Op Amp

I learned how to use a voltage divider by using a pot, and an op amp. This was for monitoring a voltage. The sensors we used were a thermistor, or a photo cell. This is a PLC class, but the instructor also teaches Electronics. These were 5V inputs.

Anybody use an Op Amp for "Real World" PLC input applications ?

5 volt inputs or outputs are common for TTL devises
this allows for very fast switching.
As far as OP Amp applications this is used but
usually they are in some form of 'Black Box'
like a converter for analogue I/O or Thermocouples etc.
we do not usually make our own control with OP Amps

iant said:

5 volt inputs or outputs are common for TTL devises
this allows for very fast switching.
As far as OP Amp applications this is used but
usually they are in some form of 'Black Box'
like a converter for analogue I/O or Thermocouples etc.
we do not usually make our own control with OP Amps

Click to expand...

Thanks, I guess it's just a few bucks in Electronics to make one.

Would the Thermistors on the busbars of a large rectifier be converting that temp to the PLC by using this method ?

It depends on the application. You can buy controllers that take the thermistor input directly and produce a digital output.
Normally you would not waste PLC analogues for that task

there were a couple of times in the beginning of plc's early 80's where I had to use op-amps for a couple of custom applications, but now there is a converter or instrument to do anything you need to do. I imagine your instructor is trying to get you above the norm and not at the norm for a programmer. Understanding what you are interfacing to, and how it works, and doesn't work will make you better than somebody who just understands the code. My background before PLC's was electronics as applied to industrial applications so I have had a lot of head's up on why something wouldn't work even though the program / PLC was perfect.

Learn everything you can now, and keep learning as you go, that's the best way to stay in front.

Also always remember, no matter what that you'll never know everything, there is always something new to learn!

old PLC do not have many types of input, only 4/20 mA etc.
So yes i have always used opamps, on boards made myself to get other inputs. I made a general input board to get a nice 4/20 mA out of any type input like RTD,TC,potmeter etc.
Due to this i still use it as my price is a lot lower as for a inputcard from PLC.
i have one for 25 $ and inputs costing about 100 $.
i also use relais multiplexing for analog signals.
on RTD side and on the mA side too (my input is a standard 0/10 Volt with some high precision resistors.

My use of Op Amps was mostly pre-PLC machines with analogue controls for motor speeds and torque limits. Once PLCs became fast enough, they took over from the old analogue controls

Knowledge of op amps and analogue electronics still comes in handy every now and then. About 18 month back I was called to a paper mill to help fix a winder that had been down for two days at a cost of £10,000/hr in profit. The machine still had analogue drives. A capacitor had gone down and was clamping the integrator in the ramp circuit.

Nick