Power-On Time Delay Relay

Description: Here's a power-on time delay relay circuit that takes advantage of the emitter/base breakdown voltage of an ordinary bi-polar transistor. The reverse connected emitter/base junction of a 2N3904 transistor is used as an 8 volt zener diode which creates a higher turn-on voltage for the Darlington connected transistor pair. Most any bi-polar transistor may be used, but the zener voltage will vary from about 6 to 9 volts depending on the particular transistor used.

The circuit operates by utilizing the breakdown voltage characteristics of a bi-polar junction transistor (BJT), specifically the 2N3904. When reverse-biased, the emitter-base junction of this transistor behaves similarly to a zener diode, allowing for a controlled voltage reference. In this configuration, the breakdown voltage is approximately 8 volts, which is critical for establishing the turn-on threshold for the Darlington pair, consisting of two transistors that amplify the input current significantly.

The power-on delay is achieved through the charging of a capacitor connected to the base of the first transistor in the Darlington pair. Initially, when power is applied, the capacitor begins to charge through a resistor. The time constant of this RC (resistor-capacitor) network determines how quickly the capacitor charges to the zener voltage level. Once the voltage across the capacitor exceeds the breakdown voltage of the emitter-base junction, the Darlington pair turns on, energizing the relay.

The relay can then activate a secondary circuit, controlling higher current loads. The choice of the resistor and capacitor values is crucial in adjusting the delay time; larger capacitance or resistance will result in a longer delay. Additionally, while the 2N3904 is a common choice, other NPN transistors may be substituted, but the specific zener breakdown voltage will vary, necessitating recalibration of the timing components.

In summary, this power-on time delay relay circuit effectively uses the characteristics of a bi-polar transistor to create a simple yet reliable delay mechanism, suitable for various applications where delayed activation is required.Here`s a power-on time delay relay circuit that takes advantage of the emitter/base breakdown voltage of an ordinary bi-polar transistor. The reverse connected emitter/base junction of a 2N3904 transistor is used as an 8 volt zener diode which creates a higher turn-on voltage for the Darlington connected transistor pair.

Most any bi-polar transistor may be used, but the zener voltage will vary from about 6 to 9 volts depending on the particular transistor used.