Self-shutdown power supply. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Self-shutdown power supply

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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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Do you want to fall asleep to music without fear that the AC adapter of your radio or player will remain plugged into the outlet all night? Then make the power supply described in the article, which automatically disconnects from the network when the load is turned off. Or modify the existing network adapter ...

When using equipment in which the power source is made in the form of a separate unit (adapter) plugged into a power outlet, there is always an inconvenience that, after using the device, you have to get up, go to the outlet and remove the power supply from it. And if you like to listen to the radio or tape recorder next to your bed before going to bed, there is a risk of falling asleep and leaving the power supply plugged in all night, even if hitchhiking works in the tape recorder. And this is not safe - it may happen that you wake up in the middle of a burned-out apartment.

The power supply offered to the attention of readers will not allow this: at the moment when the load stops consuming current, it will disconnect itself from the network. If, for example, a tape recorder with hitchhiking is powered from it, then you can safely fall asleep, and if a device without hitchhiking or a radio receiver is connected, then you just need to turn them off with the power switch - the unit will turn itself off in this case. Agree, this is much more convenient than getting up half asleep and going to the outlet.

Schematic diagram of a self-switching power supply is shown in the figure. Its basis is a classic stabilized rectifier, consisting of a T1 step-down transformer, a VD1-VD4 rectifier bridge and a DA1 microcircuit voltage regulator. Turn on the device with switch SA1. If left closed, the unit will operate normally - always on. If, when the load is running, it is turned to the "Off" position, then the power supply will remain on only as long as the load consumes current.

(click to enlarge)

As can be seen from the diagram, the load current, passing through the silicon diode VD6, creates a voltage drop of about 0,6 ... 0,7 V on it. This voltage is enough to open the transistor VT1, in the collector circuit of which relay K1 is connected. When triggered, it shunts the SA1.1 switch with K1 contacts, and the power supply remains connected to the network. When the load is turned off (for example, by its switch), the current through the VD6 diode stops, the transistor closes and the relay, releasing, disconnects the unit from the network. Diode VD5 compensates for the loss of output voltage due to the introduction of diode VD3 into the output circuit. LED HL1 - indicator of device connection to the network.

Any diodes suitable for forward current and reverse voltage can be used in the block rectifier. Diode VD6 - any silicon capable of passing (preferably with a margin) load current, VD5 and VD7 - any low-power silicon. Relay K1 - with a response voltage of not more than 7 ... 8 V, an operating current of not more than the maximum allowable for the transistor VT1 and contacts designed for switching an alternating voltage of 220 V. Switch SA 1 - any small-sized, capable of switching voltage of 220 V.

The auto-shutdown function can be endowed with any ready-made power supply by introducing only three diodes (VD5-VD7), a transistor and a relay into it.

Author: S.Kosov, Ryazan

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