Power supply with timer. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Power supply with timer. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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The main difference between this power supply and the "typical" network adapter is that it can be disconnected from the network after some time after being turned on. This time is set smoothly (with a variable resistor) in the range from 10 minutes to 2 hours. The unit can be used where it is necessary to turn off some battery-powered equipment powered by a network adapter after a while, which does not require great installation accuracy. For example, many people like to fall asleep to music or listening to the radio. To prevent the receiver from ringing all night, it can be turned off with this timer. The diagram of a "typical" network adapter is shown in fig. 1.

Power supply with timer
Fig. 1

Such sources for battery-powered equipment are most often commercially available. There is nothing here but a transformer and a rectifier with a smoothing capacitor. Of course, it is desirable to modify such an adapter, make an adjustable stabilizer on KR142EN12 or another microcircuit. But this article is about something else.

Schematic diagram of the adapter, supplemented by a timer, is shown in fig. 2.

Power supply with timer
Ris.2

In the break of one of the conductors of the primary winding of the transformer, the starting switch S1 and the contacts of the relay K1 are connected.

If you are not going to use the timer - turn on the S1 and use the adapter as usual. To use the timer, you need to plug the adapter into the network as usual, then turn the S1 on and off. After that, the countdown will begin, and then, after its expiration, the adapter will turn off completely. Accordingly, the load will also turn off.

When the power is turned on, the counter D2 is forced to reset.

Zero from its 14th digit (Q14) is inverted by inverters D1.5-D1.6 and the high-level voltage from their outputs opens the transistor VT1, relay K1 closes the contacts. Now if you turn off S1, the voltage from the network will pass through the relay contacts.

From the moment of switching on, the countdown of the time set by the variable resistor R3 begins. The pulses from the output of the generator D1.1-D1.2 (their frequency depends on R3) are counted by the counter D2. After some time, which can be from 10 to 120 minutes, depending on the resistance R3, a unit appears at the output Q4 of the counter, and, therefore, the outputs D1.5-D1.6 will be zero. The transistor will turn off and the relay will disconnect the adapter from the mains.

An indicator is made on the remaining two elements of the CD4069A chip - the HL1 LED blinks, and its blinking frequency depends on the set time (the longer the time, the lower the blinking frequency).

Electromagnetic relay TRG5-9VDC-FA-CL, in a miniature case measuring only 12x8x10 mm (such relays are used in some computer modems). Relay contacts are rated for current up to 0,25 A at 220 V.

Relay winding for 9 V. You can use another miniature electromagnetic relay with parameters no worse than these.

The CD4069A chip can be replaced with another xx4069x or domestic K561LN2. You can also use the CD4049, but it has a 16-pin package and a different pinout. The 14-bit counter NJM4020B is a complete analogue of the domestic K561IE16A. You can use CD4020 or any other IC xxx4020, as well as counters K1561IE16, EKR561IE16 and other analogues.

The adjustment consists in marking the scale around the handle put on the resistor R3, in units of time.

Literature

Radioconstructor 2008 №1

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