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Old March 12th, 2017, 11:01 AM   #1
busta9
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Testing 4-20 mA analog inputs and outputs

Hello,

I am wondering what the best practices are for testing analog 4-20 mA inputs and outputs.

I would like to buy or make a 4-20 mA source to connect to the inputs and watch the changes in the plc program. My question is what voltage should the source be? is 24Vdc too much? What spec on the data sheet for the cards do I look at to verify acceptable input voltages? Based on input range spec I see 10V if I am using the voltage option of the card so is 10Vdc the limit of the 4-20 mA source I am using as well.

For testing 4-20 mA outputs, can I simply disconnect the wires to the output im testing and use my DMM on mA setting to see the values change between 4-20 mA?

If I do not have a meter thats able to measure mA can I disconnect the output wires and use a resistor across the terminals and measure voltage without damaging the card?

How do you guys test analog mA input/output cards and what equipment do you use?


Thanks for your help.

Last edited by busta9; March 12th, 2017 at 11:08 AM.
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Old March 12th, 2017, 11:12 AM   #2
Bob O
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I use this meter that I purchased from Mark. He also has 4-20 signal generators.

http://www.plccable.com/fluke-789-pr...on-multimeter/

http://www.plccable.com/brands/Troubleshooter.html

HTH
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Old March 12th, 2017, 11:52 AM   #3
rQx
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Use a potentiometer with a resistor in series to simulate inputs 4-20mA

24VDC power source and 4mA correspond to 6000ohm and 20mA correspond to 1200ohm.

So ideal would be an 0-4800ohm potentiometer with a 1200ohm resistor in series.
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Old March 12th, 2017, 01:07 PM   #4
Mickey
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Buy don't build... $10

http://www.plctalk.net/qanda/showpos...4&postcount=65

http://www.ebay.com/itm/DC-12V-24V-4...AAAOSw8oFXzdCR
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Last edited by Mickey; March 12th, 2017 at 01:09 PM.
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Old March 12th, 2017, 01:20 PM   #5
cardosocea
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This is quite possibly overkill, but it was my go-to tool for any current loop:

http://www.fluke.com/fluke/uken/proc....htm?pid=56133

There's also this possibility:

http://www.fluke.com/fluke/uken/Proc....htm?PID=69405

The current clamp was pretty cool in some cases to avoid disconnecting wires or breaking connections.

Both of them also simulate loops and the first one Voltage signals if I remember correctly.

This way you have a solution for both sides of the price spectrum.
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Old March 12th, 2017, 03:00 PM   #6
geniusintraining
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Quote:
Originally Posted by Bob O View Post
I use this meter that I purchased from Mark. He also has 4-20 signal generators.
Thanks Bob!

Busta9, we sell a lot of these kit http://www.plccable.com/electrical-t...th-case-leads/

We can't come close to the one Mickey pointed out, our cases cost more then that.... but ours are made here in the states by a PLCTalk member, they also have 0-10VDC and 4-20MA you also do not need to supply power, that said I have also told customers about the ebay ones, if you just need 4-20 and you can wait a month its a great tool and unbelievable price, you can have ours Tuesday we ship daily to Canada
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Old March 12th, 2017, 03:52 PM   #7
danw
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Quote:
Originally Posted by busta9 View Post
Hello,I would like to buy or make a 4-20 mA source to connect to the inputs and watch the changes in the plc program. My question is what voltage should the source be? is 24Vdc too much? What spec on the data sheet for the cards do I look at to verify acceptable input voltages?
Whatever device sources the current will tell you what voltage it needs to do so. Some DVM's use s 9Vdc battery. The $10 cheapie mentioned above can use 9-30Vdc.

Quote:
Originally Posted by busta9 View Post
Based on input range spec I see 10V if I am using the voltage option of the card so is 10Vdc the limit of the 4-20 mA source I am using as well.
No, wrong interpretation. The voltage limit is the voltage reading limit of the AI card. For any AI card to read a current, the current is converted to a voltage by forcing the current through a resistor and then reading the voltage drop across the resistor. A handheld or bench meter does the same thing.

In your specific case, the limit is the 10Vdc voltage drop across whatever resistor you pick for your voltage range (assuming the current source voltage is sufficient to drive the current through the resistor. If the source voltage is too low, the current will not get to 20mA).

Quote:
Originally Posted by busta9 View Post
For testing 4-20 mA outputs, can I simply disconnect the wires to the output im testing and use my DMM on mA setting to see the values change between 4-20 mA?
Yes, if your meter has a current range suitable for 20mA.

Quote:
Originally Posted by busta9 View Post
If I do not have a meter thats able to measure mA can I disconnect the output wires and use a resistor across the terminals and measure voltage without damaging the card?
Yes, but any error in the resistance value of the resistor from what you think the resistance is will produce a proportional voltage drop error. For instance, if you think the resistor is 250 ohms and it should drop 5.00V at 20.0mA but the resistor is really 245 ohms, then 20.0mA will drop 4.90V, not 5.00V.

Quote:
Originally Posted by busta9 View Post
How do you guys test analog mA input/output cards and what equipment do you use?
I assume new one AI works properly unless something indicates otherwise.

If I need to check a mA input to prove that it is seeing a signal, I use either a battery powered cheapie $10 source with a digital meter in series or I use a handheld calibrator source, depending on which one is closer at hand.

I use a precision 250 ohm resistor (0.1%) when the analog input requires an external dropping resistor.
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Old March 12th, 2017, 06:39 PM   #8
busta9
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Quote:
Originally Posted by danw View Post
Whatever device sources the current will tell you what voltage it needs to do so. Some DVM's use s 9Vdc battery. The $10 cheapie mentioned above can use 9-30Vdc.

No, wrong interpretation. The voltage limit is the voltage reading limit of the AI card. For any AI card to read a current, the current is converted to a voltage by forcing the current through a resistor and then reading the voltage drop across the resistor. A handheld or bench meter does the same thing.

In your specific case, the limit is the 10Vdc voltage drop across whatever resistor you pick for your voltage range (assuming the current source voltage is sufficient to drive the current through the resistor. If the source voltage is too low, the current will not get to 20mA).

Yes, if your meter has a current range suitable for 20mA.

Yes, but any error in the resistance value of the resistor from what you think the resistance is will produce a proportional voltage drop error. For instance, if you think the resistor is 250 ohms and it should drop 5.00V at 20.0mA but the resistor is really 245 ohms, then 20.0mA will drop 4.90V, not 5.00V.

I assume new one AI works properly unless something indicates otherwise.

If I need to check a mA input to prove that it is seeing a signal, I use either a battery powered cheapie $10 source with a digital meter in series or I use a handheld calibrator source, depending on which one is closer at hand.

I use a precision 250 ohm resistor (0.1%) when the analog input requires an external dropping resistor.
Thank you for this very detailed response. You answered all my questions.

I appreciate everyone else for taking the time to respond. It has been very informative.

Thank you all.
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Old March 13th, 2017, 12:26 AM   #9
Geoff White
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The most ghetto tester I ever saw - but didn't try - was from a guy that insisted that 9V battery delivers a max of 20ma. He may have been right - we has about a lot of things.
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Old March 13th, 2017, 02:38 PM   #10
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look in this forum and find a simple schematic with a LM317
make a few of them.
Standard input is a resistor of 250 or 500 ohms on the input, Here the current is important, but it is measured as a voltage.
for the outputs you can use simple resistors. like 250 ohms and measure the voltage as you say.
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Old March 14th, 2017, 01:27 AM   #11
briancr
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@Busta9.

What do you require?

A simple go / no go tester. If so just get a cheapo current source. The LM317 unit works well for simple testing.

If high precision is required, look at the Fluke's mentioned above
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Old March 17th, 2017, 10:51 AM   #12
Jsu0234m
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Quote:
Originally Posted by cardosocea View Post
This is quite possibly overkill, but it was my go-to tool for any current loop:

http://www.fluke.com/fluke/uken/proc....htm?pid=56133

There's also this possibility:

http://www.fluke.com/fluke/uken/Proc....htm?PID=69405

The current clamp was pretty cool in some cases to avoid disconnecting wires or breaking connections.

Both of them also simulate loops and the first one Voltage signals if I remember correctly.

This way you have a solution for both sides of the price spectrum.
The Fluke 773 sources 0-10volt and 4-20mA as well as reads both of them. Its definitely worth it if you do a lot with analog signals. We have the 773 and it works great.
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Old March 18th, 2017, 11:01 PM   #13
MrAnonEMoose
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Quote:
Originally Posted by Geoff White View Post
The most ghetto tester I ever saw - but didn't try - was from a guy that insisted that 9V battery delivers a max of 20ma. He may have been right - we has about a lot of things.
9V applied to a total loop resistance of 500 Ohms is 18mA. Pretttttty close.
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