# Auto poweroff for power supplies circuit

Posted on Jul 28, 2012

Probably the easiest way of doing automatic switch off is with relay logic. In the diagram, the box marked RL1 is the coil, the 2 in the coil box tells you there are two sets of contacts somewhere on the circuit, operated by this coil. In this simple circuit, it's easy to see them!Sw1 is a momentary push switch. When it's pushed (contacts closed), voltage from In is applied to the coil so the relay operates. RL1/1 (the first set of contacts on RL1) closes and applies power to Out. RL1/2 closes at the same time and shorts out Sw1, maintaining power to the relay coil. So Sw1 is a push to switch on and the circuit is a relay bistable - bistable as it has two stable states, on and off.

There are lots of ways of doing this type of switching and the diagram below shows but one This system is the way many of 4QD's controllers are powered, though this particular circuit is most like that of the 4QD series. It is essentially a switched current source feeding a zener diode which acts as a stabilizer. The current though the zener is monitored by one of the transistors that switches the current source so that as the output voltage gets overloaded (so that the zener current falls to zero) the circuit switches itself off. There is more information including a discussion and component values in the 4QD-TEC members area page. There is an article on site on two transistor regulator circuits. The third of these circuits is a bistable regulator. As often happens when thinking of one circuit, another idea pops into mind. The switching on and off arrangement in the above circuit can also apply to this regulator, and the circuit below illustrates this idea. As in the above circuit, the series pass transistor is arranged as a current source: this is not always necessary. The regulating zener is here returned to the emitter of the transistor, which has a resistor present. This shuts off the current fed to the series pass transistor so that it conducts a variable amount to maintain the output voltage. The current source is then a current limit (which is why it can, optionally, be removed.

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