Regulated Power Supplies

Question File Number 22



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The Need For Voltage Regulation

(Please refer to Power Supplies too.)

A voltage regulator is added to a power supply to minimise the voltage droop or sag when the load is applied and when the current load varies widely.

Some loads, for example a SSB transceiver, present a wide-changing current requirement. The power supply current for a SSB transceiver, supplied from a car battery, can fluctuate while the operator is speaking from a few amps to 50 amp or more, depending upon its transmitter power rating. The battery voltage must remain at a constant level throughout.

Similarly, a mains-powered power supply must be able to keep a constant voltage throughout a wide current range.

A regulated power supply has another stage added to follow the filter:

Regps

A Simple Regulator

A zener diode is a silicon diode with a special level of doping to set its reverse break-down voltage level. It forms a simple regulator for low-voltage and small-current loads. The zener diode is reverse-biased and the reverse current is determined by the break-down voltage which depends on the doping level of the silicon. The breakdown voltage is repetitive provided the maximum power dissipation is not exceeded. There is a catalogue choice of zener diode across a wide range of voltages. The zener effect occurs below 5 volt, above 5 volt the avalanche effect is used.

Zenereg

The resistor R is to limit the current through the diode and the load.

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The Three-Terminal Regulator

This is an example of a regulator package, a 78LO5. It looks like a standard transistor but it is a complete regulator for supplying a 5 volt output from (say) a 12 volt DC input. There are many other similar devices available for similar purposes. The pin-connection details are given. (Three-legged regulators.)

5vreg

The diode D1 is a hold-off diode, for protection against the possibility of the input connections being inadvertently reversed.

The diode will not conduct with reverse input potential so the regulator is protected. Diode D2 is protection for the device itself.

The Series Pass Regulator

Seriesreg

A power transistor can be used to control the output voltage from a supply.

A power transistor (or several in parallel) is in series with the output. The base is fed from a separately-regulated supply such as a three-terminal regulator or a zener diode. The transistor is in an emitter-follower configuration. Its emitter contains the load and the emitter follows the voltage at the base.

Protective Measures

All the regulator circuits considered above require the input voltage to be considerably higher than the output. If the regulator fails, there is the distinct possibility that excessive voltage will be applied to the load. Over-voltage could damage the load and be very expensive if the load was a transceiver.

An electronic device known as a crowbar is usually installed to protect the load as a last ditch measure in the case of a regulator failure. The crowbar senses an over-voltage condition on the supply's output and acts instantly, firing a shorting device (usually a silicon-controlled-rectifier) across the supply output. This causes high currents in the supply which blows the mains fuse and effectively turns the supply off.

Current-limiting is another protective measure usually incorporated in a regulated supply. This is to reduce the current through the regulator to a low value under excessive load or short-circuit conditions to protect the series pass transistor from excessive power dissipation and possible destruction.



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Compiled Sun Nov 28 2010 at 8:41:44pm


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