1

I have three LEDs (RGB), with the following forward voltages and current:

  • red 2.25 V @ 0.35 A
  • green 3.3 V @ 0.35 A
  • blue 3.1V @ 0.35 A

I plan on using three (3) RFP30N06LE MOSFETs in parallel @ 0.35 A each using independent TTL-based PWM dimming from the PWM pins of an Arduino, each MOSFET connected in parallel to a shared 9V - 12V @ 1 A power rail fed with a wall wart supply.

What would the optimal method be to wire these MOSFETs in parallel and use the same shared power rail?

user1316349
  • 11
  • 3
  • The optimal way is to wire them in series (see the circuit diagram, text is in german): http://www.mikrocontroller.net/topic/147925#1383502 – starblue Jun 24 '13 at 07:00

1 Answers1

2

Just like that, in parallel:

schematic

simulate this circuit – Schematic created using CircuitLab

However, if all three LEDs can be on at once, you might want a bigger power supply. The maximum current to power the LEDs is:

$$ 0.35A + 0.35A + 0.35A = 1.05A $$

This is 5% more than your power supply's rating already, plus you need to allow for some current to power control circuitry probably, and probably you want to allow some margin for reliable operation.

Phil Frost
  • 56,804
  • 17
  • 141
  • 262
  • So are each of the resistors different values to current limit each MOSFET to 0.35 A? I had previously tried a variant on this schematic (without resistors unfortunately) and somehow ended up destroying the LEDs, just at 0.35 A total output on the bench supply. – user1316349 Jun 23 '13 at 22:57
  • @user1316349 you need resistors, or some other current-limiting element, to power an LED, unless these are LED packages with the limiting already built in. But, if you managed to destroy one with only 0.35A from your supply, perhaps your specifications are wrong. If there was 0.35A from the supply, there couldn't have been more than that in the LED, unless you had a buck converter or some other non-linear power converter. – Phil Frost Jun 23 '13 at 23:09
  • @user1316349 and yes, each resistor is a different value, because each LED has a different forward voltage. There are hundreds of questions here about that, and millions of thing on the internet at large, but maybe start here: http://electronics.stackexchange.com/questions/55823/how-can-i-efficiently-drive-an-led – Phil Frost Jun 23 '13 at 23:11
  • Thanks, I will try that next. Sorry I can't upvote yet, I don't have 15+ feedback. – user1316349 Jun 24 '13 at 01:38
  • 350ma is rather a lot for most leds, are you sure it's not 35ma ? – pjc50 Jun 24 '13 at 13:41
  • No these are high output LEDs, hence the requirement for separate MOSFETs for each LED. The Arduino can only source about 30ma if I am not mistaken. – user1316349 Jun 24 '13 at 21:01
  • Phil, these MOSFETs actually act as a type of buck boost driver. There are three pins on the input side (positive, negative, and control), with a positive and negative for the output which feed the LED. With that said, should my current limiting resistors be on the MOSFET input side instead of the output leg of each MOSFET? – user1316349 Jun 24 '13 at 21:03
  • I doubt this is a valid schematic but here goes: [https://www.circuitlab.com/circuit/3d9feu/mosfet_led_driver_1/](https://www.circuitlab.com/circuit/3d9feu/mosfet_led_driver_1/) – user1316349 Jun 24 '13 at 21:39
  • @user1316349 I don't see anything about buck-boost in the datasheet, or these three pins on the input side. You could use these in a buck-boost converter, but you'd need to add an inductor at least. Are you using some part beyond what you specified in the question? – Phil Frost Jun 24 '13 at 23:58
  • Sorry I should have been more explicit. The current design I am using for the RFP30N06LE MOSFET is based on Adafruit's MOSFET power control kit: https://www.sparkfun.com/products/10256 , which has the (+-C) input with (+-) output for the LEDs. – user1316349 Jun 25 '13 at 04:23
  • @user1316349 that's still not a buck-boost converter. All that does is give you screw terminals. – Phil Frost Jun 25 '13 at 11:31