Power SuppliesSecurity & Saftey

Electronic Power-On Delay Schematic Circuit Diagram

Power Supply and Delay Circuit Design:

Electronic power-on delay: The delay circuit’s power source is directly derived from the mains through a bridge rectifier, D1-D4. It is essential that the rectifier can handle currents of up to 1 A. To ensure safety, resistor R1 is incorporated to facilitate the rapid discharge of capacitor C1 when the mains power is switched off. For safety compliance, the resistor should be rated at 250 Va.c. or 400 Vd.c.

Voltage Stabilization with Zener Diodes:

Zenzer diodes D6 and D7 play a crucial role in maintaining a constant 24 V output from the rectifier. The use of two Zener diodes contributes to the stability of the voltage, ensuring a steady 12 V for IC1. This configuration serves to stabilize the rectifier output, and the voltage is further smoothed by capacitors C2 and C3.

Electronic power-on delay

Circuit Timing Mechanism and Relay Activation:

The circuit’s timing mechanism is synchronized with the mains frequency, where the clock signal is rectified by D5 and extracted from junctions R2-R3 at an approximately 11 V level. This signal is then directed to pin 10 of IC1. Following 2–10 clock pulses, the Q11 output goes high. If contact 2 of DIP switch S1 is closed, the elevated signal (12 V) is sent to the gate of T1, an n-channel VMOS FET, initiating transistor conduction and consequently energizing the relay.

Configurable Delays and Circuit Initialization:

In scenarios where all contacts of S1 are open, as depicted in the diagram, a T1 gate is continually linked to the positive supply rail. This arrangement ensures that upon power-up, the clock input of IC1 is low through D9, disabling the counter. Delays, selectable via the DIP switch, are detailed in the accompanying table. The DIP switch can be substituted with a binary-coded decimal (thumb-wheel) switch.

Safety Measures and Current Regulations:

Upon power-up, the IC is momentarily constrained by a pulse, limited to a safe value by D8. Subsequent count cycles always commence with the IC in the zero state. D10 serves as a freewheeling device to suppress voltage peaks resulting from relay coil current switching off. The circuit’s current consumption is primarily determined by C1, which is approximately 30 mA in the diagram.

Safety Considerations for Mains Connection:

Due to its direct connection to the mains, caution is paramount when dealing with this circuit. Testing should be conducted using an isolating transformer, and the main carrying parts must be inaccessible to prevent accidental contact.

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