250 ohm vs 255 ohm resistors

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TwoBlocked

TwoBlocked

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Hey folks, been noticing that some process equipment uses 255 ohm resistance in their 4-20 mA sense devices instead of the typical 250 ohm resistance. Any idea why?
 
230527-1125 EST

It does not matter. Within a reasonable range, it has no effect other than the calibration constant used in the receiver where the resistor is located.

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Perhaps so at the low side of a 1% or 2% tolerance they're still >= 250 ohms? Well 249.9 for a 2% 255 but thats close enough.
 
The 250 ohm resistors I purchased were 1/4 % tolerance. At the equipment EG recorder, the resistor converts the mA into voltage, which is what the recorder input needs. 25O or 255 ohms, it’s just a matter of scaling the input to 100% at the resistance
 
I have not seen this but I don't think it makes any real difference. None of these resistors are perfect and you have to calibrate the input for what the resistor actually is no matter what resistance you actually use.
 
tom baker said:
The 250 ohm resistors I purchased were 1/4 % tolerance. At the equipment EG recorder, the resistor converts the mA into voltage, which is what the recorder input needs. 25O or 255 ohms, it’s just a matter of scaling the input to 100% at the resistance
Click to expand...
1/4% tolerance? Or 1/4w power rating?
 
winnie said:
If you are using resistors for instrumentation, 1/4%, 0.1% and 0.01% are quite common.
Click to expand...

Yeah, I have a reel of surface mount 0.1% 10K resistors I used for voltage dividers to measure DC voltage with microcontrollers. I recall them being a little pricey, one of those things left over from 15 years ago that I can't manage to get rid of.
 
230528-2007 EDT

What one wants for the sensing resistor for a current loop system is stability. The resistance can be any reasonable, but stable value.

Other factors than resistance are what define a value as reasonable. For example the voltage rating of components in the loop.

..
 
If it didn't matter, I don't think the manufacturers would start specifying 255 ohm. I am wondering if it is an easier way to switch from voltage input to mA input. Maybe having a high impedance resistor built into the input device to read 0-5 volts from the input, and then the customer adds a 255 ohm resistor in parallel with the input to achieve the desired 0-5volts with a 0-20 mA input.

But has anybody else seen this 255 ohm resistor requirement? I believe it was ABB devices.
 
230529-2112 EDT

TwoBlocked:

20 MA thru a 100 ohm resistor produces 2 V. 20 MA thru a 1000 ohm resistor produces 20 V. Most solid state operational amplifiers will directly work with +/- 12 V or +/- 15 V as supply voltage. Thus, it is usually reasonable to work with signals up to +/- 10 V.

So the actual voltage from the current sensing resistor has a lot of latitude. What is important is the stability of the current sensing resistance at some reasonable value.

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TwoBlocked said:
If it didn't matter, I don't think the manufacturers would start specifying 255 ohm. I am wondering if it is an easier way to switch from voltage input to mA input. Maybe having a high impedance resistor built into the input device to read 0-5 volts from the input, and then the customer adds a 255 ohm resistor in parallel with the input to achieve the desired 0-5volts with a 0-20 mA input.

But has anybody else seen this 255 ohm resistor requirement? I believe it was ABB devices.
Click to expand...
I have not noticed, but I installed an ABB i\p last week and now I'm curious to what it was...
gar said:
230528-2007 EDT

What one wants for the sensing resistor for a current loop system is stability. The resistance can be any reasonable, but stable value.

Other factors than resistance are what define a value as reasonable. For example the voltage rating of components in the loop.

..
Click to expand...
Why is it, on some Hart smart instruments, you have to add a resistor in line for the communicator to "see" the device?
 
Some temperature transmitters we bench test and set up need a resistor in line with the communicator to for it to see it and change parameters. Usually, our DCS has it one place when connecting the communicator. But on the bench, sometimes we have to add it to a 2 wire 24vdc device. Jist trying to get an actual understanding, I realize it needs it to see it, but why is what I'm wondering now.

Otherwise, it will say "no device connected".
 
230531-1237 EDT

4-20 mA:

In the real world there is no current source that has infinite dynamic range. In other words there is only a limited range over which some device can operate as a constant current source.

A constant current source is some system that can provide a constant current output independent of of load resistance.on that source. Why is this an important capability in an instrumentation system? Because you have widely different line resistances from transducers to receivers. It reduces wire size and produces high accuracy.

At the receiving end one usually wants some particular voltage range. Thus, a sensing resistor is needed to convert the current signal into a voltage signal.

.
 
gar said:
230531-1237 EDT

4-20 mA:

In the real world there is no current source that has infinite dynamic range. In other words there is only a limited range over which some device can operate as a constant current source.

A constant current source is some system that can provide a constant current output independent of of load resistance.on that source. Why is this an important capability in an instrumentation system? Because you have widely different line resistances from transducers to receivers. It reduces wire size and produces high accuracy.

At the receiving end one usually wants some particular voltage range. Thus, a sensing resistor is needed to convert the current signal into a voltage signal.

.
Click to expand...
hence the 1 volt @ 250ohm gives the 4mA and makes it 'steady/repeatable' and 20mA @ 5? I think I'm catching on there. Thanks for some clarification.
 
230531-1515 EDT

4-20mA:

Yes.

Now add 1 ohm wiring to the series current loop, and so long as the current source can maintain the 20 mA current when the source wants to send 20 mA, then you will still get the 5 V at your input.

..
 
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