\begingroup\$ As for IRL3705, it has a rise and fall time in nanoseconds so switching 10 times more slowly would still be faster than microseconds. as 1/38000 = 36.315 microseconds, \$\endgroup\$ – Informal Limits Lad Jul 14 '20 at 19:37
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    \$\begingroup\$ Pakistan, other ways could be if i amplify my signal using a op-amp to 5v or 12v and supply if to the mosfet or i could add a transistor like 2n2222 with the emitter connected to BD139. \$\endgroup\$ – Informal Limits Lad Jul 14 '20 at 20:09
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This is best dealt with a N channel enhancement mode mosfet. The current draw from the GPIO pin on the ESP32 will be much lower and the on/off of the transistor will be better/faster.

380mA is not that much in the world of most mosfets. A quick digikey search finds a 3.2A part in a SOT-23 package for $0.43/each. NTR4170NT1G. With 3.3V on the gate it'll have about 55mΩ Rds-on or about 8mW of dissipation.

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  • \$\begingroup\$ Digikey, sparkfun, amazon and ebay doesnt ship to our country. \$\endgroup\$ – Informal Limits Lad Jul 14 '20 at 19:58
  • \$\begingroup\$ @HermanZahid Good point. You'll have to work with suppliers that work with your country. But you should be able to do a similar search for small mosfets and find a suitable part. \$\endgroup\$ – Aaron Jul 14 '20 at 20:04
  • \$\begingroup\$ currently discussing it with the Sphero Pefhany (The one who answered above), we are looking for other ways. \$\endgroup\$ – Informal Limits Lad Jul 14 '20 at 20:07

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