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Power supplies with automatic switch off.

If you have not read it, start with the first part of Power supplies with automatic switch off.

Current source and zener stabilizer

This is the circuit of the first transistor circuit in the public area 'front end' of this page.


Initially consider the situation with Vout at zero. There is no voltage on Tr4's base so it does not conduct and feeds no current through Tr3, so Tr2 is off and the circuit is 'busy doing nothing'.

Now apply a signal to the On input. Anything more than about 3 volts will do. Tr3 is biased on and C3 charges up simultaneously turning on Tr4.

Tr4 and Tr3 are arranged as a cascode pair. This is quite a useful configuration, which may well get written up in future. Here it is used as an AND gate: both Tr3 AND Tr4 must be conducting for the output to be present.

So the input signal will cause current to flow through Tr3 and Tr3, biasing on Tr2 which passes current to the output. Tr1 is arranged with Tr2 as a current source. Tr1 will maintain about 600mV across the sense resistor (unmarked) so a value of 10 ohms will give about 60mA.

There's also shown an unmarked resistor in the base of Tr2. This is probably not required, but it drops some of the voltage between Tr2's base and Tr3, reducing the power dissipated in Tr3, which may be useful at higher voltages.

The (constant) current from Tr2's collector will be shared between the load and D1, which I have here as 9v1. The current through D1 will keep Tr4 conducting after C3 has charged so the circuit will stay on as long as the On input is high and as long as the load doesn't pinch all of the available current. If the On input is removed, Tr 3 will turn off and if the load gets too high, there will be no current through the zener and Tr4 will turn off. Either event will switch off the circuit.

A BC 546, which you may well use for Tr4, has a maximum base current of 100mA and as (with no load connected) all of the available current may flow through the zener and into Tr4's base, this circuit shouldn't be used as shown for more than 100mA.

A stabilized version of the circuit

While writing the above, it occurred to me that one could relatively easily change this arrangement from a current feed zener into a current limited stabilized supply. The circuit that follows is a result of that 'occurrence'.


A quick glance will show that it is remarkably similar to the above, but here the zener has been returned to the emitter of Tr4 and not to its base. Tr4 is biased via two resistors so that it is biased on by the output. The formula for the arrangement has been given in the article Two transistor regulators.

Operation is then similar to the above circuit. However - in 4QD controllers, the battery voltage (and hence the On signal voltage) may be up to 48v. Putting 48v into the control input will, while C3 charges, over-ride the voltage stabilizing feedback, so I have included a zener diode in the control signal to limit the voltage here and thus to limit the maximum value of the output voltage during initialization.

Any circuit such as those described herein must be calculated with values to suit the actual conditions under which it may be used. So certain values have not been shown.

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© 2002-2011 RJT
First published: 30th April 2002
Page's Author: Richard Torrens