Controllogix analog output OF8H not reaching 20mA

[danw]

I'm not an A-B guy, but I doubt A-B's A/O card in its high end PLC line can not handle a zero ohm load.

A 4-20mA output regulates the current in the circuit up to a maximum load, typically a load spec'd from 500 to 1200 ohms.

Industrial analog outputs can regulate current in a zero ohm load. Zero ohms in a 4-20mA circuit is NOT a short across a power supply, it's part of current regulated circuit (power supplies are voltage regulated, not current regulated).

It is entirely feasible to take the load all the way to zero ohms for a regulated current analog output without a negative consequence, which includes the low resistance of a milliameter current input.

I don't have a solution, but the absence of a resistor load is not the issue.

 

[daba]

I'm not an A-B guy, but I doubt A-B's A/O card in its high end PLC line can not handle a zero ohm load.

A 4-20mA output regulates the current in the circuit up to a maximum load, typically a load spec'd from 500 to 1200 ohms.

Industrial analog outputs can regulate current in a zero ohm load. Zero ohms in a 4-20mA circuit is NOT a short across a power supply, it's part of current regulated circuit (power supplies are voltage regulated, not current regulated).

It is entirely feasible to take the load all the way to zero ohms for a regulated current analog output without a negative consequence, which includes the low resistance of a milliameter current input.

I don't have a solution, but the absence of a resistor load is not the issue.

The only answer I can give at this time...

V = IR

If you make R = 0, then both V and I must be zero also.

The driving circuit has to have a reference, something it can measure, and that usually is an internal resistance it measures the voltage across.

So the driver circuitry already has a load, across which it derives the current it is delivering. It is the capability of the driver circuit that determines the additional load (the receivers) that it can supply the regulated current to.

I agree it is technically possible for a 4-20mA transmitter to deliver 4-20mA into a zero-ohm load, but what would be the point? The receiver has to measure current, and that means putting a resistance in the circuit, across which it measures voltage. It is so much easier to measure voltage than current, nearly all current measurement technology uses volt-drop across a resistance.

 

[danw]

>I agree it is technically possible for a 4-20mA transmitter to deliver 4-20mA into a zero-ohm load, but what would be the point?

2 reasons for allowing a current regulated AO circuit to drive a (near) zero ohm load:

1) a circuit short should not damage the output, and it never does. An AO will sit there all day driving up to 20mA through a near zero ohm short piece of copper wire jumpered across its output terminals without any damage to the output. The load will see the current and will not operate as intended, but a wiring short will damage the AO (loop powered transmitter loops can be damaged if the transmitter is shorted because the current regulation is eliminated and a voltage regulated power supply drives the load.)

2) the use of a milliameter with its low resistance input for basic performance measurements

If a minumum resistance load were required, it would be spec'd; yet a minimum load spec never is found because AO's can easily drive zero ohm loads.

I've run into single loop controllers that use a 1 ohm analog input resistor and a 2 wire transmitter will easily regulate the 4-20mA current in the loop.

I haven't counted the number of times I have measured loop current by directly connecting either an analog or a digital milliameter to an analog output card (with no other circuit load) with reliable and predictable results, but the number is in the hundreds. A milliameter load on an AO is just not an issue.

The issue with a zero ohm load is near zero ohm short in a voltage regulated circuit, when a short circuit current skyrockets attempting to maintain a regulated voltage; which can't happen across a zero ohm load. Hopefully, crowbar protection or fusing saves the wiring, but that's not case with current regulated analog outputs.

 

[Ron Beaufort]

this is going to sound silly - but when all else fails, etc. - etc.

are you ABSOLUTELY SURE that you have the wiring correct - and that you are using a DC milliamp meter - and not an AC type? ... double-check the settings ...

also, make SURE that you have the test probes (the leads) inserted into the correct terminals on your meter ... I've seen some guys leave the leads connected to the COMMON and to the VOLT/OHM terminals by mistake while trying to make a CURRENT test ... on many (most?) digital multi-meters you would want to use the COMMON and the MA terminal for the test you're trying to make ...

tell us what brand and model meter you're using ... maybe that will lead to something useful ...

do you have any other "working" 4 to 20 mA circuits that you could measure - just to make sure that your testing equipment - and your testing techniques - are being used correctly? ...

you mentioned that you are "forcing" values into the output tags while trying your tests ... are you sure that you have the forces ENABLED? ... are you entering the values into the "Force Mask" area? ...

 

[Bering C Sparky]

I've seen some guys leave the leads connected to the COMMON and to the VOLT/OHM terminals by mistake while trying to make a CURRENT test ... on many (most?) digital multi-meters you would want to use the COMMON and the MA terminal for the test you're trying to make ...

Good point here Ron, this can be a real inconvenience, in not being able to see your current measurement if the meter is not set properly.

Not to take this thread off subject but; If I may also add:

I have seen some guys forget to switch the leads back and then attempt to test high voltage.
It is even more important to be sure and switch the leads back to Volts and Common before attempting to test High Voltage.
If you dont you will get much more than inconvenienced.
The magic smoke will come out, you may see some very pretty lights and whatever process you were testing will more than likely be stopped until you reset the over current protection, and possibly make repairs to the area where you placed your test leads.

BCS

 

[bit3399]

Dear All,

Thank you for your support/directives.

The analog output card is now giving out 20mA with respect to the EU i.e
@EU=100=20mA
@EU=0=4mA

The problem was with the multimeter.

• As earlier recommended, I initially tried a 250 Ohms across the analog output terminal but still gave me the same 4mA @ EU=100
• Again, based on another recommendation, i tried using the meter to measure another current source (4-20mA) and that was it! I discovered the meter still displays 4mA even when the injector is set to give out 20mA. At this point , i now discovered its the multimeter since i have been using the same multimeter for the test with the signal injector before.
• I opened it but might be that some of the circuitry for measuring current is damaged (not sure), though its still being able to measure voltage correctly
• The multimeter (UNI-T, Model: UT39A) was replaced with a new one (UNI-T, Model: UT39C)
• After this, i tried forcing the output again, and it starts working correctly, giving the expected measurement from the analog output module.

Now that im sure the card is working, the next is to configure my PID block.

Thank you all once again, it was a great learning curve for me.