How does a relay work?

I read the pdf

http://stevenengineering.com/pdf/45RELAY_CA2K_CA3K.pdf

For example, a CA4KN-40BW3 has power input (should be 24VDC) and ground for itself. It also have input and output terminals.

If one of the input terminal is ON (high voltage 100VDC, high current 2A), then the corresponding output terminal will be ON. The output terminal will also have 100VDC and 2A current. Then what is the purpose of the relay??

Note: I must have some misunderstanding. Please help.

The relay is performing 'a function' whether it has power applied to the 'coil' (what you referred to a 'power input') or not. It is not like a system which needs continous power to even operate.

In the table refer to the 'Contact Configuration' columns. One is labeled N.O. (Normally Open) the other is N.C. (Normally closed). The 'Normally' refers the the relay when no power is applied to the 'coil'.

If no power is applied to the 'coil' then the 'Normally Closed' contacts pass any power which is applied to them. (The particular model you noted does not have any of these.)

If power is applied to the 'coil' then the internal structure of the relay is pulled toward a magnetic coil. This causes the 'Normally Closed' contacts to open and the 'Normally Open' contacts to close and pass any power which is applied to them. (The particular model you noted has 4 of these contacts.)

Tofo said:

I read the pdf

http://stevenengineering.com/pdf/45RELAY_CA2K_CA3K.pdf

For example, a CA4KN-40BW3 has power input (should be 24VDC) and ground for itself. It also have input and output terminals.

If one of the input terminal is ON (high voltage 100VDC, high current 2A), then the corresponding output terminal will be ON. The output terminal will also have 100VDC and 2A current. Then what is the purpose of the relay??

Note: I must have some misunderstanding. Please help.

Click to expand...

I didn't read the article, but Wikipedia has a pretty extensive description of Relays. Search for "Relay"

This discussion is for the simplest type of relays, those which have only two states determined by power applied to the 'coil'.

There are more complex type of power controllers which do have power applied at all times to work. These will have other input to determine the power control mode.

I think Tofo's misunderstanding is in thinking of these simple relays as needing power applied to the 'coil' to function at all (like a PLC's power for example).

Tofo said:

has power input (should be 24VDC) and ground for itself. It also have input and output terminals.

Click to expand...

actually i don't see a ground terminal and i don't think relays have them, they have auxiliary contacts, some have also power contacts and coil contacts with difference that power contacts, if present, are for slightly higher load than aux contacts. for much higher load there are contactors but the opertaion is the same.