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Old October 22nd, 2018, 07:25 AM   #1
bitmonkey
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Types of analog, when to use which one?

Hi Guys,
I'm wondering if there are applications where 0-20mA ( 0r 4-20) might work better, or be more appropriate than 0-10v?
I have noticed that with some of my analog inputs that I seem to get more noise on the 0-10v than I do on the 0-20mA circuits, especially around the low end of the range. Even when my mA signal wire is unshielded, and the 0-10v signal is shielded. I've tried it both ways, with no significant improvement.
I guess I'm looking for a pros and cons list comparing 0-10v to 0-20mA?
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Old October 22nd, 2018, 07:33 AM   #2
James Mcquade
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as you have already noted, the 4-20 ma signal is less subject to noise.
this type of loop is current dependent, not voltage.

the 0-10 volts input is voltage dependent and is subject to noise from vfd's motor wiring, and other sources.

if I am incorrect, someone please correct me.

james
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Old October 22nd, 2018, 07:44 AM   #3
gclshortt
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https://automation-insights.blog/201...0v-vs-4-20-ma/

http://www.plctalk.net/qanda/showthread.php?t=37904

http://www.plctalk.net/qanda/showthread.php?t=91443

+1 James
The above links may help you out. If I had a choice it would be 4-20mA.
Regards,
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Old October 22nd, 2018, 08:28 AM   #4
Tom Jenkins
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Another advantage of 4-20 mA transmitters is the availability of many in 2-wire loop powered configurations. Voltage outputs are 3-wire or 4-wire.
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Old October 22nd, 2018, 08:35 AM   #5
roxusa
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Another advantage I've found is the 4-20mA is the same for a sensor at 100' of cable vs 10' of cable which makes setting the scaling up nice. the 0-10v you would have to account for the voltage drop.
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Old October 22nd, 2018, 09:06 AM   #6
mk42
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Another advantage is that 4-20 gives you easy diagnostics on a broken wire. If it reads 0, the PLC knows that theres some kind of error.
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Old October 22nd, 2018, 09:36 AM   #7
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It is an old analogy, but still true.

If you have a water pipe and you run water through it there will be a pressure drop down the pipe, so here pressure is the equivalent of volts. The volts at one end can be different than the volts at the other.

But the flow of water going in one end has to be the same as the flow going out of the other, here flow rate is current. The current at one end has to be the same as the current at the other.

It isn't a perfect analogy and as frequency increases you can get capacitive effects so that the pipe can briefly store current.

What if the pipe expands or springs a leak? The answer to those one will cost you extra.
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Old October 22nd, 2018, 10:13 AM   #8
Tom Jenkins
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Quote:
Originally Posted by roxusa View Post
Another advantage I've found is the 4-20mA is the same for a sensor at 100' of cable vs 10' of cable which makes setting the scaling up nice. the 0-10v you would have to account for the voltage drop.
I hear that a lot, but that is really less often a problem than it is usually thought to be.

Let's say your PLC input impedance is 150 k-Ohm. At 10 VDC the current is only going to be 67 micro-Amps. If you have 1,000 ft. of #24AWG your voltage drop at the input is only about 2 milli-Volt. That's negligible.
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Old October 22nd, 2018, 10:21 AM   #9
danw
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0-20mA only applies to a 4 wire device, like an analog output on a PLC, where the power to run the output electronics is supplied by the PLC.


4-20ma devices use the 3.6mA of the loop current 'below' the 4.0mA signal 'zero' threshold to power the device electronics. That's why they're 2 wire devices. The same two wires going out to the field instrument carry both the device power and the signal.
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Old October 22nd, 2018, 03:09 PM   #10
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4-20mA everyday, suitable for very long runs, often 2-wire is available, 4mA starting point allows you to easily determine signal loss over low signal (low level for instance)
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