Circuits: A simple current limiter

Here’s the schematic for a simple current limiter based on an op-amp, a N-channel MOSFET, a current sense resistor and a voltage to set the current. This circuit will allow the flow of current to be limited, and it’s maximum current will depend on some factors like the MOSFET, the voltage supply and the resistance of the current sense resistor.

This circuit can be integrated in a power supply, but the voltage reference for controlling the current, as well as the op-amp supply should be constant, preferably regulated.

You can calculate the maximum voltage drop by adding the resistances of the MOSFET and the resistor at the peak current (the maximum current tolerated before starting to limit it) and maximum voltage. In my case I used a resistor divider to set the maximum current.

To calculate the maximum current: the voltage drop across the current sense resistor Rcs is 10 times less than the current flowing through the circuit. With that in mind we deduce that the voltage going into the non-inverting input of the amplifier is going to be the same as the voltage drop across the resistor. This is because the op-amp will adjust the voltage going into the gate of the MOSFET until both of it’s inputs have the same voltage. This is called negative feedback.

Now we have a little circuit that can adjust the current. Every 100mV going into the non-inverting input can make an amp of current flow into the MOSFET. So here’s the equation:

I = Vni * 10

• I is the current flowing through the MOSFET,
• Vni if the voltage of the non inverting input
• And 10 is the quantity of fingers an average person has in a hand multiplied by 2.

By the way, make sure the gate of the MOSFET supports the maximum voltage the op-amp can deliver (look for Vgs on the MOSFET’s datasheet). Also be aware of the power rating of the resistor and the MOSFET.

So that’s it! Is was meant to be quick but I made it all long. But if you where going to use it I think you like it!