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4QD-TEC Electronics Circuits Reference Archive
Regulated, Variable Power Supply

In case you have not read it, Part 1 of Regulated Power Supply


There is, in 4QD's circuits, public area, a circuit diagram for a simple power supply regulator. This is a rather more sophisticated circuit and full details are given here. There is also a layout available for a suitable circuit board.


A circuit diagram cannot really give full specifications for a supply such as this, to specify the circuit's performance. That is up to the components you use to build it and in particular, the transformer, power transistor(s) and the heatsink. However this circuit is suitable for the following types of specifications

  • Voltage - 6 - 40
  • Current up to, say, 10 amps Current limited
  • Re-entrant current limit
  • Thermal shutdown

A circuit such as this is a general purpose one in that you can chose components so build a version for any particular need. It is not however a general purpose adjustable supply you can build and then adjust for anything! This is because one of the tricks it uses is re-entrant current limiting. If you make a power supply capable of giving 40v at 10 amps, them, if there is a fault on the output, the power supply transistor can be asked to dissipate 400w into such a fault. That will need a bank of power transistors to do the job, mounted on a huge heatsink. This supply, instead of doing that, shuts down its output current so that, into a shorted load, it delivers only a small amount of current.

The circuit


How it works

Tr1 is the main regulator transistor. It is driven by a pair of smaller transistor, Tr2 and Tr3, arranged in a Darlington configuration. Note that the circuit uses an NPN transistor here: because of this, the negative output is electrically connected to the power transistor's collector. Now the negative output is usually a sensible place to earth the power supply chassis to: so the power transistor has its collector (case) not isolated from the earth, which improves heat transfer to the heatsink and simplifies construction. The output stage is supplied from a reference voltage - the 6v2 zener. In normal operation, Tr6 is fully turned on (shorting out R2) so the current available from Tr4 is determined by the voltage (6v2-Vbe) across R3: about 4.7mA. Enough of the available current is allowed through into the Darlington's base so that the output conducts enough to establish 6.2v (measured from the positive rail) on the base of Tr5. At that point the circuit stabilises: the output voltage being determined by R1, R5 and the setting of the preset.

R9, R10. R12 and R13 are chosen to carefully control the gain of the Darlington. That controls the peak output current it can give into the load, so defines a current limit for the circuit. If the load current exceeds this value, then the Darlington can deliver no more current and the output voltage drops. The output voltage is sensed by R4 and R22. With the values shown, when the voltage across R4 drops below around 6.2v, Tr6 will start to turn off. This starts to switch in R3, so the available drive current to the base of the Darlington starts to reduce. This gives re-entrant current limiting. With the values shown, the output current starts to reduce when the output voltage falls below about 17v.

Tr1 needs to be mounted on a suitable heatsink.

We will first consider the power transistor, as it is going to be doing all the work. A suitable transistor could be a 2N6254. The manufacturer's specs rate this as 15 amps max, 80v max and 150W max at 25°C. That does not mean you should try and use a single 2N6254 to build a 15 amp 60v power supply!

Printed Circuit Layout

Full details of this board (such as copper layout etc) are available. The two drawings show an overall view and the component layout.


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