OT: Diodes in control circuits?

Girevik, Ron,

Please allow me to put on my Critic's hat, one of the most useful functions that I seem to serve here. There is no personal disrespect meant here. These are only my opinions. I see some minor difficulties with each of your wiring schemes.

First, when the Stop pushbutton is pressed, power would be removed from the Reverse function. Even if the drive is stopping, power should be maintained on the Reverse terminal so that the drive does not try to reverse direction while powering to a stop. Many drives would go into fault mode, signaling a Current Overload if the drive tries to reverse while the motor is spinning. However, some drives have built-in protection against a sudden direction reversal.

Second, the PLC Stop function only works when the PLC R1 relay is enabled. It should work at all times. For safety, the PLC should have the ability to stop the drive, even in non-PLC control mode. You cannot predict what students will try. "STOP" should always perform the stop function, no matter when.

Third, both schemes would require PLCHacker to unwire BOTH terminals of the Stop button, moving it to a new location in the circuit. Why complicate the problem (without gaining any benefits)?
 
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Lancie1 said:
Girevik, Ron,

Please allow me to put on my Critic's hat, one of the most useful functions that I seem to serve here. There is no personal disrespect meant here. These are only my opinions. I see some minor difficulties with each of your wiring schemes.

First, when the Stop pushbutton is pressed, power would be removed from the Reverse function. Even if the drive is stopping, power should be maintained on the Reverse terminal so that the drive does not try to reverse direction while powering to a stop. Many drives would go into fault mode, signaling a Current Overload if the drive tries to reverse while the motor is spinning. However, some drives have built-in protection against a sudden direction reversal.

Second, the PLC Stop function only works when the PLC R1 relay is enabled. It should work at all times. For safety, the PLC should have the ability to stop the drive, even in non-PLC control mode. You cannot predict what students will try. "STOP" should always perform the stop function, no matter when.

Third, both schemes would require PLCHacker to unwire BOTH terminals of the Stop button, moving it to a new location in the circuit. Why complicate the problem (without gaining any benefits)?

Will accede the Reverse aspect, did not consider that, have to think about that.

Not sure what you mean about R1 affecting the Stop, the idea was when in manual everything is done by hand, when in Auto everything done by PLC. Not sure why the PLC should do anything in manual, nor how my drawing differs in that area from yours.

I redid your Ascii drawing, it does consider the reverse which I did not consider.

lanceascii.jpg


Just realized, the only difference between my drawing and this is my placement of the STOP PB and order of devices. Put the Stop PB after Stop Relay 2 and its the same ckt more or less.

newtrainer1.jpg


newtrainer2.jpg
 
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I will reiterate I am just a Bubba, just trying to throw some thoughts out there to help along people to develop their own ideas. Personally would do something like this completely different but...
 
Wow!



First Lancie1 is correct. I am trying to maintain the original functionality of the trainers. There is training material developed for the "out of the box" trainer, and I am not the only instructor that uses these trainers. They go on loan to the Alabama Technology Network, a group that works with existing industry in the State of AL and even into FL, MS and a few other States. Those guys are providing me with extra work, which in turn is paying for my own education. Rule 1, don't bite the hand...


Rsdoran, you are right, the PLC should know which mode the controls are in. Sense R1 is fired by the PLC, it, the PLC, should know. If there was a switch, instead, then the PLC would need an input from the switch. I may at some point wire the Drives, programmable output to allow either Faulted, or running at speed input to the PLC. In every Drive I set up in the real world, this was always on the short list. Love those 1336+2's; lots of programmable outputs, when R/IO was out of the question.


Girevik, You are very much correct, I would not have posted here if I didn't expect other ideas, and even warranted harsh criticism. One of the reasons I visit this site so much is to insure that I am keeping up with industry. I am the only non-engineer in our group, and what I bring to the table is practical experience. If I lose that edge, I would then need to change jobs.


Believe me, I am as picky as anyone about stop ckts. So much so that failure to properly set up the Estop on our trainers, or failure to properly set up a stop circuit results in a 20% loss in lab grade. I still cannot believe that I missed this one. The stop switch and the PLC Stop Relay should be in series. Unfortunately this means using NC contacts on the stop relay, and that is not good. I'll have to mull that one over a bit.


Other than the relay contacts, I believe the rest is much better than my original solution. As for the load, it is a well protected unloaded motor. I may well put this in front of the class and see what the students find. Oddly enough, I don't teach the first mtr ctls class, but I do teach the PLC classes.
 
plc noob said:
what exactly is he trying to do with the diodes . i read these post and still do not see what he is using the diodes for ?

Blocking feedback, diodes only allow current to flow one way. His early design when it was in Auto Mode the common fed back through the STOP PB to the Start and Reverse. The diode blocked current from back flowing through the STOP PB (in Auto mode).

The early drawing that he provided with diodes shown was not properly representative.
 
Diodes in a switch matrix

Not Industrial but... I know that diodes and switches are still used in pinball machines. The switches and diodes are used in a matrix configuration. The matrix configuration is used to cut down on the amount of inputs and wiring. The diodes are used to prevent backfeed into a strobe signal.

SwitchMatrix3.jpg


Bud
 

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