Author Topic: Industrial POT replacement  (Read 388 times)

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Offline jamorais

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Industrial POT replacement
« on: December 03, 2022, 07:03:49 pm »
Hi Guys.
I have a situation where a customer has some old industrial rectifiers that were controlled by a Pot.
Now i'd like to control them with the PLC but the pot circuit pulls too much current and the 0-10v outputs of the plc max at 45mA so, if i connect directly, the voltage drops on the plc output.

if i connect an external power supply and provide the 0-10v it works fine.
Now i've seen some ideas on the internet with FET+OpAmp with feedback to get linearity, i saw digital pots which could be controlled with digital plc outputs (up down steps).
An LM317, with 24v on the input and the 0-10v plc output to control the adjustment so i get 0-10 on the LM317 output. Although this uses de 24V power supply. Not ideal but it works.

this interesting one which gives me galvanic isolation too, which is important in this situation:

What do you guys think? Any ideas?

Offline bidrohini

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Re: Industrial POT replacement
« Reply #1 on: December 06, 2022, 06:50:33 am »
Hard to give an accurate idea because these type of projects often needs to be done in trial and error basis. I think it's better to try with both LM317 and LM358. After that, you can compare their performances. 

Offline abyrvalg

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Re: Industrial POT replacement
« Reply #2 on: December 06, 2022, 07:12:45 am »
Depending on PLC processing power, perhaps you don’t even need linearity so much - just design some curve translation block and insert them before each pot output.

Offline Jeroen3

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Re: Industrial POT replacement
« Reply #3 on: December 06, 2022, 07:24:28 am »
Investigate the input circuit a bit, you can probably use a current output and perhaps some additional parallel resistor. 
Then just a universal MCR-SL-UI-UI-NC will be fine.

Offline Terry Bites

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Re: Industrial POT replacement
« Reply #4 on: December 06, 2022, 01:57:42 pm »
This circuit I imagine you've already come across. Its just an amplfier with boosted current capabilities.  Vo=(1+R1/R2)Vc, here Vo=2Cv
C1 and R3 keep this circuit stable. A rail to rail opamp is needed here. Just about any N-Ch power mosfet will work.

Cv can just be a voltage or a lowpass filtered PWM signal. Isolating PWM is easy, just use a photocoupler.


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