Electronic – Mosfet constant current


I am looking at a schematic that has a very similar form to a emitter follower but in the mosfet world. It's intended to keep the brightness relatively constant as the battery battery voltage depletes

I am trying to figure out what the current in this device would be and if the components are rated appropriately. Unfortunately, my mosfet knowledge beyond a switch is not sufficient.

How can I go about figuring out the current through the resistor?


simulate this circuit – Schematic created using CircuitLab

Best Answer

Here is an alternative method that is almost as simple and is more repeatable (and, probably at least as important, has a lot of compliance):


simulate this circuit – Schematic created using CircuitLab

Use a MOSFET rated to work on 2.5V drive if your logic input is 3.3V. It needs to turn on fully with about 2.7V to get the full compliance range. You can also use a BJT in place of M1.

The current is about 0.6V/R2.

It works by Q1 reducing the gate voltage to M1 (or base current if a BJT) as the voltage across R2 approaches 0.6V or so and Q1 starts to turn on.

It will drop out around 0.6-0.7V or so if the parts are well chosen, so a 3V LED can be maintained at fully brightness/current to about 3.7V input.