relay usage

[Guest kyle]

I always got confused of when to use a relay, I know it’s an electromechanical device that opens or closes contacts when a current is passed through a coil. Some people say that you need a relay for this project other say you don’t need a relay I guess when do we usually need a relay can any one give me a clear example Thanks

[panic mode 246]

Well, you might want to use relay when dealing with different voltages, different circuits or when load exceeds the output rating of your device. 1. different voltages Let's say you have push button wired into 24VDC circuit but the device to be turned on is something runing on 120VAC. You can use relay with 24VDC coil and contacts rated for AC and current of your load. You can also do it the other way arround with 120VAC pushbutton, 120VAC relay coil and relay contacts as part of 24VDC circuit (PLC or VFD input for example). 2. different circuits Maybe both circuits are 24VDC (just an example) but you don't want to connect them because they are different polarity (PNP/NPN) or have separate grounds so you want to keep them independant. Then you use relays to pass signal from one circuit to another and keep them isolated. 3. different rating Maybe the current of one device doesn't meet the load requirement. For example, your PLC has 24VDC, 2A max. outputs but you need to power device that requires 5A or 25A. Use relay or your PLC output might get damaged. And of course, yo can have combination of above cases like when one output (push button, PLC output etc) needs to control couple of things. Just put relay that has enough properly rated contacts and smile...

[Guest Kum]

Hi, I myself also NOT so familiar with the purpose of the relay. After I read through the explanation provided by 'Panic Mode' then now I am a bit clearer to the 'picture' of the relay. However, I still got some doubts and hope you guys can help me out on these. First, as mentioned that relay is used when using 'different circuits' like pnp to npn connection. But then I still can't really imagine of how is it done. Can someone explain slightly more on this and how was it done? Sorry... Also, relay is used when the output load doesn't match the load. An example is given by mr. 'Panic Mode' also..... mentioning that a plc with the output 24V and 2A maximum output. When connecting to the output that need 5A or 25A we need to use the relay ELSE the plc might can damaged. My confusion here is the output needed is 5A or 25A and our plc is giving out 2A only. Then, how can the plc been damaged by the output. In my mind is, the most is the output 'cannot be turned on' only as the supplying from the plc is less than the 5 or 25A. Then, how can the output damage the plc? I hope 'someone' out there or mr 'Panic Mode' still can "patiently" explaining to me as I really 'don't know' and I 'don't want' to act like I know... :-p Thank you very MUCH...

[panic mode 246]

It is quite common to get PLC with relay or transistor outputs and it is quite common that outputs are grouped (4 or 8 outputs for example have one common point like in attached picture where I show group of four outputs). Max current for each output is limited (0.5A or 1A or 2A for example). However you still have to make sure that total current doesn't exceed rating of common terminal. For example even if you have group of say 16 outputs and each is rated 2A for example you might have limitation of only max 6 or 8 Amp per common. This means that while you sure can have one or two utputs drawing a bit more than the rest, don't exadurate (16x2A=32A!!!). PLC is primarily CONTROL device (it's the brains - not the musscle). If you need to turn on load that is using 600V 3phases 100Amp you MUST put SOMETHING between PLC and load. That something is a special version of relay called contactor (most remarkable contacot feature is ability to switch high loads eg. high currents in circuits with higher voltage). PLC is only sending signal when to turn the load on. The current of the PLC output driving this contactor (or big relay if you want to call it that way) is probably going to be in 200-300mA range (0.2-0.3A) which is easily managed by any digital output. See attached picture for similar example. PLC doesn't SEND current of 2A. PLC simply closes circuit and allows current to flow. If the current exceeds rating of the circuit, something will melt (connecting wires, PLC terminals, PLC switching element which is relay or transistor for example etc.). Current in circuit can be calculated using Ohms law. If the resistance of PLC output is 0.1 Ohm and resistance of connecting wires is 0.2 Ohm and the resistance of the load is 2.7 Ohm, total resistance of the circuit is only 3.0 Ohm (0.1 + 0.2 + 2.7=3.0). If the used power supply is 24VDC for example (industrial standard), current flowing through this circuit would be according to Ohms law 24V / 3Ohm=8Amp. Although common terminal of the group of outputs might handle it, it is unlikely that the actual output which is rated only 2A would survive. PLC card would probably continue to work except one blown output. Now do you think customer will be ok with few bad outputs if you still have couple of spares? Would you like to have a Laptop with 10-15% of dead pixels or socks with only few holes? Edited 5 May 2004 by panic mode

[Guest Kum]

Dear 'sir mr. Panic Mode', First of all.... very very much thank you for your detail's explanation. I had been reading about this article for a few times already. I now got a picture in my mind. The 'picture' is like this..... Relay or Contactor must be use as the 'medium' between the plc and the say, output with high power rating. Then, the PLC simply just "turn on" the relay/contactor and the relay/contactor will 'close' and the current will flow through the output. So, the rating of the relay/contactor must NOT be exceed by the actual output rating in such a way that the relay/contactor won't be burn, is it so? Hence, indirectly the relay/contactor serve as a 'safety' purpose to protect the plc from damage by 'directly' connecting to the output, is it right also? :) Hope I really get your idea...... Really, really very much Thank You to you, sir Panic Mode... Regards,

[JimRowell 1]

That has a nice ring to it. Sir Panic Mode.

[Sleepy Wombat 27]

Good job Panic mode (sorry) Sir Panic Mode :-p Since you have been knighted mabey your avatar should be....