# Variable Resistive device

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#### Jody Gallant

Hi, we have an application where a piece of machinery can be controlled based on a resistance value placed across its remote input. Right now, we are feeding a bank of different resistors into the ouputs of a plc, and feeding the common back to the machine we are controlling. When one of the outputs are triggered (relay outputs, non-isolated) this resistive value is essentially put across the machine, and the command takes place. What I would like to do, instead, is have a 0-10V analog output card attached to something that can produce a variable resistance based on the voltage present at its inputs. (isolated from the resistance) Problem is, I don't know if such a device exists, or where to get it? I know this isn't really 100% on topic, but figured maybe someone out there has done something similar. thanks Jody Gallant CET Shadcomm Ltd.

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#### jmGiraud

Your problem looks like a piece of cake: If your PLC is not too short of calculations, simply linearise the output !!! As a bonus, this method will nicely interpolate, thus providing continuously smooth movement of the machinery under command. Obviously the above method does not work if the output is multistep. In that case, some number crunching is required. If you can provide all what I need, in comprehendable values, I will categorise your algorithm (may be in minutes). Good luck. [email protected]

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If I understood your problem, in fact you don't need to have a variable resistance. You can feed a Voltage tranducer with an AC voltage on one side and get a 0-10 V, or 0-1 mA or even better, a 4-20 mA isolated signal on the other to feed your PLC analog input card. Try one of these: http://www.rochester.com/Products/RIS/trasdu.htm http://www.metec.co.nz/Products/cewe/ http://www.a-a-c.com/0.htm http://www.softco-op.com/stitransducers/prod01.htm Or if your source is a DC voltage you can use a Signal conditioner from the ActionPak 4000-9000 series. Try: http://www.actionio.com/datasheets/home.html Cheers, Alfonso Padilla QPS Control & Automation [email protected]

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#### Bill Marsh

Xicor (and probably others) make Digitally controlled Potentiometer (IC's) that might be appropriate. A typical device would have up/down and increment inputs to control a 100 step silicone based trimmer pot (3 wire). Not going to be a plug-n-play solution however. Most require 5vdc supply voltage and place severe limits the voltage on the resistor inputs. If you can determine what the resistor is actually doing in the machinery controls, you might be able to use a standard DC voltage or current output to control the application. Bill Marsh

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#### Johan Bengtsson P&L Automatik AB

I don't know of one, but I think it could be done by using a matched pair of FET:s they act as a controlleable resistor where the gate voltage determines resistance By using a matched pair and using one for the output and one internally for controlling the proper gate voltage it could be done. If you want to buy something off the shelf I cannot help you, (others on the list might however) but if you could consider making one yourself I could perhaps help you. /Johan Bengtsson ---------------------------------------- P&L, Innovation in training Box 252, S-281 23 H{ssleholm SWEDEN Tel: +46 451 49 460, Fax: +46 451 89 833 E-mail: [email protected] Internet: http://www.pol.se/ ----------------------------------------

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#### Richard Dewees

what you are describing is a transistor by feeding voltage to one of the legs you vary the resistance across the other two or if you want you could you use a vacuum tube

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#### Ed Mulligan

A long time ago I saw this done using a FET output opto-isolator. Feeding a 0-5Vdc signal into the input varied the resistance of the output. The application I saw was to use a PC joystick port as a simple DAQ for 0-5V signals. I don't remember the part number, but it was a typical 4 pin device. Sorry I can't give more details, it was about 10 years ago. Ed Speaking for me, not for Starbucks. . .

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#### wlmostia

You could try M-Systems(214-733-1313, http://www.m-system.com/) DC/Potentiometer Transmitter Model CVR. Most major transducer companies such as Action and Transmation should also be able to supply a similar transmitter. Bill Mostia ======================================================= William(Bill) L. Mostia, Jr. PE Independent I &E Consultant WLM Engineering Co. P.O. Box 1129 Kemah, TX 77565 [email protected] 281-334-3169

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#### Johan Bengtsson

Be aware that a "normal" bipolar transistor does not act as a variable resistance but rather let thru a constant current This don't mean it would not work, just that it would be harder to set up properly /Johan Bengtsson ---------------------------------------- P&L, Innovation in training Box 252, S-281 23 H{ssleholm SWEDEN Tel: +46 451 49 460, Fax: +46 451 89 833 E-mail: [email protected] Internet: http://www.pol.se/ ----------------------------------------

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#### Pierre R. Hinse CET

I have seen such a device in a generator control system, it was made by woodward http//:www.woodward.com. I don't have part numbers, but give them a call, and they can probably help you. Pierre R. Hinse CET

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#### Ken Irving

There are relatively low tech and low cost devices that (literally) do what you describe using relays to select resistors, and present an 0-10V, 2-10V, 4-20mA, etc., interface. Kele and Associates (http:://www.kele.com/) has devices of this sort with 8 relays, i.e., providing 8-bit resolution (255 steps). This may/may not be appropriate for your application, e.g., depending on how rapidly the output changes. Ken Irving <[email protected]>

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Jody, In my previous comment it is clear that I mixed up the whole thing: I didn't get the picture at all. My apologies. Now I do. The variable resistance at your machine's input delivers a variable voltage in two possible ways: If the circuitry inside the machine has a DC voltage source (Vs) and a series resistor (R1)connected to the external resistance (R2),it is a voltage divider scheme, such that Vi= Vs*R2 /(R1+R2) . If the machine has a constant current source (I) it is straightforward Vi = IR2. Either way if you can provide R2min and R2max values, input voltage measured @ R2min and R2max and current flowing through R2 at least at one of those conditions it would ease calculations (not only ohmic values but also power disipation in R2)and facilitate the right choice Alfonso Padilla QPS Control & Automation [email protected]

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#### Jody Gallant

Well, got a lot more responses then expected, Thanks a lot to everyone. One of the responses I got, led me back to one of the ideas I had. Since the device just used a voltage comparator chip which used 12Vdc as a reference voltage, and then made a decision based on the voltage present at the input, I decided just to try using a variable voltage source with the opposite polarity. The equipment responded immediately. I was then able to setup a lookup table in the PLC to trigger an analog output card (0-10Volts). Then, with the proper sequence of MOV commands all the equipment commands are at the touch of a button. (MMI) Thanks again to everyone who responded. Jody Gallant CET Shadcomm Ltd.

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#### Michael Griffin

At 10:01 26/02/01 -0500, Jody Gallant wrote: <clip> >Hi, we have an application where a piece of machinery can be controlled >based on a resistance value placed across its remote input. Right now, we are >feeding a bank of different resistors into the ouputs of a plc, and >feeding the common back to the machine we are controlling. When one of the >outputs are triggered (relay outputs, non-isolated) this resistive value is >essentially put across the machine, and the command takes place. > >What I would like to do, instead, is have a 0-10V analog output card >attached to something that can produce a variable resistance based on >the voltage present at its inputs. (isolated from the resistance) >Problem is, I don't know if such a device exists, or where to get it? <clip> A lot of devices which take a resistance or potentiometer input will also work with a voltage input using standard voltages. This is usually not very well documented in the manual or data sheet though. A pot is easy to substitute, as the terminal for the command voltage is easily accessable. The command voltage from your analogue card is connected to the wiper input, and the analogue common is connected to the -ve terminal. The third terminal (which supplied the reference voltage to the pot) is left unconnected. When a resistor is used to generate a command signal, it is often as part of a voltage divider, with the other half of the voltage divider being an on-board resistor. This is similar to the potentiometer type input, except the reference voltage terminal is "hidden" from you, and all you see is the command terminal and the common. This type will also often take a voltage input but sometimes requires changing a jumper or switch or some such thing. These are two common ways these types of command inputs work. The device you have may of course be different in some way. If you don't have the device schematics, you should give the manufacturer a call before hooking things up to experiment. You often also need to use isolated analogue connections. I have controlled both potentiometer and resistance programmed devices using analogue outputs from PLCs. However, I always used isolated signal conditioners to give the proper signal level, provide isolation, calibration, and sometimes to provide features like reverse acting outputs (so the device would go off if the PLC command disappeared). ********************** Michael Griffin London, Ont. Canada [email protected] **********************

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#### Bob Peterson

You can also get motor driven pots. With a simple feedback loop you could create whatever resistance you want. Bob Peterson

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#### Anthony Kerstens

Hey, don't be knocking vacuum tubes. They're still in use for high-end audio amps (for audiophiles with deep pockets . Anthony Kerstens P.Eng.

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#### Willy Smith

Indeed. And it's hard to beat a tube for those rad-hard designs. For PDF datasheets, app notes, and links to spice models, see: http://www.svetlana.com/ Willy Smith

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#### Johan Bengtsson

The above statement is true, probably for most devices. I just wanted to point out that there is more possibilities, for example the one used in a standard PC joystick. A capacitor is charged thru the resistance and the time is measured until some certain voltage is reached, a transitor turns on and dischages the capacitor, turns off, and then it starts over. This type of input would not be happy with a varying voltage. Note: I don't say it won't work. It most obviously works for the original poster of this thread, but I just wanted to point out why it is not necesarily working everywhere. /Johan Bengtsson ---------------------------------------- P&L, Innovation in training Box 252, S-281 23 H{ssleholm SWEDEN Tel: +46 451 49 460, Fax: +46 451 89 833 E-mail: [email protected] Internet: http://www.pol.se/ ----------------------------------------