Dead plc why?

Thankyou for the information on the FXon PLC's guys. I do have another question though.

Recently we have had the master PLC die on us it blow internal relays, I was trying to find out why and the only thing I can think of is: when one of the vales it controlles went short circuit it took out the plc at the same time. Is this possible? and if so is there anyway of protecting it from this again? as it currently controlles about 30 valves.

Valves are big culprits for blowing plc outputs. Mitsubishi outputs are rated at only 2 amps.
You should use interface relays where the output only operates low current relays and the valve is then switched through the interface relay.

The PLC can easily be repaired by replacing the internal blown relays. The plc comes apart easily and is in seperate units inside.

The spare relays can be ordered from any electronic retailer.

Normally..pneumatic valve use 24 VDC..(I am not sure you use 220 VAC or not)
You can block by Zener diode too.(Attention MAx..Relay output contact of Mitsu is 2 A)
Want to be sure..please follow Prof.Goody said..using Contactor (Relay that can take more Amp. contect connect to Relay Output (inside plc)...
In case relay inside plc has blown out ..you have to replace it..by
using the same type )
You can find some picture about preventing Mitsu PLC in the manual..

platootod
MechatronicS RIT.Non

Cindy,

The best way to protect your PLC outputs is to isolate the output completely from the controlled device by installing an interposing relay and having the PLC output activate the relay to control the device. Another way of doing this would be to use external fusing on the PLC output circuits. When doing this the external fuse size must be several mA less than the PLC's internal fuse (if there is an internal fuse).

Although best to relay outputs you can get Freewheeling Diodes built into the plug or as little plates that fit between plug and valve. They use LED's so also show status as well. Note: once diodes are fitted the polarity is important. Using a good power supply with fold back auto reset also protects as it is so easy for coils to go down. Or good old fashion fuse. The type that din rail mount are easy to flick open and great for fault finding even if power supply closes down before they blow. A lot quicker to test in inclosure than pulling plugs to see which one is down.

Solenoids and other devices that are prone to inductive kick can cause havoc in PLC systems. It has been my practice to use an interposing relay and break both legs to the solenoid.
So I have a better understanding, did you in fact have a PLC that was damaged or did you have an output card damaged?

Roger

PLC output to solenoid valves.

I recall using Omron PLC in a project with many solenoid valaves, that is working over 4 years without blowing the output cards.
1: I used terminal blocks with embeded free wheelind diodes For the 24 VDC solenoids.
2: PLC relay output selected to meet solenoid electrical characteristic (current and inductance).
3: Each output module was connected to a number of solenoids NOT TO EXCEED the COMMON max amps. (sum of amps of all simultanously energized outputs)
For this I also figured out the solenoids that are never energized simultanously.

I am not sure what size pneumatic solenoid valves you are using but
modern air assisted solenoids only draw around 100mAmps @ 24vdc
so if an output is rated at 2 amps it would not matter if you connected up to 20 of them per output...
Might I also say a that installing an interposing relay has the same chance of shorting and taking out the PLC, as the valve...
Fuse the output if you wish that will give some safety margin.
Good luck with your projects.

David

(Quote
Might I also say a that installing an interposing relay has the same chance of shorting and taking out the PLC, as the valve...

I think that is stretching it a little, I suppose you could say that about the wiring or most anything in a circuit.
I have been working with PLC's form more than twenty years and I have NEVER had an interposing relay take out a PLC. Interposing relays are usually in a safe environvent while valves are not.

Roger

God you guys are good!!

Most of the vales our plc controlls ear solenoid vales, so write on the button guys!. When I can to the job the plc had already been blown up and replaced. They bought a whole plc because they didnt know anything about the internal relays, there is not circuit diagrams of the plant and the mechanical drawings were very and not correct either. I have since drawn all the mechanical and Electrical drawings so we have a record but I didnt get the chance to look at the old plc they blew up. so from your advice I will install relay protection and also din rail terminal fuses to the inputs and outputs. I know that normally the outputs should be ok but I think there is alot of backfeeding going on in the panel too.

Thanks

Cindy

sorry

sorry for all the bad spelling in previuos post

I didn't see if it was ac or dc solenoids. I've had past problems with AC solenoids where the plunger gets trash in it and it sticks and won't operate. During this time the solenoids draw high current. These should be fuse or relay protected from the PLC output. Also, all solenoids should have some form of surge protection because when the relay or output is opened (turned off), a high reverse voltage is produced on the line because of the collapsing magnetic field. A DC solenoid should have a reverse bias diode and an ac sol should have a cap/resistor or MOV suppressor on it. If not, this high voltage pulse can cause all sorts of problems and glitches with the PLC and any other electronics in the panel. In practice, the suppressor should be as close to the sol coil as possible.

Sorry I still need guidance!

Thanks Micah,

Where can I get these components? The problem I have is that I dont know what to specify. We are using 240V ac solenoids and the plc is also running off 240V, this keeps the current low and therefore the bills low too (not my decision).

Thanks Roger,

Same question to you too where can I get interposing relays from what sort of spec will I need and what are they? I know what a relay is and how it works but how is this one different?

Cindy

An "interposing relay" gets its name from the function, not the type of relay. An interposing relay is just a standard relay, usually SPDT (Single Pole Double Throw or Form C contacts) that is interposed or inserted between the PLC output and the load. Any standard plug in relay relay (sometimes called an ice cube relay because of the way they looked years ago) will work if you match the contact rating (10 amps is typical) to the load and match the coil (230 VAC in your case) to the PLC output voltage. I like Turk's product, but Potter Brumfield, Siemens, Finder, Idec and countless others make the same thing. Many have manual overrides and indicating LEDs on the relay, which are a good idea. Many also have plug in modules for arc suppression on the contacts to simplify that process too. For very high loads you can use a machine tool relay, which are incredibly reliable, but they are large and expensive.

Cindy, I am presuming you are in the UK - the 240V ac and Mitsubishi PLC is what I am going on.

The interposing relays I use (except they are termed 'interface relays' in UK) are made by Omron and they are extremely common here.


I will give you the parts No at the end. (Any electrical wholesaler will easily be able to get them for you.)

The benefits of these relays are;

1, They are very low current with a snubber circuit already designed and incorporated within.
2, They have an LED to show when they are actuated
3, They cannot be operated manually (ie sticking a screwdriver in to make the contacts close.
4, They are very slim so that you can fit a lot side by side on din rail.
5, The contacts are rated at 10A - 240V ac
6, They are easy to change (a little flip handle and they are out)
7, the contacts are highly visible so you can see when they need changing.
8, Relatively cheap.
9, Long lasting.

I have used them for years and they are excellent.

Part No's

Relay.......Omron G2R-1-SN 240V ac
Base unit..Omron P2RF-05-E