Functional parts of rp and a devices and conventional symbols used in them

Fig 3.8. Biased Relays



Polarized relays
This kind of relays operates exactly the same as the biased relays. The only difference is that these relays does not have the permanent magnet, instead they have a diode in series to the coil. If the diode is correctly biased, the coil will have power and the armature will be actuated. The difference that makes these two relay types different is that the biased relays will allow the current to flow through it’s coil, even it the relay is reverse-biased! Very important if someone wants to connect the coils of two or more relays in series.
Solid State Relays (SSR)
This is the modern type of relays. These relays does not have a coil, nor any other moving part, that’s why they are called Solid State. They are used for fast switching (up to several hundreds of Hz) and for controlling loads in explosive or harsh environments. They have significantly more lifetime than the conventional relays, as their contacts will not corrode due to humidity, dust or other causes. Actually, they do not have contacts! Instead, a FET or a TRIAC is used to simulate the contacts. The major disadvantage is the price…
Category 3. The contacts
The third and last category is the contacts of the relays.
There are 3 major characteristics that distinguishes the relays:
1. The max voltage: This characteristic is determined by the gap that exists between the contacts, as well as the alloy that the the contact is made of. The higher the gap the higher the voltage that a relay can cut-off.

1. 
   The max current: This characteristic is determined by the thickness of the contacts, as well as the alloy that the the contact is made of. The thicker the contacts the higher the current that a relay can handle.
   
2. 
   The switching frequency: This characteristic is determined by the mechanical construction of the relay. The lighter the construction, the faster the switching.
   
3. 
   The number of contacts:…Just the number of contacts.
   

As far as the contact number is concerned, the relays (like the switches) comes with some kind of coding. The general code form is this:
xPyT
The ‘P’ stands for “POLES”. The ‘x’ is the number of “POLES” that a relay has. Thus, if a relay has 1 contact pair (POLE), the code would be SP as for Single Pole. For two contact pairs, it would be DP as for Double Pole. Above 2 contact pairs, the x gets the number of poles, eg for 3 poles it would be 3P etc etc.
The ‘T’ stands for “THROW” and ‘y’ is the number of “THROWS”. ‘y’ can be Single or Double. Single Throw (ST) means that there is only one NO or NC contact. Double Throw (DT) means that the relay has pairs of NO/NC contact.
Relay Symbols
The relay symbols are unlimited. Each manufacturer can make his own symbol for a specific relay that has different internal connections and characteristics, carrying out a specific task. I will illustrate the most basic types of relays:

Fig 3.9. Relay Symbols