Formation of the turn-on delay. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Radio amateur designer
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In many devices, in order to switch from one mode of operation to another, it is necessary to form time delays for turning on certain nodes. For the attention of radio amateurs, I offer a simple delay driver that I used in a security alarm system.
When the power is turned on, the discharged capacitor C2 begins to charge through the resistor R2 (see diagram). The voltage drop across the resistor R2 opens the field effect transistor VT1. As a result, the voltage comparator DA1 is in a high output voltage state, regardless of the difference signal at its input.
After charging the capacitor C2, the transistor VT1 closes and does not take part in the further operation of the node. After the transistor closes, the comparator goes into operating mode.
The delay time is determined by the values of the elements C2 and R2. With the ratings indicated on the diagram, the delay is approximately 18 s.
When designing the PCB of the node, it should be borne in mind that the used voltage comparator, like many other op-amps without correction circuits, is prone to self-excitation due to the coupling through parasitic capacitance between the pin 9 circuits and adjacent zero-level correction circuits. These chains should be made as short as possible and as far as possible from one another. The capacitor C1 serves to increase the stability of the comparator.
In the security alarm system, the described node provides a time delay that allows you to leave the protected premises without haste and close the doors until the device switches to armed mode. However, this node can also be useful in other equipment where turn-on transients lead to malfunctions.
Author: V.Grichko, Krasnodar
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