OT: Diodes in control circuits?

plchacker

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Feb 2006
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Helwestern, AL
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While modifying a couple of my trainers I ran into an interesting situation. The trainer is for the AB SMC 180 drives. This little drive is similar to the 1305 VFD. For controls, it uses its own sinking 12VDC power source. In three wire controls the stop (like always) should have normally closed contacts, while Run and Rev use normally open contacts. Fine and well. Try to run one with a PLC and the obvious solution is to use relays.



So.... I ordered four DIN mounted SPDT relays for each drive. The first to select between the panel mounted PB's or allow the PLC to take control, and the other three for three wire controls from the PLC.



Remember the Stop contacts, yep, they will allow the voltage to feed back to the power side of the other switches/relays, and allow the drive to be started or reversed from the wrong control (either PLC or PB's). Stopping can only be accomplished from the selected control, PLC or PB's. BAD NEWS~!



In a mill, I would use the PB's as inputs for the PLC, and only use three relays. In our lab, with the nifty (and expensive) little trainer, this is not an option. The other thing is that in a mill I would have plenty relays on hand, and adding either a DPDT relay to control the stop, or a fifth SPDT to do the same would be no big deal. In my lab, that means jumping through the money hoops to get the extra relays. Again, this is not a mill. The trainer must function with or without the PLC.



So, I used Diodes. To keep my wiring clean I used two per trainer, isolating the stop contacts (relay and PB). This works like a champ.



At long last my question. Is this acceptable? It works fine, and will soon be documented. The students will not see this until after Christmas. I just want to cover my bases and not present my students with bad practices. In the real world I have used diodes/MOV's to contain spikes from inductors like sol. valves and the like. So I don't see any real issue with this, but above all, I want to produce students that understand good practices.



Thanks for any comments.
 
The only issue is the Stop? Got a spare SPDT Switch? Take the power off the buttons and to the relays, like an Auto/Manual control. This can let you keep the Stop PB in with the PLC control but remove the other buttons.

Not sure I like the diode idea.
 
You are right, another (fifth) relay will do the trick. I may just have to jump through the hoops. I'll try to post a drawing to show what I did tomorow. Its been a very long day. (Not the drives, I did those last week.) A good friend's father died of cancer last night.
 
You are right, another (fifth) relay will do the trick.
Well, if you are really hard-pressed for relays, you could APPLY a little of that Boolean logic that you have been teaching your students.

Let's see, you need to do two separate things:

1. Select between PB's and PLC control --> 1 control bit needed, and you used 1 relay. You could substitute a SPDT switch as Ron suggested. Use the switch to apply control power to the drive, or to send an input to the PLC telling it that it is now in control.

2. Control 3 bits of logic to the SMC 180 drive (Stop, Run, Reverse). You have to do three things, and you used three binary bits (relay outputs) to do it with. But you know that with only two binary bits, you can produce 4 different "states": 0, 1, 2, and 4. You only need 3 separate commands, so you're in luck! You could use only TWO relays to control the SMC 180! The contacts can be arranged many differnt ways, depending on if the drive is set up for momentary or maintained contacts. Here is one way.

|
| R2 STOP (to drive)
|---+----|/|----+-------------------
| | |
| | R3 |
| +----|/|----+
|
| R2 RUN (to drive)
|--------| |------------------------
|
| R3 REVERSE (to drive)
|--------| |------------------------
|
|

 
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plchacker said:
While modifying a couple of my trainers I ran into an interesting situation. The trainer is for the AB SMC 180 drives. This little drive is similar to the 1305 VFD. For controls, it uses its own sinking 12VDC power source. In three wire controls the stop (like always) should have normally closed contacts, while Run and Rev use normally open contacts. Fine and well. Try to run one with a PLC and the obvious solution is to use relays.



So.... I ordered four DIN mounted SPDT relays for each drive. The first to select between the panel mounted PB's or allow the PLC to take control, and the other three for three wire controls from the PLC.



Remember the Stop contacts, yep, they will allow the voltage to feed back to the power side of the other switches/relays, and allow the drive to be started or reversed from the wrong control (either PLC or PB's). Stopping can only be accomplished from the selected control, PLC or PB's. BAD NEWS~!

Why doesn't that work? Run the 12v thru the stop PB then to the relay, thru the NC on the relay back to the PB's, switch the relay to remove feed to the other PB's and the Stop button is still in the ckt.

Lancie, 2 bits offer 0,1,2,3 numerically but 4 states i.e. none on, 1 on, 2 on, or both on...right?
 
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I'm so dumb, I don't understand what you're doing with the diodes.

So you used diodes to do . . . ?

Dan
 
Lancie, 2 bits offer 0,1,2,3 numerically but 4 states i.e. none on, 1 on, 2 on, or both on...right?
Correct, that 4 was a typing error. I saw it later but it was past the 1-hour limit.

I see no reason why the task should take more than 4 relays per PLC, fewer if selector switches are used.

PLCHacker, if you posted a schematic of your situation, I imagine that a cheap and easy solution could be found, with a minimum number of relays. Here is one that uses 3 relays.

STATE TABLE (DC = DON'T CARE)

R1 R2 R3 FUNCTION
OFF DC DC CONTROL BY SELECTOR SWITCH
ON OFF OFF NOT USED
ON ON OFF PLC RUN FORWARD
ON OFF ON PLC RUN REVERSE
ON ON ON PLC STOP

|+12 VDC
|
| STOP
| SWITCH R2 STOP (to drive)
|---0_|_0----+----|/|----+-------------------
| | |
| | R3 |
| +----|/|----+
| FORWARD
| SWITCH R1 FORWARD (to drive)
|---0 \ 0---------|/|----+---------------------
| |
| R2 |
|-----||-----------------+
|
| REVERSE
| SWITCH R1 REVERSE (to drive)
|---0 / 0---------|/|----+--------------------
| |
| R3 |
|-----||-----------------+
|


 
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Here is a more complete version.

STATE TABLE (DC = DON'T CARE)

R1 R2 R3 FUNCTION
OFF DC DC CONTROL BY SELECTOR SWITCH
ON OFF OFF NOT USED
ON ON OFF PLC RUN FORWARD
ON OFF ON PLC RUN REVERSE
ON ON ON PLC STOP

|+12 VDC
|
| STOP
| R2 SWITCH STOP (to drive)
|---+----|/|----+-----0_|_0-----------------------
| | |
| | R3 |
| +----|/|----+

|
| FORWARD
| R1-1 SWITCH FORWARD (to drive)
|--------|/|-----0 \ 0---+-----------------------
| |
| R1-1 R2 |
|--------| |------| |----+
|

| REVERSE
| R1-2 SWITCH REVERSE (to drive)
|--------|/|-----0 \ 0---+-----------------------
| |
| R1-2 R3 |
|--------| |------| |----+
|


 
When you use diodes like this they are called "steering diodes".

The only problem I see is that most control circuits are AC and diodes won't work the same way in AC. It just happens that VFD input signals are generally DC but, as you doubtless know, that's the exception rather than the rule.

I often use diodes for this purpose on drive inputs but i am not a general controls guy. I pretty much limit myself to drives and leave the rest of the control world to someone else.
 
SMC160Trainer.JPG



This should show my the trainer as is. Remember that ther is only one set of R1 contacts. I showed this set three times to simplify the sketch.

The trouble is that without the diodes, if R1 is not energized, current travels through the normally closed switch and contacts of R1 to the Start and Reverse relay contacts. This allows the drive to be started or reversed, but not stopped with the PLC. Likewise for the PB's if R1 is energized. With the diodes everything is fine. I'm just a little shakey about using this in the classroom. I guess the squid in me is taking over.
 
Got me confused, you said SPDT:

SPDT-Switch.png



This should allow you to break the common after the Stop PB (if you want to keep it in ckt) and just feed the Drive stop from the NC contact of the relay and go to NC of 2 relays, then use the 2 relays to actuate start and reverse as desired.


In other words do not think about running everything in parallel i.e. use a relay to make 2 separate control ckts, one for manual and one for auto.

My thinking is you only need 3 relays but the 4th could be used as an input to the plc to state in Man or Auto.
 
Lancie1,

Thanks for your effort. I do have one question. Using your drawing, if R1 is energized to give control to the PLC, and R3 is energized to run reverse, and I then energize R2 to start the drive, will I not loose the stop signal? The start and stop signals are momentary, but the directional is maintained. This is a matter of the drive requirements. I should have posted more information about the drive.

DickDV, Your drive experience is key here. Until this particular situation, I never had any trouble whatsoever starting, stopping and reversing a drive. It is the mix of PB's and Relays that caused me to resort to the diodes. In every other case, I would use just three relays, all controlled by the PLC, with any number of start/stop stations both hardwired to PLC inputs and HMI.

I realize that I could have used a single diode. I was in trial and error mode when I added the first.

The original question is not what else will work, but is the use of diodes acceptable in this type of situation? Granted, if I can eliminate the use of the Diodes I certainly will.

Also, I made a mistake in the initial post, these are SMC 160 drives, not SMC 180.
 
DickDV said:
The only problem I see is that most control circuits are AC and diodes won't work the same way in AC.


Is that statement true in the States or just where DickDV works?

In the UK and from what I've seen so far in Canada, most control circuits are 24VDC.

Older (very much older in the UK) circuits were 110VAC and I have rarely seen 24VAC.
 
rsdoran said:
Got me confused, you said SPDT:

SPDT-Switch.png



This should allow you to break the common after the Stop PB (if you want to keep it in ckt) and just feed the Drive stop from the NC contact of the relay and go to NC of 2 relays, then use the 2 relays to actuate start and reverse as desired.


In other words do not think about running everything in parallel i.e. use a relay to make 2 separate control ckts, one for manual and one for auto.


My thinking is you only need 3 relays but the 4th could be used as an input to the plc to state in Man or Auto.

RSDoran, Your way should work. I'll rework the sketch after lunch and repost it to make sure I'm clear.
 
This drawing is sloppy and not precisely accurate but you should get the gist.

smc160pre.jpg


There are things to consider, this was a rush job but may get the idea across.
 
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