Hi, I need to create a circuit that flashes an LED @ approximately 2.2v & 500ma for a few micro seconds each time. I was thinking of using a mosfet. I need to be able to use 3.3v logic levels. (teensyboard).
I'm having an issue sourcing a mosfet that will work due to the sheer amount of them available, I'm a bit overwhelmed by all the info out there basically.
Any recommendations would be greatly appreciated. Thanks!
What voltage will the leds be powered from? In other words how much voltage headroom do the transistors have to work with?
How much current can the teensy pins source or sink? Have you considered BJTs as an alternative to MOSFETs? BJTs are switched by current rather than voltage.
Sparkfun and Adafruit have such a mosfet:
Sparkfun N-Channel mosfet: https://www.sparkfun.com/products/10213
Adafruit N-Channel mosfet: https://www.adafruit.com/product/355
[ADDED] I see that PaulRB asked the right questions. Switching on for a few microseconds with a mosfet is not easy because of its gate capacitance (which can be 1nF). A single transistor that can do 500mA might be better.
Hi Paul, thanks for the answer,
I have a variable power supply for the LED so I can set that to a max current and w/e voltage I like.
Output HIGH on the teensyboard is 3.3V. The maximum output current is 4mA.
I am open to using a single transistor or BJT if that is easier.
Thanks,
Dave.
I know enough about transistors to know that I don't know enough about transistors to give you a definitive answer!
The power MOSFETs suggested by @Koepel seem overkill for switching 500mA. I don't know if switching speed will be an issue with them or not.
An ordinary NPN BJT like BC337 can handle 500mA, especially as they will be on for only short periods. Normally you would want to get the BJT into its saturation region to minimise voltage drop and power dissipation, but switching is slower in that region, and there's no great need to minimise those things here.
So maybe try bc337 with a base series resistor to limit the current to less than 4mA. (3.3V - 0.7V)/4 ~= 680R perhaps. Someone check my maths here: will the base-emitter drop be as high as 0.7V?
Great, I will try as you suggested with the BC337. I will also purchase the mosfet he suggested to experiment around.
Thanks for your time .
this page (in French but google translate can help easily) has a nice list of power mosfet (P or N channel) sorted out with their key specs.
Except that to switch 500 mA, you want to saturate it, and then it does not want to come out of saturation within several microseconds.
That is one reason why we moved to FETs!
Also, the choice is much wider if you can solder small packages (SOT23 etc.).
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