Control Relay Ciruits Learning Kit

[techzone12]

I am an ME, and I work in the industrial automation industry. I am trying to learn Control Circuits Design, and I don't know exactly how to approach this.
I can grasp the theory without much effort, but I need more practical knowledge.

What I am trying to do is build some circuits, and experiment with them. Primarily I would like to focus on the traditional electromechanical relay circuits (rather that the digital ones).

I was wondering if someone can suggest an educational/training kit (accompanied with instructions) and/or a book that would get me started on this.
I realize that there is no substitute for the OJT, but I am just looking to get started here.

I had no luck looking on google. I would appreciate any advice!

Thanks

 

[Skogsgurra]

I think that relay control circuits are regarded obsolete and that is why you cannot find any training kits. But, if you want some practical training in ladder logic (which is the solid state version of relay logic) then try the Soemens LOGO! (yes, the "!" is part of the name). You can buy it at a very low cost and there is an editor runniing on a PC so that you can construct your circuits and also run them in a simulator before downloading to the PLC and testing out in real life.

The LOGO! can also work with logic diagrams, so when you feel that you know about relays (ladder), you can easily switch to logic diagrams.

The whole thing won't cost you more than 100 - 200 USD. Depending on what your relation to the vender is.

Gunnar Englund

http://www.gke.org

 

[itsmoked]

Skogs is correct. No one wants to have to use relays much these days. A PLC is much more flexible and powerful and probably cheaper when you exceed something like 5 relays.

Do read the recent post:

thread248-123418

Oh and skogs prolly ment to say "Siemens" not "Soemens" yuk yuk. His *I* key must be too close to his *O* key.

 

[Skogsgurra]

Right, Smoked. Had also had myself a night-cap... Like "prolly" - never heard that before. Useful when fingertrouble strikes.

Gunnar Englund

http://www.gke.org

 

[techzone12]

I am not sure if I can agree on the fact that "relay control circuits are regarded obsolete". I still see them a lot. I know that the PLC has replaced a lot of the functionality that used to be accomplished by relays. But the relays are still there. This is especially true in safety circuits, where you want something directly hardwired.

My problem is that I never get the chance to do the wiring myself. The control panle is usaully worked on by the techs.

Ideally I would like to buy a PLC and do a very simple project, with all the hardware required (including sensors, switches and relays, etc). Even the most primitive project can turn out to be highly educational.

I know I need to start somewhere. And I am hoping that this discussion will give me some hints.

Thank you for your help.

 

[itsmoked]

um you read that thread I listed? Had basically your question with a lot of sugestions... Any thing ELSE specific we can help with?

 

[zonecontroller]

Wiring up a small relay based circuit is a great way to grasp the basics before realising the benefit of moving on to using miniature PLCs and the like.

If you start out by specifying what small automation project you want to create, then that will get you ready to select components, figure out a circuit and then actually build, wire and test that circuit.

Actual steps involved:-
1. Define relay circuit purpose. eg open/close garage door, start/stop heating system, etc.
2. Create circuit diagram. Power supply, circuit breaker, relays, switches, etc.
3. Find a supplier. Local electronics, store, online, etc.
4. Wire up and test.

Agree with previous comments, beyond a few relays, and it's really worth investing in a mini PLC. Understanding ladder logic, and how it in effect soft-wires, what you would create in hard-wire, is a real bonus.

There are alternatives to the LOGO!, namely, Zelio, Mitsubishi Alpha, Crouzet Millenium, etc, and some of these devices offer both analogue as well as digital control.

It's best to start with an extra low voltage circuit, say a 24V DC supply for safety reasons too, in case a wiring error destroys a component!

If you want any more info then post.

 

[Skogsgurra]

techzone,

You come here for advice. And you have certainly come to the right place. We have given you advice. And you do not accept what we give you. Where do we go from here?

Your observation that you still see a lot of them (relays, that is) around is correct. But that does not mean that the PLC training - especially the ladder diagram training - is useless. You will not be able to pick up anything but the crudest AND, OR, LATCH and NOT functions from fumbling around with relays and wires. And that is so elementary that we can cover it here:

A relay (or a contactor) is a device that is controlled by a winding around a magnetic core. If the winding is connected to a voltage that is of the right type (AC or DC) and the correct magnitude (the pull in voltage), the core is magnetised and pulls in an armature that actuates a set of contacts.
The contacts may be normally open (NO) or normally closed (NC). An NO is open when coil not energised and closes (lets current through) when coil is energised. The NC does the opposite.
Connecting a voltage source and a contact and a lamp (or another relay coil, or a motor, or any load) makes a simple control circuit. Apply voltage to coil - lamp lights up.
If you use an NC contact, the operation will be the opposite; lamp lights when coil not energised and goes out when voltage applied to coil. Of course, when coil voltage is removed, the coil drops out and contacts go back to their initial state. Not much to it. Is it?
Well, yes. To high a coil voltage will start a fire. So will DC applied to an AC coil. But that is trivial, I would say.

Connecting two contacts (say contact K and L) in series means that both contacts have to be closed before any current can flow. The need is that K AND L are closed to have a complete circuit. Series connection = AND function.

Connecting in parallel (K and L again) means that either K OR L needs to be closed to have a circuit. Parallel connection = OR function.

Using an NC contact means that you already have a closed circuit as long as you do NOT activate the coil. An NC contact = NOT function.

There are two basic ways of controlling a circuit; Push-button and Switch. We already covered the Switch method above. Switch closed = relay pulls in. Switch open = relay drops out.
The other method is very common. A button is pushed to start something (i.e. pull in a relay) and another one is pushed to stop it.
We need to do something so that the relay "remembers" that we pushed that Start button. A very easy way is to connect an NO contact on the relay in parallel to the Start button. Press the Start, relay pulls in, closes NO contact and creates a parallel path. Let go the Start button - relay contact serves as a parallel path and keeps relay energised. It will stay so until supply voltage goes out. This is called a LATCHING circuit.
To stop same circuit? Just wire a Stop button with an NC contact in series with the coil. Press Stop to interrupt current to coil. Relay drops out and opens NO contact parallel to Start button.


This is about all you will learn from using relays and hard-wire them. Everything after this is refinements and application specific knowledge. Nothing that you couldn't learn much quicker by using a little PLC with simulation capabilities.

Of course, there are subtleties like special contacts (close-before-break, impulse contacts and so on) and special relays with several windings for different purposes or delay arrangements using capacitors or diodes or even pneumatic devices, but that is something that you will pick up once you need it. It is not something that you are likely to learn from a basic relay learning kit. Actually, I can't remember ever having seen such a thing (my memories go back to late 1950) and the reason is probably that it all is so self-evident that it needs no training kit. A few mistakes on the job and you have learned what you need to know - AKA "job training" or "learn by doing".

Good luck. I hope that I have been very clear.


Gunnar Englund

http://www.gke.org

 

[itsmoked]

Relay training site...

http://www.bcae1.com/relays.htm

 

[Skogsgurra]

Grrreat, Smoked.

Sometimes, I'm not all that convinced that a picture is worth a thousand words... Not even an animated one.

Gunnar Englund

http://www.gke.org

 

[frankiee]

I am also trying to learn Control Circuits Design.

I know what you are saying because I am just learning also.
Relays can look so intimidating.
I think a good place to learn control because it is physical and you can see the path of electricity.
I am in marine engineering and on ship (Great Lakes) There are still a lot of relays because of the harsh enviornment- heat, vibration, moisure.
Can you find a community college that has a tech wing and ask if you can participate in some of the labs?

I have the benefit of being in college where we have a lab board where we hardwire various circuits both with relays and PLC control.
The insructor comes along and puts in faults (Which the apparatus is designed to do) and we have to figure out what the problem is.
I say that is the best place to learn because if you fry a make a mistake the instructor will catch it and explain your error. If he does not catch it and a relay gets fried then he puts another relay in and you do it again.
It is also alot safer then playing with electricity on your own.
I know what you mean that theory is alot different when you get to the real world of masses of wires and relays.
I too have searched for computer programs that more realisticly looks and acts like a circuit aboard a ship.
I look for a program where I can put faults in and then take electrical measurements and see how it affects the whole system.
We learn in our school simulator how the complete ship system works. But that simulator costs 1.7 million and the software is $85,000 Canadian.
A simulator is an good way to learn.
There must be an electrical simulator out there some where.
If you find one soon let me know please.
I will ask my electrical instructor when he comes back. That wont be for another month or so.
I will let you know if I hear of one.
I think finding a local college is your best bet though.
Stay safe and dont make an ash of yourself.

 

[techzone12]

Thank you all for the valuable advice.
When I say that I don't agree with some thing posted, I don't mean to say it out of disrespect. I apologize if I sounded that way. I do it to provoke a further discussion.

I guess what I am looking for is some sort of a guide on how to translate ladder logic into physical wiring diagrams and visa-versa. As ladder logic diagrams are not all that hard to grasp for me, but physical wiring can be some-what challenging. Feel free to suggest a book too!

This is what I am planning to do:
Buy a low voltage PLC with simulation software and play with it for a little while.
Meanwhile I will be looking for a relay circuit’s simulation software
Finally I will buy some switches and relays and wire them up to the PLC IO.
And I did not exclude the possibility of going for a training course.

frankiee,
I will work with low voltage, and I will be very careful. Thank you for your concern.
I will also post once I find simulation software for control relay circuits.

Thanks

 

[toiap]

I have not used it but CMH makes a simulator for control circuits. It is called Constructor. They also have instructional material. Good luck.

http://www.cmhsoftware.com/index.htm

 

[Skogsgurra]

techzone12,

There is hope for you! You have finally told us what you think that your problem is and it seems to be to how wire a relay circuit once you have the ladder diagram.

I can vaguely remember that feeling. It was a very long time ago and after having built a few panels, I found it to be very natural and self-evident. That's probably why I couldn't understand your problem.

This is how I was told to do it. An elderly electrician with lots of patience guided me.

1 Make sure that you really (REALLY) know what your circuit is intended to do. This is where many designs start to go wrong. Discuss with collegues to have a second view. Take your time.

2 Write down the specification. Including voltages, wattages, where things shall be put, colors of lamps, push-buttons. Do you need delays or special relays - specify their functions and find a supplier. What cabling, terminal blocks and junction boxes you need. Do not forget motors, thermal protection, fuses, control voltage transformers. Decide on a box or cabinet to put it all in. You also need conduits, flanges and glands. Remember that wiring takes a lot of space. You probably need about three times the foot-print of the devices. Leave spare room so that you can modify the circuit when needs arise. And then think. Anything forgotten? When everything is there, you have a Bill of Material (BOM).

3 Buy the material. Check that you get the right things and that everthing is there

4 Draw a diagram (it will probably be a ladder diagram), mark all components, number all connections and terminals and "dry-test" it. Will the motor reverse when it hits that end stop? Will that remote push-button work under all circumstances? What happens when a black-out hits? What happens when power returns? Lots of such things (this is where a simulator is useful).

5 When everything looks OK. Put your devices on the plate in a logical order. Put labels where they can be read even after the wiring is complete (never put labels on the devices themselves, the get lost when you need to change a relay or whatever) Mark and drill. Make sure that you don't get shavings in the relays and other devices. (Yes, you can have shavings between pins of an ice-cube relay if not properly seated). Fix all devices very well. Remember that your panel probably will be handled by someone who doesn't care much. Transformers are especially troublesome. Use oversized screws.

6 Now, the wiring. It is easy. First do all heavy-gauge wiring (motor main circuits, heater circuits and so on). Then connect up all control voltages. Starting with control "ground" (the potential to the right in the ladder diagram) and then control "phase" (sometimes there is a master relay, remember to include the input to the MR). Then connect one rung at a time. Tick off in the diagram as you proceed. When there is nothing to tick off. Finished!

7 Testing. I think that I will write about that when you have finished building the circuit. BTW: you need a multimeter with ohms range and voltage. Nothing fancy, but make sure it works before using it. Every time!



Gunnar Englund

http://www.gke.org

 

[itsmoked]

Nice synopsis Skogs. I think I would reverse the order of (3) and (4) as drawing the diagram before buying may illuminate any omissions.

 

[Skogsgurra]

Yes. I probably should.

Gunnar Englund

http://www.gke.org

 

[Skogsgurra]

Remember now, delivery time. It took some time to get all goods delivered. It is faster nowadays - how time changes...

Gunnar Englund

http://www.gke.org

 

[mc5w]

What you want to do is to understand logic systems and finite state machines in terms that are INDEPENDENT of what technology you are using. For practical purposes all programmable controllers are programmed to imitate the control relays that they replace. You should also feel just a comfortable with hardwiring Transistor Transistor Logic chips and you would relays.

I disagree with the idea that control relays are obsolete. In some applications such as explosionproof areas the control relays are PNEUMATIC and the motor controllers actually use an air piston in place of the electric coil. This also requires absolutely clean and dry compressed air which can only be achieved by attaching dehumidifiers to both the intake and output of the air compressor pump especially with rotary screw compressors.

Also, using a PLC to control a hazardous machine such as a punch press or hydraulic press can be a liability hazard. What somebody at Link Electric and Safety Controls told me is that if the voltage regulator for the logic power creates an overvoltage condition, all of the outputs of a PLC will jam on. What this means is that a PLC for a hazardous machine needs to be fully dual channel INCLUDING the power supplies. What somebody at OSHA did was that they tested everybody's PLCs for this condition by cutting the reference wire in the voltage regulator and this is what happened.

I have also seen instances where an individual output of a PLC jammed in the on position due to a blown transistor. I have also seen an instance where OPTO-22 input relays would lose noise suppression resulting in false input signals. The system then goes bonkers. When that happens ALL of the input relays somehow lose noise suppression and I do not see how spikes can kill the noise suppression and only the noise suppression but I have seen it happen. Another failure mechanism is that in servo computers digital to analog converters can short the +12 volt analog power into the 5 volt TTL and send 0.7 amp high voltage pulses into the microprocessor.

Of couse, PLCs are extremely useful for applications such as die automation systems where the control program needs to be changed every time you stick a different die into a punch press. However, never ever use that PLC to automatically start the press. Connecting a PLC output to a realy that is in series with the operator buttons is OK for enforcing start permissives such as correct material position.

You should also gain some experience with a wide variety of PLCs because Allen-Bratley is h#!!atiously expensive versus Aromat a.k.a. Panasonic or Eagle Signal. For instance the Allen-Bratley PLC software disk is $1,200 versus Aromat's disk which is more like $159. You should also become familiar with mini PLCs such as the Idec Micro-1 for applications such as PLC that replaces a few timing relays and the program likely never changes for 20 years. That is, you can pay through the nose for a name.

If you really want you can also get into Wonderware but that costs $2,000 for a 3 day course just to get started!

Another type of logic system is a mechanical load monitor such as for bolt forging dies. The reason for force monitoring is if a piece of the dies breaks of a piece of blank material sticks in the die the force on the next hit will be out of acceptable parameters therefore the forging press must stop. Same thing if the die approaches a wear limit. This consists of solid state or other strain gauges connected to an electronic monitor that does trending and other monitoring algorithms to determine what is reasonable and when to stop the machine.

By the way, a lot of machine builders do not allow customer access to a programmable controller program, not even to a written copy, because of copyright, trade secrecy, and safety issues such as the customer defeating safety interlocks. In that case PLC debugging has to be done using the Garbage In Garbage Out principle. Turns out that 99% of failures in a machine that has a PLC are things like broken wires, misaligned limit switches, and jammed gearboxes. Normal vibration will loosen up limit switches and wiring connections. A lot of new sensors have very thin wires that break easily due to machine vibration.

Never ever let anybody convince you that PLCs are a form of sorcery or some other black art. Sure, electrical engineering can sometimes be more like witchcraft but that is because God and fools find ingenious ways to hand you a homework problem.

 

[techzone12]

mc5w,

Thank you for the follow up. I see your point. I realize that when safety is an issue relays are a must. In my work many times Relay circuits are used in conjunction with the PLC's.
I understand "finite state machines" because I took a course in "Digital Design". So I perfectly understand binary logic, etc.

I understand the concepts, I never had a chance however to wire up relay circuits myself. If I look at engineering drawing however I can understand the wiring diagrams, etc.
I know enough to get my job done, but I am trying to take myself one level up.

I am going to buy an affordable micro PLC and some cheap relays/sensors and play with them? I am going to use simulation too for more advanced applications.

Thanks a lot for the feedback