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Old January 2nd, 2018, 09:47 AM   #1
Ne_plc_newbie
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how to check mA

I have a senor wired in one wire goes to my 24volts and the other wire goes into a terminal that has a wire coming out from the other side connecting it into a PLC Analog card and In my programming we cant get it to read accurately for temperature so they want me to check the mA but I am not sure how? do I put the black lead of my meter on the 24v and take out the wire from my sensor going into the terminal and put my red lead from my meter on that and have it switched to mA?
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Old January 2nd, 2018, 09:57 AM   #2
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I am guessing a bit at your question since it isn't stated with proper grammar, and doesn't even state correct facts. To re-state, the sensor has a 24 VDC input and a "sig" output which goes to a PLC analog card. You didn't state PN's. Presumably, this card is for reading mA signals. If true and if wired correctly, it will return the current to the power supply negative terminal.

Yes, you can read the current with a multimeter in mA mode. It should work by wiring it in series on the high side (if battery operated) as you state, but better to wire it on the low side. Simply disconnect the wire that goes to the PLC card, connect it to the "mA" input and connect the multimeter common (black lead) to the PLC card. Typically, sensors output a 4 to 20 mA signal, but some are 0 to 20 mA or other values, so read the datasheet.

BTW, mA type sensors have a "compliance voltage" spec. This means if your multimeter causes too much voltage drop in reading the mA, the sensor may stop working correctly. If <2 V drop, it should be OK. You could measure that drop with another multimeter in "V" mode.

Last edited by RocketTester; January 2nd, 2018 at 10:00 AM.
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Old January 3rd, 2018, 02:08 AM   #3
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Not trying to be funny, but make sure the leads on your multimeter are in the correct place for measuring current.

Also, some multimeters will be able to power the instrument and simulate a loop, this can be interesting to ensure the instrument is good and not being brought down by the wire.

What PLC card are we talking about? I seem to remember that Siemens, for example, sent out their cards with a default to measure voltage instead of current. This caught a couple of people where I worked previously.
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Old January 3rd, 2018, 09:17 AM   #4
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no offense meant.

what is your background?

please state the model # of the sensor, plc and input card.
is the input programmed for ma of voltage?
is the programming for the input scaled correctly? if its not, that is a lot of your issue.

james
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Old January 3rd, 2018, 09:43 AM   #5
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what PLC & analog module are you using?


also, what type of meter are you using for reading the mA?
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Old January 3rd, 2018, 10:57 AM   #6
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In many cases, a "current input" is just a precision resistor which the current develops a voltage across, which the analog input measures to determine the current flowing through it.

So, you can connect your multimeter, set for voltage, across the input directly, and measure the volts dropped across the resistor.

Typical values for this resistance is 250 ohms, (but check the specs on your particular module), so a 4 - 20mA signal would be measured as 1 - 5V.

An advantage of measuring the voltage on the input, as opposed to breaking the loop and measuring current, is that the voltage measurement will have much less effect on the current flowing in the loop, because the multimeter voltage measurement will be much higher impedance.
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Old January 3rd, 2018, 11:29 AM   #7
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Quote:
Originally Posted by daba View Post
...So, you can connect your multimeter, set for voltage, across the input directly, and measure the volts dropped across the resistor.
+1 for this method

And if you need to do this often, consider a clamp-on milliamp meter such as the Fluke 771, 772, or 773. Though quite costly.
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Old January 3rd, 2018, 02:28 PM   #8
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Quote:
Originally Posted by daba View Post
In many cases, a "current input" is just a precision resistor which the current develops a voltage across, which the analog input measures to determine the current flowing through it.

So, you can connect your multimeter, set for voltage, across the input directly, and measure the volts dropped across the resistor.

Typical values for this resistance is 250 ohms, (but check the specs on your particular module), so a 4 - 20mA signal would be measured as 1 - 5V.

An advantage of measuring the voltage on the input, as opposed to breaking the loop and measuring current, is that the voltage measurement will have much less effect on the current flowing in the loop, because the multimeter voltage measurement will be much higher impedance.
I typically use that method, but I do it for convenience. I can't really imagine a multimeter measuring mA in series adding so much impedance to a circuit that it would affect the output of a transmitter.

Using the voltage measurement, one can also measure at the instrument. Assuming that the PS is calibrated to 24 VDC, you will read a voltage between 23 and 19 VDC across a two wire instrument as 1 to 5 VDC is dropped across the analog input.
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Old January 4th, 2018, 04:22 AM   #9
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Quote:
Originally Posted by Bit_Bucket_07 View Post
I typically use that method, but I do it for convenience. I can't really imagine a multimeter measuring mA in series adding so much impedance to a circuit that it would affect the output of a transmitter.

Using the voltage measurement, one can also measure at the instrument. Assuming that the PS is calibrated to 24 VDC, you will read a voltage between 23 and 19 VDC across a two wire instrument as 1 to 5 VDC is dropped across the analog input.
The convenience extends to not breaking the circuit to insert the mAmmeter. This breaking of the circuit could, in certain circumstances, have a negative effect on the process, as it will be seen as an alarm condition. The programmer may have instigated some form of safe shut-down procedure in the event of an over-temperature, or unreliable, reading.

Measuring the Volts is the most non-invasive method, and after all is only doing what the analog input module is doing anyway.
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Old January 4th, 2018, 10:24 AM   #10
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Quote:
Originally Posted by daba View Post
the convenience extends to not breaking the circuit to insert the mammeter. This breaking of the circuit could, in certain circumstances, have a negative effect on the process, as it will be seen as an alarm condition. The programmer may have instigated some form of safe shut-down procedure in the event of an over-temperature, or unreliable, reading.

Measuring the volts is the most non-invasive method, and after all is only doing what the analog input module is doing anyway.
+1
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Old January 4th, 2018, 12:30 PM   #11
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measure the voltage over the input, if reading is anything above 1 Volt, you can use this to measure the sensor.
otherwise you will need to put the meter inbetween the input terminal, put the meter on mA, and be sure the internal fuse is oke.
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Old January 5th, 2018, 10:20 AM   #12
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Checking back. Great answers, but the OP appears to be a drive-by poster who never returned to give more details or even thanks.
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Old January 6th, 2018, 06:55 AM   #13
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Originally Posted by RocketTester View Post
Checking back. Great answers, but the OP appears to be a drive-by poster who never returned to give more details or even thanks.
Agreed, but he's a registered member, so will get email notifications of replies.... if he chooses to ignore them though, what can we do ?
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Old January 7th, 2018, 03:09 PM   #14
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An advantage of measuring the voltage on the input, as opposed to breaking the loop and measuring current, is that the voltage measurement will have much less effect on the current flowing in the loop, because the multimeter voltage measurement will be much higher impedance.
Unless you happen to have a fluke process clamp meter, which you could stick on the loop.
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Old January 8th, 2018, 05:37 AM   #15
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Unless you happen to have a fluke process clamp meter, which you could stick on the loop.
At around 350 a throw, I'm sure everyone has one lying around somewhere, just for the one time in 3 years when a loop doesn't seem to be working correctly....

Just jesting, but you get my meaning. A cheap (13) multimeter from eBay can tell you if the voltage across the input resistor is within range.
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