The principle of the relay I know. I found some information. Said it is often closed to protect the circuit. How to protect I do not understand this sentence.
The relay is not a small current to control the high current thing. If a 12V control power is connected to the relay, keep the relay coil constant. As long as there is always a current, the relay should not be broken off normally open point.
So the machine is a failure "such as: connected." Often closed point should not be broken into a very open point ah.
what is the problem?
You are not very clear about this problem, but on the question you asked I can say that the working principle of the relay. Not to say that normally closed than usual to open, depending on what circuit you want to switch, try to use the normal state access circuit, for example, you want to switch the circuit for a long time to pass the state, then use normally closed. The principle of the relay you know, mainly rely on the coil to produce a force to pull the coil to pull the spring or shrapnel force to open the contact open (this is often open to say). If you want to say just now, your circuit requires a long pass, if you use normally open, you need to always supply the relay coil (except magnetic latching relay). The spring (shrapnel) is always in the stretched state, the metal is also tired, long spring (shrapnel) may not play or other conditions. If the use of normally closed, through itself is his initial state, so the problem is very much reduced. Have nothing to ask
Simply said: relay in the non-power state, the closure of a pair of contacts is often closed point, disconnect a pair of contacts is often open point, the relay coil voltage, the normally closed, normally open closed to achieve conversion The If the coil is always in the power state, the coil will be hot, of course, the relay itself is allowed to a certain temperature rise, but often when the power plus the ambient temperature, the relay life will be affected, heat for too close to other electronic components, Will be affected. Therefore, in the application should be considered on-off time ratio.
You describe the problem of equipment failure is not clear, you should look at the specific application of reanalysis.
Relay connected to the power, the normally open and closed, normally closed off. Rather than what you said, "as long as there is always a current, the relay should not be broken off the normally open point." Normally open and normally closed when the relay coil is not energized when the state of contact. As for the normally closed connection protection circuit, such as the motor forward and reverse control circuit is not allowed to forward contactor and reverse contact with the situation at the same time. It is necessary to forward contact contactor normally closed auxiliary contact string to reverse contactor coil circuit, the reverse contactor of the normally closed auxiliary contact string to the forward contactor coil circuit. In this way, the forward contact contactor is closed when the auxiliary contact is closed, so that the reverse contactor coil circuit open circuit can not be connected. vice versa. This plays a protective role
