Electrical Doubt

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Nov 2009
Location
India
Posts
116
Hi

My PLC control panel is supply with 24 VDC, its quiet common.

I am using Analog outputs which is configured to 4 to 20 ma. I can also measure 4 to 20 ma by forcing various values to analog output channel.

But the problem is when i measure the voltgae across the thse 4 to 20 ma terminal i getting 30V, how can it be possible, then i check the power supply it is measuring 24V only. I restarted the panel, later i measure it showing 24V.

I am in doubt why this happened before and why not again after restarting power supply and what is the normal voltage across these 4 to 20 ma terminal, its 24V right?
 
What you must understand is that you are working with 4-20 ma loops, not voltage.
This means that your current output devices will produce 4-20 ma signals regardless of the voltage if your loop is connected correctly.
That being said, there is typically a voltage limitation to these devices, usually 30 volts.

So you will measure different voltages at those terminals,
the key is how your loop is connected to the load and its associated resistive load.

For example, a 500 ohm load,
using ohms law, volts= current * resistance.
at 4 ma, you will read 2.00 volts
at 20 ma, you will read 10 volts

regards,
james
 
Your best bet is to get a milli-amp reader/generator but most of your better multi meters will have milli-amp function. When checking milli-amp you must put your meter/reader in line with the circuit. You may not just probe the circuit. If the meter is not in line with the circuit you will not get an acurate reading.
 
When checking milli-amp you must put your meter/reader in line with the circuit.

And unless your memory is better than mine, you need to stock fuses for your meter. Because I very often forget to change the meter lead plug from the ma socket to volts/ohms socket after I'm done reading the ma.:whistle:
 
And unless your memory is better than mine, you need to stock fuses for your meter. Because I very often forget to change the meter lead plug from the ma socket to volts/ohms socket after I'm done reading the ma.:whistle:

And
I am so glad to see that someone else makes the same mistake also. The fuses are pricy so I started buying in the five pack.

Dan Bentler
 
Later i found thers is problem in wiring. The analog ouptut channel i am using can give voltage and also current output.

There are 3 terminals V+,I+ and Comm. So for current output it should connect between I+ and Comm but it was connected between V+ and I+.
My one more doubt how can i get 4 to 20 ma when forcing the Analog output channel with these terminals because there is no common both the terminals are positive, how this possible...................
 
If you use I+ to COMM you will get 4-20ma. If you use V+ to COMM you will get 0-10 VDC. I don't know which PLC your using but I almost positive you will need top go into your processor I/O setup and select the outpuit you want to use, 0-10 or 4-20. Hope this helps.
 
I am using GE Fanuc PLC, the mdule is configured for 4 to 20ma but the wiring in the module made wrong between V+ and I+.

Still i can able to read 4 to 20ma how is possible.............
 
I suggest that you look at the manual for the GE card you are working with. You also might want to get in touch with the local GE sale rep for tech support.

Only then, can you determine what's right and what's not.

regards,
james
 
When refering to Ohoms Law V=voltage I=Current and R=resistance. You said your module has V+ I+ and Comm. V=voltage output, I=Current output and Comm=Ground. You will not ever put I and V together in a circuit as they are bassically the same output, there is no potential. If you use I+ to COMM you will read 4-20 ma. If you use V+ to comm you will read 0-10 vdc. If you use I+ to V+ you will get nothing or really weird readings. If you want to send 4-20 to a device like a drive for a speed reference then hook the I+ and the comm to the drives 4-20 reference input.

NOTE! Make sure the drive or device is set for external reference. Most drives let you select if you want to use an externel source or the drives own internal source. Some drives and devices require you to provide a source, Others require you to use thier internal source.

Note! Make sure your PLC is internally sourced and does not require source from the drive or device.

In the end result there can be only one source. The plc and device can not both be the source.

If you hook the red lead from your meter on to the I+ and the black lead to the comm of your PLC you should be able to read a milliamp output. If there is a device hooked to the plc then you will need to hook the red lead of the meter to the Negative lead of the device and the black from the meter to the comm of the plc.

NOTE! make sure you switch the meter lead to MA on your meter or the aforementioned of this post will happen.

I hope this clears this up for you.
 
When refering to Ohoms Law V=voltage I=Current and R=resistance. You said your module has V+ I+ and Comm. V=voltage output, I=Current output and Comm=Ground. You will not ever put I and V together in a circuit as they are bassically the same output, there is no potential. If you use I+ to COMM you will read 4-20 ma. If you use V+ to comm you will read 0-10 vdc. If you use I+ to V+ you will get nothing or really weird readings. If you want to send 4-20 to a device like a drive for a speed reference then hook the I+ and the comm to the drives 4-20 reference input.

NOTE! Make sure the drive or device is set for external reference. Most drives let you select if you want to use an externel source or the drives own internal source. Some drives and devices require you to provide a source, Others require you to use thier internal source.

Note! Make sure your PLC is internally sourced and does not require source from the drive or device.

In the end result there can be only one source. The plc and device can not both be the source.

If you hook the red lead from your meter on to the I+ and the black lead to the comm of your PLC you should be able to read a milliamp output. If there is a device hooked to the plc then you will need to hook the red lead of the meter to the Negative lead of the device and the black from the meter to the comm of the plc.

NOTE! make sure you switch the meter lead to MA on your meter or the aforementioned of this post will happen.

I hope this clears this up for you.

I don't think you're getting it...He understands how a 4-20 signal works and how to use it. He is saying that he realized that his module was wired incorrectly. BUT...even though it was wired incorrectly, he was still getting a current reading of around 4-20 milliamps. He is asking how this is possible with it wired incorrectly. And, the answer is exactly what you already said...you can get really weird readings when using the wrong wiring setup. It looks like he just happened to get something that was kind of close to what it should have been had it been wired correctly. Coincidence I guess...
 

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