I need to add an active current limit to the LEDs from the previous post. The main reason for the current limit is so that the brightness of the LEDs remains uniform across the lead acid battery voltage range of 11.8 to 14.4V. The circuit I am using is a very simple and only uses 4 components to limit current.
See the schematic here:(to see a bigger version of any of these pictures click the image, then click the image on the following page, it is a weird wordpress issue)














The circuit functions as follows:
For this example we will set the current limit at 100mA so R2 is 6 Ohms.
The NMOS is turned on when the voltage from Vin through R1 exceeds the NMOS Gate threshold level. As voltage at Vin rises so does current through the circuit and so does the voltage drop across R2. The current limit kicks in when the R2 voltage drop exceeds the base voltage threshold (VBE) of approximately 600mV on Q1. So if the current limit is set for 100mA the voltage across R2 = 6 Ohms * 0.1A = 600mV. When Q1 turns on it starts to pull the voltage down at the gate of the NMOS causing the FET to go into its linear region, this limits the current through the circuit. The NPN transistor is so much faster than the NMOS that there is no problems with oscillation in the circuit ( this assumes that the circuit is built properly with the current limit components in a very tight group )

There is one more ‘benefit’ to this circuit. If you want some limited form of over heating protection for your load circuit you can place the transistor next to the hottest part of the load. The VBE of the transistor will drop as it heats up, effectively dropping the the current limit in the circuit, this will all depend on the current in the transistor and the temperature. The greatest analog guru of all time Bob Pease wrote an article that explains VBE in detail just click here to read What’s All This VBE Stuff, Anyhow? Of course if you want to keep your current limit circuit constant keep the transistor away from the heating of the load.

Here is a simulation run of the circuit: Red line is the current going through the LEDs and R2, Blue line is the Voltage into the circuit ramped from 0 to 14.4V in 20mS, Green is the gate voltage of the NMOS, Light blue is the Voltage across R2 and at the base of Q1.

This circuit can be used for a constant current source / sink for LEDs and laser diodes (the circuit can be used on the high or low side of a load). It can also be used anywhere a current limit function is needed.

The NMOS can be just about any N-Channel FET that is rated for 1.5X the voltage of the circuit, 2X the Wattage of the circuit and at least 2X the current of the circuit, and should have a reasonable RDSon. The FET may need to attached to a heat sink, if you plan on running more than a 1/4 Watt.

The current limit function comes into play only if the input voltage is high enough to meet the following:

Vin > Load Circuit Voltage Drop + ( NMOS RDSon * Ilim ) + ( R2 * Ilim)

Here is an example of the current limit setting a constant current for two different voltage levels:

For example A the current limit is set for 30mA and is barely on with just 0.7V dropping across M1. This is derived with : 11.8V Vin – (LED forward voltage 3.5V at 30mA * 3 LEDs) – 0.6V across R2
The power dropped on M1 = 0.7V * 0.03A = 21mW

In example B the lead acid battery is fully charged and the power drop is at it highest with the power dissipated on M1 = 4V * 0.03A = 102mW