Do you really understand relays?

1、 Brief introduction to relay

Relay （ English name ：relay） It's an electrical control device , It's when the input is （ Incentive quantity ） When the change of the system meets the specified requirements , An electrical device in an electrical output circuit that causes a predetermined step change in the controlled quantity .

As a control element , Sum up , The relay has the following functions ：

1. Expand the scope of control ： for example , When the control signal of multi contact relay reaches a certain value , It can be in different forms of contact groups , Simultaneous connection 、 Disconnect 、 Turn on the multiplex circuit .

2. Zoom in ： for example , Sensitive relay 、 Intermediate relay, etc , With a very small amount of control , A circuit that can control a large amount of power .

3. Integrated signal ： for example , When multiple control signals are input to the multi winding relay in the specified form , After comparison and synthesis , Achieve the predetermined control effect .

4. Automatically 、 remote control 、 monitoring ： for example , The relay on the automatic device works with other electrical appliances , It can form a program control circuit , So as to realize automatic operation .

2、 Relay principle

The relay consists of a moving contact B And two static contacts A and C.

When it's normal , Moving contact B Static contact and static contact A Contact , That is, it is in a closed state , It is called normally closed contact ; When the coil is energized , Its moving contact B Static contact and static contact A Immediately disconnect and contact the static contact C closed , Cut off the static contact A Control circuit , Contact static contact C Control circuit of .

It can be seen from the control principle of electromagnetic relay , There is no difference between positive and negative relay coil voltage , Because both forward and reverse currents , The coil will generate suction .

3、 Schematic example

Example 1

This figure shows the double switch relay , In fact, only one . When below switch1 Output high level , Triode conduction , Thus, the relay also turns on , Turn the switch over .

Example 2

left P13 Output high level , Triode conduction , Thus the relay turns on .

4、 Question and answer

1、 Why should a diode be connected in parallel to the relay coil ？

explain 1：

Diode acts as freewheeling . say concretely , When the relay coil is de energized , A self induced electromotive force is generated at both ends of the coil , This self induced potential can cause damage , A diode is connected in reverse parallel at both ends of the relay coil , A discharge current circuit is provided for the self induced electromotive force （ Afterflow ）, And then play a protective role , Functionally, this diode is called freewheeling diode .

explain 2：

When the relay coil is de energized , The residual energy must be released in a suitable way . If there is no diode , Then the energy is released in the form of sparks , The electronic switch is badly damaged , The mechanical switch will be obviously damaged after a long time . After the diode is connected in parallel with the relay coil , The negative pole of the diode is connected to the positive pole of the DC power supply , When the relay coil is de energized , The diode makes good use of the situation , Provide a way for the coil current to continue to flow , The residual energy is slowly consumed in the circuit composed of coil and diode , The switch is effectively protected .

explain 3：

The wire package of the relay is equivalent to an inductance , The characteristic of inductance is that the current cannot change suddenly , When the relay is switched off, the current of the wire package generates a high voltage on the switching device , It will break the closing device . This diode provides a circuit for the current of the wire package , Gradually reduce the current of the wire package , The closing device is protected .

2、 Why do parallel diodes have fast switching speed ？

Because the unidirectional conductivity of ordinary diodes depends on P Type C semiconductors and N Type semiconductor contact PN junction , Due to the existence of junction capacitance , The reaction time is not too short , The moment when the switch is off , The diode hasn't had time to turn on yet , It is equivalent to no diode .

So we need to choose Schottky diode for diode （ Also known as Schottky barrier diode ,Schottky Barrier Diode,SBD） The unidirectional conductivity of is formed by the contact between metal and semiconductor , It is characterized by fast switching speed .

3、 Electromagnetic relay and solid state relay （SSR） What's the difference? ？

Solid state relays are widely used in digital program control devices 、 Micromotor control 、 Temperature regulating device 、 Data processing system and computer terminal interface circuit , Especially suitable for frequent movements 、 Explosion proof and moisture resistant 、 Corrosion resistance and other special occasions .

Solid state relays have high life 、 Highly reliable 、 High sensitivity 、 Low control power , Good electromagnetic compatibility , Fast conversion , Small electromagnetic interference ;

Disadvantages of solid state relays ： The pressure drop of the pipe after conduction is large , Generally, the on resistance of power FET is larger than that of mechanical contact .