[Flash Relay]The Working Principle Of The Flash Relay How To Wire The Flash Relay

Working Principle and Wiring of a Flash Relay

Working Principle of the Flash Relay

A flash relay is an automatic control device that activates when the input quantity (electricity, magnetism, sound, light, or heat) reaches a predetermined value. When this set value is reached, the output changes, causing the relay to “jump.”

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The relay’s characteristics can be described as follows: when the input signal $x$ increases continuously from zero to the action value $x_x$ , the armature starts to pull in, and the output signal of the relay immediately jumps from $y = 0$ to $y = y_m$ . This means the normally open contact changes from off to on. Once the contact is closed, if the input quantity $x$ continues to increase, the output signal $y$ will no longer change. When the input quantity $x$ drops from a value greater than $x_x$ to $x_f$ , the relay starts to release, and the normally open contact disconnects. This characteristic is called the relay characteristic, also known as the input-output characteristic of the relay. The ratio of the release value $x_f$ to the action value $x_x$ is called the feedback coefficient.

How to Wire a Flash Relay

To wire a flash relay:

L is connected to the light.
B is connected to the power supply.
E is connected to the ground.

Types of Flash Relays

There are three types of flash relays commonly used in automobiles:

Capacitive flasher
Wing flasher
Electronic flasher

Pin Definitions for Flash Relays

The three pins of the flasher are defined as follows:

B: Power terminal, connected to the power supply (+)
L: Flasher control terminal, connected to the lamp (+), and the lamp (-) is connected to the ground
E: Ground terminal, connected to the vehicle body and the power supply (-)

Working Principle of the Electronic Flasher

The electronic flasher uses the switching characteristics of a transistor, along with the charging and discharging delay characteristics of a capacitor, to control the on and off states of the relay coil. This, in turn, connects and disconnects the contacts, causing the turn signal lights to flash.

Electronic flashers are widely used in automotive turn signal systems due to their reliable operation and long service life. They are available in contact and non-contact types, integrated circuits, and transistors.

Detailed Circuit Description

The three pins of the electronic flasher are:

B: Power terminal
L: Flasher control terminal
E: Ground terminal

The charging circuit involves:

The battery’s positive power switch (SW) terminal $B$
Emitter $e$
Base $b$
Capacitor $C$
Resistor $R 3$
Terminal $S$
Turn signal light $K$
Right turn signal lights
Ground (battery negative)

When the car turns to the right:

Turn on the power switch $S W$ and the turn signal switch $K$ .
The current is supplied by the battery’s positive pole to terminal $B$ , resistor $R 1$ , and the normally closed contact $J$ of the relay.
Terminal $S$ , turn signal switch $K$ , and right turn signal grounding complete the loop, causing the right turn signal to light up.

As the current passes through resistor $R 1$ , a voltage drop occurs, turning on the transistor $V T$ due to forward bias. The collector current passes through the relay coil $J$ , causing the normally closed contact of the relay to open, turning off the right turn signal light.

When the transistor $V$ is on, the base current charges capacitor $C$ . As the capacitor charges, the charging current decreases, and the collector current $I_c$ of the transistor also decreases, causing the relay contacts to close again, turning the turn signal lights on. The capacitor $C$ then discharges through resistor $R 2$ , the normally closed contact $J$ of the relay, and resistor $R 3$ . The voltage drop generated by the discharge current on $R 2$ provides a reverse bias voltage for the transistor, accelerating its cutoff. When the discharge current is close to zero, the voltage drop across $R 1$ provides a forward bias for the transistor to turn on again.

This continuous charging and discharging of the capacitor, along with the switching of the transistor, causes the relay contacts to repeatedly open and close, making the turn signal flash.