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Jim Granville
11/17/13 12:13
Read: 870 times

#190149 - test bench vs field design
Responding to: Raghunathan's previous message
Raghunathan said:
This whole requirement arose beacuse I was trying to verify a data acquistion channel meant for a 2 wire loop powered transmitter. In the absence of a calibrator I replaced the transmittter with a combination of 5k/1.5W potentiometer + 1kOhm series resistor. It did the job - quick and dirty - but I was not at ease about the potentiometer when near the 20mA setting. [ loop was powered by a 24V dc source ]

If it is a test-bench use only, the bar is rather lower than for something that has to work z000 hours.

One aspect of wiper current, will be noise degradation, important in field-lifetime design, but a don't-care on a quick test bench.

The other detail with POTs is they tend to have a mW/mm rating - ie the element here will manage 1.5W across the whole material, but that means 50% in variable resistor mode, is 750mW if the power density is kept the same.

Since you know 24V and 20mA, another approach is to use fixed resistors to get within say 20% of your target and then pass only the 20% needed thru the pot.

List of 15 messages in thread
Power rating of potentiometers      Raghunathan      11/14/13 21:24      
   Not the point      Andy Neil      11/15/13 11:47      
      show me such        Jan Waclawek      11/16/13 13:48      
         (was) incorrect      Erik Malund      11/16/13 18:02      
            not your average "pot"      Andy Neil      11/17/13 07:40      
         Specs that are not normally told..      Raghunathan      11/17/13 03:37      
            power      Joseph Hebert      11/17/13 04:10      
            test bench vs field design       Jim Granville      11/17/13 12:13      
         That was kind of my point      Andy Neil      11/17/13 07:50      
   pot limits      Joseph Hebert      11/17/13 04:00      
      While I don't disagree ... there are different failure modes        Richard Erlacher      11/17/13 14:50      
   Bourns says: "yes."        Kai Klaas      11/18/13 16:08      
      Tesla says: no      Jan Waclawek      11/19/13 14:11      
      cermet or wirewound      Jan Waclawek      11/20/13 01:02      
         No big difference, if it all      Kai Klaas      11/23/13 06:10      

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