Measuring Current

efigalaxie

Member
Join Date
Aug 2015
Location
Salem, VA
Posts
23
I just was hit with something that threw me for a loop.

With a three wire 4-20ma loop, another tech simply placed his Fluke meter (set for measuring DC ma current, leads on the meter correctly) across the negative and signal lines for the sensor and read current. I broke the loop and inserted the same meter into the loop and it read the same. He told me on 2 wire loops that this would blow the meter fuse, but on the 3 wire, it wouldn’t. This is really bothering me. Why can we get an accurate reading both through the meter and placing the meter across the loop - in a 3 wire loop?
 
I've never measured a current loop across terminals, only breaking the loop.

I would be interested to know how he got the same reading.
 
Did you use the same meter in both options? I would say the meter is broken... but that's me.

Edit 1:

Just noticed it is the same meter... did you do something to see the value change in the meter?

Edit 2:

Not sure if this is it, but on a two wire loop if you put it across the + and - of the loop you are effectively shorting it and the input card may allow more than the fuse in the meter.
On a three wire instrument, you are shorting the output of the instrument which should be limiting it's output to 22 or something mA... This however doesn't make it right as you are creating a current divider between the meter and the input card on the PLC. However, the input impedance on the analog card may be much higher than the meter.
 
Last edited:
It was the same meter. Both across and in series with the sensor gave the same reading. The sensor is an AB IO-Link. 45LMS-D8LGC1-D4 Diffuse 0 - 2.8M eye. Yes, this messed me up too. Wait..... can it be that a 3 or 4 wire sensor’s return is a constant current source for the level of detection? IE if it is putting out 9ma for a particular stimulus, it will do so as long as the power supply can handle it. I have an idea. I am going to set this up and read the voltage off the output with only a resistor loading it, only a meter loading it, and both......
 
Last edited:
...
On a three wire instrument, you are shorting the output of the instrument which should be limiting it's output to 22 or something mA... This however doesn't make it right as you are creating a current divider between the meter and the input card on the PLC. However, the input impedance on the analog card may be much higher than the meter.

This is what's happening. Fluke 87 happens to be 1.8ohms on mA scale, where most input cards are 200-250ohms... negligible current split.
 
That didn’t work. I can read voltage as long as the other meter isn’t across the output...set for ma. Trying to read across for voltage with the other meter across set for ma is essentially reading voltage across the short presented by the meter set for ma........nothing. The reading is being taken and is right. I am betting if you watched this on a plc analog input, the input would show nothing when using the meter across the loop return. I think there is a happy coincidence here. The input card presents 250 ohms to the loop. The input impedance across the meter is 2ohm. The meter uses voltage across that internal resistance to calculate amperage. Since the sensor is outputting a constant current relative to the stimulus, I am betting voltage changes relative to the load presented to the output of the sensor.
 
This is what's happening. Fluke 87 happens to be 1.8ohms on mA scale, where most input cards are 200-250ohms... negligible current split.

+1

Note that on a two-wire loop the entire current capacity of the power supply will go through the Fluke and blow its fuse. On a three-wire transmitter, the power supply is on the third wire, and the other two are signal only. Draw out the circuit and it will make sense.
 
This is what's happening. Fluke 87 happens to be 1.8ohms on mA scale, where most input cards are 200-250ohms... negligible current split.
Yes...exactly......so if I were to look at the output of the sensor through the plc input.....it would be nothing because the entire current is essentially flowing through the meter. Since the sensor maintains a constant current output for a given stimulus, the voltage driving the current drops....ergo....we get a current measurement close enough for Jazz.

This works, but boy do I see possible issues
 
Interesting, I never thought about it that way. So you're essentially "stealing" current from the other device. I guess it works if you don't mind the momentary loss of signal.
 
So you're essentially "stealing" current from the other device.

The 'Path of Least Resistance' is a fundamental physics concept; it is also socially true...:D...

This works, but boy do I see possible issues

Major issues I might say...It is a 'trick of the trade'...Always open a circuit prior to (non clamp-on) current measurements...
 
Last edited:
If you put it across the two terminals of the 2wire transmitter, you will get 24V/(1.8+200 ohms) ~ 120mA.

If you put it between the return and the 0V* of your system, you should get the same reading as your 3 wire system.

*0V will not be sent out to the transmitter so you will make this measurement in your cabinet.
 
The tech involved was rather pleased with agitating me until one of the top guys advised him that the consequences could be bad. Thank you all for your help. BTW.... we use the blade terminals a lot here. Those things are expensive too. We get the Phoenix brand....more than the automation direct.
 
The tech involved was rather pleased with agitating me until one of the top guys advised him that the consequences could be bad. Thank you all for your help. BTW.... we use the blade terminals a lot here. Those things are expensive too. We get the Phoenix brand....more than the automation direct.

I use a lot of Phoenix and Automation Direct. But I really like Phoenix terminal blocks.
So... I show the Automation Direct pricing (on the their terminals) and make Phoenix compete. They get very close..
 

Similar Topics

Hi, I need a program for measuring the current consumption of Equipments ( Pumps, Compressor,etc). and data logging of these measurement should...
Replies
13
Views
3,967
i want to measure current from CT using multiple options. at present my CT is connected to Analog Amps meter. i want to plan for update to PLC AI...
Replies
8
Views
4,163
I have done some tests to measure the current at the output of a variable frequency drives. Of course you should look at the current that the...
Replies
24
Views
15,389
hi, can anyone help,i have 6 x 3phase (415v) heater circuits which i would like to view the current on each phase to see if any heaters are...
Replies
3
Views
1,830
Hello, We are about to deleiver a system to the US. Since we are running at 400V 50Hz here in Sweden I have connected a frequency converter to...
Replies
7
Views
4,094
Back
Top Bottom