Designing an electronic circuit that introduces a power delay after the switch is turned on can be quite useful in many applications. This type of circuit allows the system to wait for a specific period before turning on or off again, which is especially important when dealing with sensitive components or ensuring stability. If you want to avoid using a relay, a transistor-based solution can effectively delay the DC voltage turn-on.
In some cases, it's essential to control when the power is applied to a device. For example, in audio amplifiers, it’s common to wait until the amplifier has stabilized before powering up the speakers. Similarly, in other systems, you might need to ensure that the power is applied only after certain conditions are met, such as when a circuit has reached its operating voltage.
Figure 1 shows a typical delay circuit using a 555 timer IC as the time base. This circuit drives a relay, creating a delay after the power switch is turned on. The delay time is determined by the values of resistor R and capacitor C. When R is 100kΩ and C is 10μF, the delay is approximately 1.1 seconds. This setup is ideal for applications where a mechanical relay is acceptable.
However, if you prefer not to use a relay, a transistor-based design can serve the same purpose. Figure 2 illustrates such a circuit. Here, a PNP transistor is used to control the output voltage, introducing a delay without the need for a relay. This approach is more compact and often more cost-effective.
When the switch K is connected to the +12V power supply, the NE555 monostable circuit activates, causing the output at pin 3 to go high. In the first circuit, this triggers the relay, connecting the load to the power supply. In the second circuit, the PNP transistor turns on, allowing the output to connect to the +12V supply and power the external load.
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