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:
Relay Characteristics
A sample of a label from a relay
The things that characterizes a relay are the followings:
The coil voltage: This is the voltage that the coil can actuate the armature. This value should indicate also if the current is AC or DC
The coil current: This value indicates the current that the coil will draw when it is powered with the indicated coil voltage. Very important characteristic to take into account when designing the driver of the relay. The current that goes through the driver must be enough to actuate the armature.
The off-voltage: This characteristic shows the minimum voltage that the armature is pulled by the electromagnet. If the voltage goes bellow this value, the spring will overcome the magnetic force and the relay will change state.
The number/type of contacts: Is it an SPST? An DPST? DPDT? Or what?
The power for the contacts: This characteristic indicates the maximum power that the contacts can handle. Some manufacturers will use the voltage and the amperes, some others the voltage and the kilo-watts, while some others will indicate all three values.
The operating temperature: The temperature that the relay can operate without problems
Switching frequency: The maximum make-break frequency
The package: Last but not least is the package. Some packages (like the octal type) comes with an appropriate base, while some others are directly soldered/connected to the PCB/electric cabinet.
Well here is the deal. Long long time ago, the switches were like "knife" switches (check this page http://upload.wikimedia.org/wikipedia/commons/4/4b/Open_knife_switch.jpg)
See the similarity of a relay contact? The blade connects to the common, the other end that the blade goes in is the NO contact. So, it was common place to connect the live to the NO contact as safety measures, so that when the switch was off (open) the blade that was exposed would have no power.
BUT, this is not common use an more. Here is the other deal. If you want the relay to switch power between two loads, then the the live should go to common. Say for example that you want to power a traffic light for pedestrians, it can either be GREEN or RED, never both. So you connect the common to live power, the NC to RED and the NO to GREEN. Arm and disarm the relay to turn each of the lights ON or OFF...
So, it has to do with the operation that you want to perform.
At 28 December 2014, 11:15:29 user Pammi wrote: [reply @ Pammi]
Thanks a lot Giorgos..
I am new to working with Relays. Therefore, I have just one more question :-
In various other tutorials on Relays, I saw that LIVE wire was always connected to the 'Common' port of relay, and neutral to NO/NC (via Bulb/Microwave).
Is there any specific reason for that ?
I personally think that even if we connect the LIVE wire to NO/NC, and COM to neutral wire, it does not matter. It is one and the same thing.
But still wanted to have your view on that. Anything related to safety or something ?
At 28 December 2014, 10:20:19 user Pammi wrote: [reply @ Pammi]
This is so clear. Thanks !
I have a small Doubt: Why is the microwave (or any other AC component) is connected to power via NO/NC? Can we not the AC component to power via Common port of the relay ? Will there be any issue?
At 7 November 2014, 17:33:08 user mahdi wrote: [reply @ mahdi]
its the best explain about work relay.TANKS
At 18 October 2014, 11:01:45 user sri wrote: [reply @ sri]
will u please tell me brief about armature.
At 18 October 2014, 10:59:11 user sri wrote: [reply @ sri]
Thank you very much. It is good and the practical which u have shown is excellent and very understandable to me.
Hi, I have a usb relay current - Up to 16Amp @24vdc or 250vac, I wanted make it work like a light switch to turn on a light on my front door! How would i wire it? Thank you in advance!
explaniation is very good.. keep going on... thanks a lot...from hereafter i can do the mini projects in my home... this website very useful for all... thanks alottt....
I just want to say. THANK YOU. I've been researching about relays for 5 hours straight only to be left confused. You pretty much showed it in such an easy way and EVEN STATED that their are different kinds and purposes...no wonder i'm god damn confused the whole time
very useful information ! thanks a lot, you really cleared my doubts regading relay opertaion. now i can do my home automation project without any fear of damaging my arduino.,
At 11 July 2013, 13:06:14 user owen wrote: [reply @ owen]
thanks i found it very virtal for i was confused on how i can make an automatic gate closer. If u do mind please asist me wth more illumination to this e-mail okamphata@hotmail.com thank u pipo
Hi I have too signals to be brought back using relay? or relays? one is 24vdc for a plc signal and the other is volt free for a telemetry alarm how can this be done?
Can you please explain me what is meant by the "Potential free relays".
As i want to integrate my device with the SCADA system they asked me to provide signal to SCADA device for 5 sec( means i want to complete there circuit using a potential relay) with the help of potential relays only.
Or can i use any kind of relay to complete there circuit.
your explanation really help me understand what is relay about, I want to unerstand it from long time ago, but I just can't find other source that can help me understand easily like this!
i am a lecturer in a polytechnic. Now i can explain this relay topic in such a easy way to the students. i trust that every student can understand this topic about relay.
dayalraj
really i like this of explanation.if we are reading in book.we can,t understand fully.sometimes it will be confusing.if we read the operations of instruments on animation,it will be usefull.so you have a good sense.please continue this.
At 7 September 2011, 8:39:39 user sushil wrote: [reply @ sushil]
Really nice explanation.One of the simplest explanations for understanding relays
Thank you for the serail flow for the process in detail......
At 28 June 2010, 19:01:31 user ravi wrote: [reply @ ravi]
i like the way you discuss i really thankful that i got such a gud explanation. but i would like to request to discuss more on induction motor for industrial application.