Power supply current limiter circuit

- a simple circuit for a power supply current limiter using two diodes and a resistor

In any power supply there is always the risk that the output will experience a short circuit. Accordingly it is necessary to protect the power supply from damage under these circumstances. There are a number of circuits that can be used for power supply protection, but one of the simplest circuits uses just two diodes and an additional resistor.

The circuit for the power supply current limiter uses a sense resistor placed in series with the emitter of the output pass transistor. Two diodes placed between the output of the circuit and the base of the pass transistor provide the current limiting action. When the circuit is operating within its normal operating range a small voltage exists across the series resistor. This voltage plus the base emitter voltage of the transistor is less than the two diode junction drops needed to turn on the two diodes to allow them to conduct current. However as the current increases so does the voltage across the resistor. When it equals the turn on voltage for a diode the voltage across the resistor plus the base emitter junction drop for the transistor equals two diode drops, and as a result this voltage appears across the two diodes, which start to conduct. This starts to pull the voltage on the base of the transistor down, thereby limiting the current that can be drawn.

Basic power supply current limiting circuit

Basic power supply current limiting circuit

The circuit of this diode current limiter for a power supply is particularly simple. The value of the series resistor can be calculated so that the voltage across it rises to 0.6 volts (the turn on voltage for a silicon diode) when the maximum current is reached. However it is always best to ensure that there is some margin in hand by limiting the current from the simple power supply regulator before the absolute maximum level is reached.


Use in other circuits

The same simple diode form of current limiting may be incorporated into power supply circuits that use feedback to sense the actual output voltage and provide a more accurately regulated output. If the output voltage sense point is taken after the series current sensing resistor, then the voltage drop across this can be corrected at the output.

Power supply circuit with feedback and current limiting

Power supply circuit with feedback and current limiting

This circuit gives far better regulation than the straight emitter follower regulator. Also voltage drops in the series current limit sense resistor can be accounted for provided that there is sufficient voltage drop across the series pass transistor in the power supply circuit. Finally the output voltage can be adjusted to give the required value using the variable resistor.


Summary

The diode form of current limiting can be incorporated into a power supply circuit very easily. Additionally it is cheap and convenient. However if superior performance is needed than a transistorised form of current limit may be used. This gives a sharper limiting that is more suitable for more exacting power supply requirements.

By Ian Poole

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