Isolating relays on PLC output?

[bce123]

DBIGBY, wow you are lost! relays are used to isolate the outputs of the plc. lets say you have 24 v outputs and want to switch a 120 v load you would use a relay or if you had a 5 amp rated output and needed to switch a 10 amp load you could use a relay......
Get the Idea?

 

[[email protected]]

To turn on a relay takes magnetomotive force=

B=magnetic flux density x area perpendicular to the surface.

Magnetomotive Force (F or mmf) - The magnetic potential difference between any two points. Analogous to voltage in electrical circuits. That which tends to produce a magnetic field. Commonly produced by a current flowing through a coil of wire. Measured in Gilberts (C.G.S.) or Ampere Turns (S.I.).

 

[bce123]

to turn on a relay takes magnetomotive force=
b=magnetic flux density x area perpendicular to the surface.

Magnetomotive force (f or mmf) - the magnetic potential difference between any two points. Analogous to voltage in electrical circuits. That which tends to produce a magnetic field. Commonly produced by a current flowing through a coil of wire. Measured in gilberts (c.g.s.) or ampere turns (s.i.).

and your point is?

 

[[email protected]]

I meant to ask why would it take a 120 volt ac card to drive a 120 volt relay, if it just takes current to cause the magnetic field to move the armature.

 

[bce123]

I meant to ask why would it take a 120 volt ac card to drive a 120 volt relay, if it just takes current to cause the magnetic field to move the armature.

YOU HAVE TO APPLY VOLTAGE TO IT SOME HOW UNLESS YOU HAVE INVENTED A WIRELESS WAY TO APPLY THE CURRENT! the intent of an isolation relay is to isolate volts or amps... from the plc to the end use device

 

[Rube]

1. If you have a 24V load, you send it 24V from an output card.

Example: If you have a 24V coil in your relay, you send it 24V when you want it to close it's NO contacts (or open it's NC contacts).

2. If you have a 120V load, you send it 120V from an output card.

Example: If you have a 120V coil in your relay, you send it 120V when you want it to close it's NO contacts (or open it's NC contacts).

3. If you have a 120V load but only 24V available at your output card, you install a relay with a 24V coil to the output card. Then you wire your 120V load through the relay.

4. If you have a 24V load but only 120V available at your output card, you install a relay with a 120V coil to the output card. Then you wire your 24V load through the relay.

If you send 24V to a 120V load, good luck getting the desired results.
If you send 120V to a 24V load, good luck getting a new job.

 

[yam785]

I usually put in as much protection, as the customer wants to pay for.

 

[[email protected]]

Thanks, a lot it clears up some confusion.

 

[Rube]

Good deal---time well spent then...! Makes us smile.

 

[iant]

what about an analogue input to a solid state relay output - should work

 

[OkiePC]

what about an analogue input to a solid state relay output - should work

I did that once. I even sorta PWM'd it in school in '96. I programmed the SIN function and then played with making it conditional and trended the scan time.

It was hooked up to a heater, so I just made my SIN magnitude larger to get more heat from the PID instruction.

Later I used what I learned to replace hand calculated angle set up on a beam-shear to a simple condition trig routine.


Here's how I decide whether to use interfacing relays when the PLC already meets the requirement and I have room:

I just ask myself:
How much money comes out of my pocket and how many screws do I have to turn to swap it out?

If it's my money, I use them. Other peoples, money, I go with the old saying "A Lazy Maintenance Tech is a Good Maintenace Tech".

A co-worker explained that by saying, "He will apply himself to making the machine reliable, and minimize downtime".

That's probably why I am a big believer in diodes and quencharcs...cheap and maintenance free.

 

[iant]

i know your right - and agree with the original comments - of why would you.

but the idea is their

 

[OkiePC]

I did once connected a discrete interlock between two machines which had a spare analog and no spare digital I/O. Later I used the LIM to send four different status to the upstream machine with a 0-10vdc.

 

[iant]

not bad for out of the square thinking

 

[abolfazl]

Relays should be used to optimize