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    circuit design help analog to Potentiometer

    Looking for help to build a circuit to convert analog input to potentiometer output. I am sending an analog signal 0-10vdc via a PLC card. i need to send out a 0-6.2vdc signal through a potentiometer. The reason being, the device (Lincoln welder) is currently being tuned with a potentiometer and has a built in safety circuit. So it is monitoring all three connections of the physical potentiometer and if it detects an issue the welder shuts down. What i am trying to do is use the analog signal to remotely tune the welder and use the digital potentiometer to simulate the physical one. I have 10 welders and limited amount of space in the panel so trying to keep it small.

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    Re: circuit design help analog to Potentiometer

    Hi,

    That sounds very hard to implement. I wouldn't want to try with my limited experience.

    Not sure if voltage to pulse count + up/down counter + motor driver (h-bridge) + servo or stepper motor would be relevant parts/blocks. Others will suggest a uC +???

    Good luck.



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    Re: circuit design help analog to Potentiometer

    Not sure what you’re talking about: “send 0-6.2 V through a potentiometer”, but then you say the welder is “monitoring all three connections...” . Do you want an analog voltage to control a pot? Do you want to convert 0-10 to 0-6.2?



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    Re: circuit design help analog to Potentiometer

    Using the digital potentiometer i need the welder to think it is the physical potentiometer. More or less "relay" the analog signal to the welder.
    so yes, the analog value will control the pot.



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    Re: circuit design help analog to Potentiometer

    I’d use a small microprocessor and a digital pot. They do make motorized pots that can be controlled by an analog voltage, but they are big and expensive. You could also use a servo, but then you’d have to design some mechanical connection to the pot.



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    Re: circuit design help analog to Potentiometer

    So it is monitoring all three connections of the physical potentiometer and if it detects an issue the welder shuts down.
    You need to know which parameters of the potentiometer interface are monitored to design an appropriate circuit. Not sure what you have actually tested. I guess you tried to drive the controller by an external voltage and it failed, but that doesn't necessarily mean that "all three connections are monitored". To use a digital potentiometer, you need to know the required voltage range, is isolation required?

    Besides a digital potentiometer, there are also possible analog solutions, e.g. using analog multiplier and equivalent circuits. We have a similar problem when emulating analog sensors in calibrators and testers.



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    Re: circuit design help analog to Potentiometer

    the welder voltage is 0-6.2 VDC. I figure using an isolated digital Potentiometer would clear the circuit of all the safety checks, so i would not have to determine what is being monitored.



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    Re: circuit design help analog to Potentiometer

    Quote Originally Posted by rbenh View Post
    the welder voltage is 0-6.2 VDC. I figure using an isolated digital Potentiometer would clear the circuit of all the safety checks, so i would not have to determine what is being monitored.
    You mean the welder CONTROL voltage is 0-6.2? It’s still unclear what you’ve got going on here. Saying “it monitors all three connections” is doubtful. And what “issues” could it possibly detect from a pot? Overvoltage?



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    Re: circuit design help analog to Potentiometer

    yes, the control power for he welder is 0-6.2vdc. the pot controls the output of the welder. So likely, it is monitoring for high voltage so the output does not go to high. i looked into the Power Pot element and found a few, good suggestion there. that will work but i really would like to be able to change the "output" on demand, by sending out a value via the analog. I added a post of the welders print i am working with. R1 on bottom right is the pot. there is a switch to change the terminals to 77, and 78. All that does is make those terminals a connection for a remote pot and still uses 75 (the power source) as the remotes pot's source.Click image for larger version. 

Name:	welderP.png 
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    Re: circuit design help analog to Potentiometer

    What is the model of that Lincoln welder?
    Make no mistake between my personality and my attitude.
    My personality is who I am.
    My attitude depends on who you are.



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    Re: circuit design help analog to Potentiometer

    From the schematic it looks like you don't need a potentiometer at all, just a variable voltage.

    The "CVS MAX TRIM" and "CVI MIN TRIM" circuits just set the voltages at the ends of R1 to restrict its range. At most the range can be -6.2V to 0V but the trim controls can narrow it down to about -4V to -2V.

    The problem you will face is the polarity. On its own the polarity is not important, you can just produce a positive voltage and swap the output wires but there will almost certainly be a ground in the PLC and probably in the welder and linking them together will cause problems. They are probably linked already through the power cords.

    There are many solutions to the problem, the simplest is probably to use an inverting op-amp circuit as the load current seems to be quite small. Your PLC would produce say 0V to +6.2V which I assume is within it's capability and the op-amp basically reverses its polarity with respect to the same ground. You can do it using only resistors if you are very careful by balancing a positive output from the PLC card against the negative voltage supply but the control will be reversed, that may be something you can work around in the PLC control program.

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    Re: circuit design help analog to Potentiometer

    As Brian notes, it's a pretty simple matter to just replace the pot with an analog voltage. There is nothing in the pot circuit that has anything to do with "detecting issues".



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