Transistor current limiter

- a simple one transistor circuit to act as a current limiter in a power supply

A current limiting circuit is almost an essential element of any modern power supply. There is always a risk that the power supply rails may be shorted. The inclusion of a current limiter circuit will prevent any further damage occurring to the external circuit as well as preventing damage to the power supply itself.

It is possible to implement a power supply current limiter with just diodes, but this one uses a single transistor and a current sense resistor. This circuit forms the basis of most power supply current limiters used today. The circuit is shown below incorporated into a simple regulator circuit.

The operation of the current limiter is very straightforward. When the power supply is supplying current below the maximum level, current flows through the sense resistor and a small potential difference develops across it. The value of the resistor is chosen so that at when the maximum allowable current flows from the power supply, a voltage equal to the turn on voltage of the current sense transistor is developed across it. This is typically 0.6 volts, assuming that a silicon transistor is used.

As the voltage across the current sense resistor reaches 0.6 volts, so the current sense transistor starts to turn on. When it does this, the voltage it the base of the main power supply pass transistor is pulled down, thereby preventing any increase in the output current of the power supply. In this way it is very easy to calculate the value for the sense resistor using Ohms Law. It is simply 0.6 / maximum current. The current sense transistor should have a sufficiently large current capacity to be able to take the current the base of the main series pass transistor.

Power supply with feedback and transistor current limiting

In view of the fact that the regulator sense point occurs after the current sense resistor, any voltage drop across the resistor will not affect the output voltage of the circuit as this will be compensated for by the regulator. (This assumes that there is sufficient voltage across the series pass transistor for it to regulate correctly.) In this way the current sense resistor will not cause any reduction in the voltage output from the power supply regulator circuit.

The power supply current limiter circuit is shown within the circuit of a very simple regulator. However it can be placed within most regulator circuits made from discrete components with little change. For regulator circuits using regulator integrated circuits, these are virtually certain to include current limiter circuitry based around this principle.

By Ian Poole

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