We know that the contact relay should be more cruel than the general Mosfet, the relay load is much larger than Mosfet.
Common DC load DC motor, DC clutch and DC solenoid valves, these perceptual load switch is closed, hundreds or even thousands of volts of back electromotive force caused by the surge will contact life to reduce or even completely damaged. Of course if the current is small, such as when near 1a, back electromotive force will cause arc discharge discharge will lead to pollution contact of metal oxides, which led to the failure of the contact, the contact resistance becomes large.
Here to mention, the relay is always fail, we do protection, mainly want to extend the using time of the relay, because contacts are always will carbon deposition, aging, its surface as originally as clean. The contact resistance will increase rapidly when the relay life is near the end of the relay.
General under normal temperature and pressure, air key dielectric breakdown voltage is 200 ~ 300V. so our goal in general is to control the voltage below 200V or smaller voltage.
Standard diode can significantly extend the dropout time, conventional diode and zener diode series and not too much influence on reverse time. If it is inductive load. When the contacts apart, longer reverse time longer arcing time, and will shorten the service life of the contact. For example, a relay connected to the diode on a coil requires 9.8ms time to release the contacts. The zener diode and small signal diode together, it can be shortened to 1.9ms. The coil is not connected to the diode reverse time relay 1.5ms.
Although the inductive load is difficult to handle than resistive load, the use of good protection will make the performance better.
There are two ways to be very bad, do not use the.
Under DC load, high frequency switch can cause abnormal high corrosion (electric spark).
When the higher frequency control DC solenoid valve or clutch, contacts may occur blue-green corrosion. The reasons for this situation is when generated by EDM (electrical arc discharge), nitrogen and oxygen in the air in the reaction generated.
Material transfer phenomenon
When the material is in contact, the transition occurs when the contact part is melted or damaged. With the transfer of the passage of time, and even the phenomenon will appear below. After a period of time, the uneven contacts will stick together.
The load usually occurs in large current (capacitive and inductive) inrush current, the arc generated will cause sticking phenomenon.
For sticky and only two strategies:
Contact protection circuit and material transfer material such as silver, tin oxide, silver tungsten or AgCu in the use of contacts.
Generally the concave shape appears in the cathode, the convex shape appears in the anode.
Post time: Jun-03-2020