Transistor or mosfet

Projects General Guidance
1

Power source is 5v. Will be controlling with a atmega328. I will be using .5w led that is 3.4v at 120 mha. Will drop from 5v to 3.4 by resistor. I have been playing with transistors but not mosfet. Some pins will have one and some will have two LED connected. I have read where some mosfet when switched from 5v will not open or close. Looking for input.

2

You can use a logic-level MOSFET.

See

for a very thorough presentation of various solutions.

HTH

Added: I didn't see Gammon name the part, here's two

AO3400A Up to about 2–4 A, compact

5–30+ A loads, TO-220 heatsinkable designs

added: I dropped the actual part number of the second suggestion! (THX @jim-p)

IRLZ44N - 5–30+ A loads, TO-220 heatsinkable design

AO3400A or IRLZ44N

As your currents are moderate, I'd probably follow @kmin and just use a 2N2222. Actually a 2N3904 would be my choice, I have a lifetime supply of them.

a7

3

120mA LED switching is fine with transistor which you are more familiar with. Logic level mosfet can have benefits over transistor though, if you are looking for some improvements?

4

You can use an IRLZ44N MOSFET, it's very common, can be used in a breadboard and can handle large currents.

5

Use whatever you have, either will work in the proper circuit. Almost any N-Channel logic level MOSFET will work. Be sure to put a resistor (10K - 57K) from the processor port pin to ground, that guarantees the MOSFET is off until your software turns it on. For a transistor you will need a resistor from the port pin to the base, assuming both solutions are low switching.

6

120mA is fine and easier with a 2N2222 or BC337 with ~330 Ohm base resistor.
A mosfet won't have any real benefits at that current.

My preferred LED driver is a 2-transistor constant current circuit.
BC547/BC337 or 2N3904/2N2222.

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7

Another point of consideration is: where is how much heat produced? (Types are for example.)

  1. Simple transistor (2N2222): Current 120 mA, saturation voltage = 0.3 V -> P = 0.03 W
    Drop over series resistor 5 - 3.4 - 0.3 = 1.3 V -> R = 10 Ohm, P = 0.144 W
    Base resistor: 330 Ohm, 0.075 W
  2. Mosfet (AO3400) Channel resistance negligable -> P = negligable (loss during switching is ignored)
    Drop over series resistor 5 - 3.4 = 1.6 V -> R = 12 Ohm, P = 0.18 W
  3. Current source (2x 2N2222): Emitter resistor 5.6 Ohm 0.08 W
    Drop over second transistor: 5 - 3.4 - 0.7 V = 0.9 V -> P = 0.1 W
    Base resistor: 1k, 0.025 W
    Advantage: tolerances have little effect on luminosity.

If two LEDs are controlled by one pin, they can be controlled by one transistor (or mosfet) but each LED needs its own series resistor.
If chosen for current source, each LED needs its owm current source

8

I realise that with a constant current source most of the power is dissipated in the pass transistor, but 0.1W is well within the safe zone of that transistor.
The main advantage is a constant luminosity, independent on supply variations.