Current limiting circuits can be confused with current/circuit breaker. Unlike a fuse that breaks a circuit connection, a current limiter limits the current (to the designed value). Current limiting circuits can be as simple as a single resistor but here I present an active current limiting circuit. With a resistor (a passive current limiter) its voltage drop increases with increasing load current. The higher the load current the higher the voltage drop on the resistor, resulting in a decreasing load voltage. This is usually undesirable.

Active current limiters have two major advantages over fuses and current limiting resistors. First, they are much faster acting than fuses so do a better job of protection (not to mention that they don’t have to be replaced). Second, they do not cause a reduction in load voltage.
Figure 1. Current Limiting Circuit

In this active current limiting circuit, the load voltage remains constant when load current is within the allowable range. It is also desirable that the limiting circuit not dissipate additional power, which means almost all the power is delivered to the load. But when the load draws more than allowed, the current limiting circuit begins to act as a resistor. The value of resistance increases to limit current to the designed level.

Without the current limiter, the voltage source in Figure 1 is directly connected to R load. R load here represents a resistance that draws a variable current. Batteries under a charge or an amplifier circuit are examples.

How the circuit Works
See Figure 1. The key to this circuit is that emitter followers (Q1 and Q2) act like a switch, turned on and off by voltage at their base. When base voltage is high, transistor current is high and it acts like a closed switch. When base voltage is low, there is no current flow – switch off.

Normally the base voltage of Q1 (controlled by R bias and V ce across Q2) is sufficiently high for Q1 to operate in saturation as a closed switch (I ce is at maximum). This is because R bias and the V ce across Q2 act like a voltage divider, where the higher the resistance – the higher the voltage across that resistor. When Q2 is turned off, it has a high voltage drop (acts like an open switch). The open switch condition of Q2 – V ce, when compared to the R bias voltage, is so large that Q1 is turned on by the large V b of its base.

How load current is controlled is the function of the R sense resistor. Its value is small, so under normal conditions load current does not create an appreciable voltage drop. This means that full current and voltage are presented to the load. But when current approaches the maximum design limits, load current through R sense is sufficient to develop a voltage at the base of Q2. This voltage is large enough to begin to turn Q2 on. Because of the current gain of Q2, this transition from an off to an on switch is rapid.

When the Ice current through Q2 increases, its current must come from the source and that effectively shunts current from R sense while increasing the voltage drop across R bias. Two things begin to happen. The voltage drop across Q2 (V ce) falls rapidly and the voltage drop across R bias increases. This reduces the base voltage of Q1, turning it off and closing the Q1 switch. When the Q1 switch is closed, current flow to the load stops. Transistors are capable of operating millions of times a second, so the transition from off to on actually controls the load current at a level set by the design value of R sense.
The remainder is unchanged.