PLC's, power supplies, and wireing

JOLTRON · May 13, 2008

Hello All,

Got a few questions for you guys. I have been programming for a few years now but have not had much hands on with wiring and electrical theory. First couple of questions:
1) If you have 2 plc's running off of the same power supply is it ok to wire discrete communication without isolating them thru relays?
2) How about if the 2 PLC's are running off of separate power supplies, I was told direct discrete is fine if the grounds are tied together?!?!? sounds weird to me.
3) When building a new system what steps are taken to figure out what size power supply you will need?

Thanks everyone,
-joel

cjd1965 · May 13, 2008

1) I would have seperate MCB's between the power supply and the PLC. Cant think of a reason for relays on the power supply of a PLC.

2) What you are told is essentially correct. I would earth bond the 2 systems so that 0 volts is the same across both systems

3)Total up the maximum current, allow for diversity, and multiply for future expansion. I usually goto 150% of what I total up.

Cheers

plchacker · May 13, 2008

Use caution with AC sources. You may get different phases...

mordred · May 13, 2008

cjd1965 said:
1) I would have seperate MCB's between the power supply and the PLC. Cant think of a reason for relays on the power supply of a PLC.

2) What you are told is essentially correct. I would earth bond the 2 systems so that 0 volts is the same across both systems

3)Total up the maximum current, allow for diversity, and multiply for future expansion. I usually goto 150% of what I total up.

Cheers

to add to step three make sure you allow for line length current drop

cjd1965 · May 13, 2008

I agree AC is a problem. I forgot that one because here in UK it is pretty much 24V DC control for all new installations

keithkyll · May 13, 2008

I agree with all of the answers so far. In addition:
The input for a PLC is an optoisolator. It has an LED driving a phototransistor with a piece of glass in between. When you activate a PLC input, all you are doing is lighting an LED. The phototransistor is on the PLC side. The glass gives you 4000 volts of isolation.
All you're doing is shining a flashlight through the window of the PLC CPU inside. Just make sure the light doesn't flicker, and you'll be okay.
When you have optical isolation, there are many things you don't need to worry about.

JOLTRON · May 13, 2008

In question 1) I was wondering if relays were needed for discrete communication from 1 plc to another or if there would be any issues wiring an Output from 1 plc directly into another?

Also in 2) if I tie the earth grounds together of 2 powersupplies and use discrete communication,is it not a problem if 1 power supply puts out around 23 volts and the other 25ish. Trying to understand if this would cause any issues with a system or it would still be happy

As for question 3) and totaling up the total maximum current is there a rule of thumb to I could use to guesstimate what would be needed for proximity switches and solenoids? Just trying to get a rough basis before I start diggin in too deep.

Thanks again,
-joltron

tragically1969 · May 13, 2008

Do you mean an output from one PLC to the input of another, if so there is no need for a relay.

JOLTRON · May 13, 2008

Yes that's what I meant, sorry got a little ahead of myself. I remeber seeing a project in a robotics class where they took an output from one 24vdc item and ran it into another 24vdc input and there was an issue with it. When they measured voltage they had about 48vdc, so they had to isolate them thru relays,

keithkyll · May 13, 2008

Study the subject of sinking and sourcing. Both outputs and inputs. This will explain why they had 48 volts.
Do a search right here on this site.

Proximity switches only use about 10 to 20 mA, or .48 watts each.
Solenoids have a wide range. If everything is 24 VDC, just add up the watts, and size your supply at least 25% more.

JOLTRON · May 13, 2008

I have worked with both sinking(NPN) and sourcing(PNP). I currently work with Siemens 300's setup as sourcing integrated with Motoman robot's that are sinking, and have a pretty good grasp at it, but still can't figure out the 48 volt issue.

keithkyll · May 13, 2008

Sounds like more of a trick question than anything else. Two 24DC supplies, but one of them has + as the common. That's the only way you could get 48DC on the input.
PLC1 has relay output and soucing the -. +24 is wired to common return.
PLC2 has sinking inputs. -24DC is wired to common return (shared between the 2 supplies), and +24DC is wired to input common.

Does anybody tie +24 to ground? I don't think so.

JOLTRON · May 13, 2008

If I remeber correctly the issue I saw wasn't on a 2 plc system, that was just an example for my question. The setup I saw with the issue was a PLC5 and an Adept robot. The adept may be an NPN system.

keithkyll · May 13, 2008

I'll buy that. Many times, designers will use an inverted supply on remote inputs for noise immunity.
For example, Logic is +5, valves and motors are +24 relative to ground.
Remote inputs are 24 DC, but wired internally as 0 to -24DC. That puts noise for all inputs below the zero rail (ground reference).

JOLTRON · May 13, 2008

What do you mean 0 to -24vdc??? I don't understand how you can get -24 volts.

PLC's, power supplies, and wireing

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