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from the Automation List department...
4-20 to resistance convertor
Sensor technologies. topic
Posted by Kevin Mc on 9 May, 2003 - 10:48 am
I need to convert a 4-20 mA signal to a three-wire pot, 0-5k ohms. I am outputting a 4-20mA signal from a controller and need it to manipulate a resistance. In other words I am trying to control a device that has a potentiometer input(manual) with an automatic control. I know of many converters that will perform this in reverse however I have never seen one that would pull it off in this direction.

Any help would greatly be appreciated.


Posted by Bob Peterson on 9 May, 2003 - 3:28 pm
Motorized pots are available. I have seen them used occasionally. Sometimes they will gang another pot onto it as feedback for the pot output.

Bob Peterson


Posted by Anonymous on 9 May, 2003 - 4:11 pm
<P>It is most likely that the actual input to your device is a differential voltage input, and the pot was an easy and cheap way to vary that voltage. It is doubtful the device measures resistance of the pot directly. One end of your pot is probably connected to a voltage source from your device, with the wiper providing the commanded value and the other end of the pot the reference. Use a voltmeter to determine which terminal is supplying the voltage, then connect your 4-20mA input to the other two terminals (match polarity!) with a resistor parallel to the terminals. Use ohms law to determine what size resistor to use to convert the mA signal to volts to match the pot source voltage provided by the device.
eg, if the pot source voltage is 5V then use a 250 ohm resistor.
<PRE>
POT 4-20mA
|------0 (+5v) 0 (+5v)
\
/
\<-----0 (wiper) +4-20mA--------|--0 (wiper)
/ \
\ / 250ohm
|------0 (ref) -4-20mA--------|--0 (ref)

</PRE>
<P>Your actual device terminals and voltage may differ but the same principles apply</P>


Posted by Kevin on 10 May, 2003 - 11:15 am
Thanks for the feedback.

Based on the feedback and looking at the circuit that I am trying to control it appears that resistance is not the key, voltage is what the device I am trying to control is looking for. I know of several devices that will isolate power supplies and provide the conversion I need.

Thanks again!


Posted by Landaus@attbi.com on 11 May, 2003 - 1:07 pm
Check out http://www.kele.com

They have all sorts of funky signal converters, 0-135 ohm is a common one used in HVAC control circuits.

S. Landau
SPEC


Posted by Anonymous on 9 May, 2003 - 4:19 pm
Does the existing input really use resistance? Most Pot type inputs are really 0-10Vdc or similiar voltage. Basically a voltage divider with the wiper of the pot returning a voltage between the voltage across the pot. If you can use voltage then just build a fixed voltage divider and run the 4-20mA through it. ( be sure to be within the load specs for the 4-20mA output and that you don't need voltage isolation between the devices.


Posted by Curt Wuollet on 10 May, 2003 - 11:25 am
The general case would be pretty interesting to tackle. Your specific case might be simple or difficult depending on what the pot actually does and the voltages involved. In some cases a dc controlled attenuator would work. In some cases you might just need a variable voltage. If the pot is for feedback or similar it would take two FETS, control, isolation, etc. etc. What's it do?

Regards

cww


Posted by Paul P on 10 July, 2003 - 9:11 pm
Sounds like something I'm working on as well. Does your project involve an electric vehicle? Mine does.

Sorry for the late reply, but if your providing a control signal to a similar motor, than I can assume that I too can simply provide a voltage to the motor controller and forget about the current/resitance or voltage/resitance problem, as long as I can get the input impedence of the controller input.

Paul P.

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