Fan switch with timer
Encyclopedia of radio electronics and electrical engineering / Clocks, timers, relays, load switches
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After 3 ... 5 minutes after turning on the fan, this device turns it off and turns off by itself, no longer consuming energy from the 220 V network. The development is based on A. Vyzhanov's "Simple Time Relay" ("Radio", 2007, No. 6, p. 54). The scheme of the device is shown in fig. 1, and its appearance - in Fig. 2
Fig. 1
When you press the SB1 button, the mains voltage is supplied to the winding of the transformer T1. The second group of contacts of the button closes the charging circuit of the capacitor C1 to the voltage at the output of the rectifier, consisting of a diode bridge and a smoothing capacitor C2. As a result, the transistor VT1 opens, the relays K1 and K2 are triggered
Contacts K 1.1 block the "network" group of contacts of the button, and you can release it, the device will continue to work. Contacts K2.1 close the control electrode circuit of the triac VS1. The triac begins to open in each half-cycle of the mains voltage. through it, an alternating voltage of 220 V is supplied to the fan and the fan works. Thanks to a triac that can withstand current up to 5 A, it can be quite powerful.
After the SB1 button is released, the voltage across the capacitor C1 slowly decreases. But the transistor VT1 remains open and the fan is on until it drops to approximately 1,5 V. The transistor then closes, and the relays break the circuits they held closed. Triac stops opening. The fan and the entire device are turned off until the next press of the SB1 button. If necessary, you can turn on the fan for an indefinite time with switch SA1, without resorting to the "services" of the machine.
The design uses relays RES49 version RS4.569.421-02 (passport RS4.569.425) with a winding resistance of 270 ohms and an operating voltage of 12 V. They can be replaced by other low-power relays - the current switched by their contacts does not exceed a few milliamps.
Transformer T1 should be selected with an alternating voltage on winding II 1 ... 2 V more than the direct voltage of the relay. And the rated voltage of capacitors C1 and C2 should be 1,5 ... 2 times the voltage on this winding. Resistor R2, which limits the charging current of capacitor C1 to a safe value, can be made from a piece of nichrome wire. There is no need to accurately select its resistance.
With the ratings of the elements indicated in the diagram, a fan turn-off delay of about 5 minutes was obtained. By selecting capacitor C1, it can be changed.
Author: A. Subbotin, Zvenigovo, Mari El; Publication: radioradar.net
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