Hello,
I need suggestions on which type of mosFET to use.
I will be driving 8 LED assemblies, each of which draw 246mA each at 12.8 volts. Since they are arrays built for automotive use, they have resistors built in. The base of the mosFET's will be connected to Logic Gates, so the input to the base of the mosFET's is only 5volts.
So, I need a mosFET with a continuous draw of 2A. It will be wired like the picture. I need them to have a diode to help with any noise issues if they arise. A small surface mount device as well, NO through hole.
There is a reason for the diodes at the bottom of the LED's. I actually have more LED arrays, and they all have a common ground connection, so the diodes there keep the from turning on/off for the rest of the circuit (which has SCR's in it) Basically, the LED's assemblies have two colors, 3 wires out. One is ground, and one is for power for each color. So the ground is common.
Anyway, I have drawn the diodes to illustrate the voltage drop in the circuit, as well as the transistor at the bottom. There is a reason that transistor needs to be there, so that the other LED arrays in the circuit (different color) can be turned off via another transistor, via another logic gate.
In simulation, I can't get more then 54mA or so through each LED. I don't know why. Maybe because the simulation program I am using are not Logic Level mosFET's? Is it just that they are not turning on all the way? I can't edit the perimeters for them in the simulation program.
I even tried driving the mosFET's in the program using a transistor to switch on the mosFET, and I get 212mA through each LED, UNTIL I put in the diodes and transistor at the bottom of the schematic, then it drops to like 23mA. The diodes will be a schottkey diode with a voltage drop of around 0.42volts.
The DIP switch is so that each LED can be turned off in the array for situations where all 8 LED's won't be used. I need it to connect to the bases of the mosfet, because the DIP switch can only have a little current through it.
Would it be better to replace the NPN Transistor with an N-Channel mosFET? I am not sure on the values needed.
Thanks for any help given. I appreciate all your guys help over the last couple months. I've been racking my brain with this circuit, and though I've learned alot, there's still more to learn.