Delay switch circuit designed by LM431

Probe domestic switch needle KG-300K, needle head diameter is 3.0mm, normally open switch needle

Tantalum capacitor

The following is the circuit diagram of a delay switch circuit designed using LM431.

Delay Switch Circuit Using LM431

This circuit uses the LM431 to create a time-delay function. The key components are R1 and C1, which together form a timing network. For a 1-second delay, it's recommended to use a tantalum capacitor that can operate within a wide temperature range, from -55°C to 125°C.

Upon power-up, the input of the LM431 is at a low level, causing its output to rise to approximately 23 volts. This turns on transistors Q1 and Q2, allowing current to flow through the ignition coil for a short period—around 24 amps. After one second, the voltage at the LM431’s input increases to 2.5 volts, reducing the output voltage to about 2 volts.

To ensure proper operation, a Zener diode (Z2) with a breakdown voltage of 6.2V should be connected in series with the LM431 output. This allows Q1 and Q2 to turn off, de-energizing the ignition coil. The ignition coil has an inductive load with a DC resistance of about 1 ohm, and it operates on a 24-volt battery. The measured current during operation is approximately 25 amps.

This circuit is designed to function reliably in a wide operating temperature range, from -40°C to 121°C, making it suitable for automotive electronics applications where environmental conditions can vary significantly.

(Editor: Circuit Diagram)