Simple linear supply

A simple supply with 1% current and voltage regulation.

The circuit.



The above circuit is certainly NOT a low dropout regulator, we loose about a minimum of 4.5 volts across Q1! However, in some instances this is inevitable or not a problem, and then the simplicity of this design becomes a great asset.

Indicators

A red LED indicates when the current limiter is active and a green led indicates active voltage regulation. Both the current and voltage is regulated to about 1%.

Loads

The design was not intended to driver dynamic loads and has not been tested for dynamic stability. On static loads (like the battery indicated in the circuit) however, the design works wonderfully.

The TL431

The TL431 is a lovely device to use in power supplies. It is very cheap, has a small footprint, contains a 2.5 Volt reference, a comparator and a open collector like output. The two things to keep in mind when using the TL431 is that it may oscillate under capacitive loads, and it needs about 1ma before it operates linearly.
R3 and R4 provide the running current, while the volt drop of the LEDs ensure no premature regulation occurs.

MOSFETS

The use of MOSFETS in linear power supplies provides unique challenges and benefits. Under static loads, MOSFETS have low drive current requirements, simplifying control circuitry.

MOSFETS can be paralleled as they have a positive temperature coefficient, the only drawback being the increase in gate capacitance.

Used in common drain applications, MOSFETS have a low voltage gain. As a result, open loop regulation is not as good as we have come to expect from bipolar transistors, this simply means we need good, fast closed loop control.

The high turn on voltage of MOSFETS compared to bipolar transistors results in a high minimum volt drop when used in common drain applications.

Conclusion

If you need a simple linear power supply with current limiting for static loads, this circuit is a winner.
Future designs in this series are sure to come.
Watch this space.

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