Power supply current limiter
- 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
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.
Using 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 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.
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