4 - 20 mA read in parallel

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realolman

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I have been working on some equipment that involves a PLC and some 4-20mA circuits. The guy from the equipment manufacturer put a DMM set on mA on the terminals to which the 4-20mA circuit was connected and read the amperage.

He is not the first to tell me that that can be done. I don't see how you can place the leads in parallel with the 4-20mA circuit instead of in series, and get the correct reading... why doesn't it just short the circuit out?
 
he did short it out. all the current went through the meter while he had the meter hooked up that way. a common practice, but don't do it on something that will trip quickly.
 
petersonra said:
he did short it out. all the current went through the meter while he had the meter hooked up that way. a common practice, but don't do it on something that will trip quickly.
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Why would something trip?...the transmitter is a current transmitter it should reduce the output voltage to drive the correct current through the parallel loads. The meter will have much less resistance than the PLC input and most of the current will flow through the meter and a small amount though the PLC input.
 
don_resqcapt19 said:
Why would something trip?...the transmitter is a current transmitter it should reduce the output voltage to drive the correct current through the parallel loads. The meter will have much less resistance than the PLC input and most of the current will flow through the meter and a small amount though the PLC input.
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That is exactly what will happen. The PLC will see close to zero current. Many times that will be programmed to trip, think open wire detection.
 
petersonra said:
That is exactly what will happen. The PLC will see close to zero current. Many times that will be programmed to trip, think open wire detection.
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We are used to seeing "trip" referring to power source protection rather than system fault detection at the receiving end. For those who work a lot with control and alarm circuits the other meaning is more likely to be recognized.
 
GoldDigger said:
We are used to seeing "trip" referring to power source protection rather than system fault detection at the receiving end. For those who work a lot with control and alarm circuits the other meaning is more likely to be recognized.
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While I do a lot of control work, I was thinking like you...a power source trip.
 
GoldDigger said:
As I understand it, one reason it is a 4-20ma. loop rather than a 0-20ma. loop is to allow open and short circuit detection.
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Yes, most control systems are set up to give an alarm on current that is outside of the 4-20mA rannge, but we don't call it a "trip". We call it an "alarm". It may or may not shut down the process. It just depends on how critical to the process the information from that transmitter is.
 
Alarm or no alarm you are changing the load on the circuit, and I don't see what value any reading on the meter may have as you are not really measuring current through the load you intend to check. That said there may be some cases where it does work though. If you short an output device you likely will read the true or very close to true output current, but the load you shorted across will see little or no current and respond accordingly, so if that response disrupts production enough you may not want to do so. Kind of similar will happen with an input type device. Either way something will see current that does not correspond to actual conditions and corrective action will be attempted by the controls to make it right or possibly go into an alarm status.
 
realolman said:
I always disconnected the wire and put the meter in series ... Apparently that is not necessary
Hmmm
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Never gave any thought to the process but with the number,
I had to go find a fluke technical reference. Meters such as 771 - 773.
can hand it.

PS just remember that " - " means through. I frankly didn't recall.

I thinking what is a 4-20mA :)

Nice thread, thanks
 
kwired said:
Alarm or no alarm you are changing the load on the circuit, and I don't see what value any reading on the meter may have as you are not really measuring current through the load you intend to check. That said there may be some cases where it does work though. If you short an output device you likely will read the true or very close to true output current, but the load you shorted across will see little or no current and respond accordingly, so if that response disrupts production enough you may not want to do so. Kind of similar will happen with an input type device. Either way something will see current that does not correspond to actual conditions and corrective action will be attempted by the controls to make it right or possibly go into an alarm status.
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If you leave out the complication of sensor data changes based on upsetting the process, you will read different values to the extent that the transmitter is not a perfect current source.
But if the transmitter is not a perfect current source, then any voltage drop across the series ammeter is going to upset the circuit too. :)
Not a dramatic an effect, but still there.
Meanwhile, with a parallel meter, if you do not know the resistance of the loop and of the receiver you are shorting out you will not know whether you have gone out of the operating range of the transmitter or not.
I have to agree with you based on that alone!
When you allow for the possibility of a line-powered transmitter, where the input terminals are providing the current, then placing an ammeter in parallel with the input is going to really mess things up. :)

So in that specific case, where the PLC is providing the loop power, realolman's question
why doesn't it just short the circuit out?
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is very much on point!
 
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Disconnecting the wires to put the meter in series will also disrupt the signal to the PLC. Fluke has a nice little "clamp-on" that will read the 4-20mA signal.
 
GoldDigger said:
As I understand it, one reason it is a 4-20ma. loop rather than a 0-20ma. loop is to allow open and short circuit detection.
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That's one reason... :happyyes:

A more basic reason is an electronic-control device requires power to implement control. Zero current equals zero power equals zero control. :p
 
Many of the devices that the transmitter is connected to have a resistor across the inputs to convert the current signal to a voltage signal. Many of these resistors are 250Ω and you will read 1-5 volts for a 4-20mA signal.
 
cadpoint said:
Never gave any thought to the process but with the number,
I had to go find a fluke technical reference. Meters such as 771 - 773.
can hand it.

PS just remember that " - " means through. I frankly didn't recall.

I thinking what is a 4-20mA :)

Nice thread, thanks
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Are you asking what is a 4-20 mA control circuit? Or something else?

Shorting this through the meter will not harm any multimeter that I am aware of, it is low voltage and low current control circuit. Short circuit current should be in the milliamp ranges.
 
Many 4-20ma circuits are actually voltage circuits. The ma signal passes through a resistor, converting it to a voltage signal. He's just reading the voltage drop across the resistor. But a series test is a much better test, because it does not change the impedance of the circuit much. With a high impedance meter it really doesn't load the circuit much, but with an analog meter, it will give you bogus info. At least that's my experience. If you know the resistance, you can calculate the ma signal, but why bother. Series for me!
 
meternerd said:
Many 4-20ma circuits are actually voltage circuits. The ma signal passes through a resistor, converting it to a voltage signal. He's just reading the voltage drop across the resistor. But a series test is a much better test, because it does not change the impedance of the circuit much. With a high impedance meter it really doesn't load the circuit much, but with an analog meter, it will give you bogus info. At least that's my experience. If you know the resistance, you can calculate the ma signal, but why bother. Series for me!
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The original poster said that the meter was set for current, not voltage.

One of the advantages of reading the voltage drop across the input resistor is that you don't have to open the circuit to take the reading. Yes, it will have, as you said some impact on the reading, but not much and not near as much as if you set the meter to current and put it across the input terminals.
 
kwired said:
Are you asking what is a 4-20 mA control circuit? Or something else?

Shorting this through the meter will not harm any multimeter that I am aware of, it is low voltage and low current control circuit. Short circuit current should be in the milliamp ranges.
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I got stuck on the phrase 4 thru 20 mA. I don?t normally deal with LV.

With modern equipment one does not need to be opening live wires to take measurements.
 
cadpoint said:
I got stuck on the phrase 4 thru 20 mA. I don?t normally deal with LV.

With modern equipment one does not need to be opening live wires to take measurements.
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If you want to know if you are getting the correct response from say a 10 mA signal wouldn't you want to verify that you indeed do have a 10 mA signal?
 
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