Linear Power Supply Circuits Primer & Tutorial Includes:
Linear power supply Shunt regulator Series regulator Current limiter 7805, 7812, & 78** series regulators
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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.
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.
Transistor current limited circuit
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.
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.
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.
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