Output current requirement

kvogel

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Join Date
Jan 2011
Location
Colorado
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I am assisting a coworker with the specification of a replacement valve controller and found this in the valve's project planning information.
I don't understand what it's asking for and what I need to be looking for on the output side. How is the output current determined, I thought the load determined the current and all I needed to worry was if the maximum load was within the current rating of the output. Looking to learn something new.

5.1.2 Digital inputs
The digital inputs require a current of 3 mA ±1.5 mA. To
avoid interference, it may be necessary to increase the
output current by using an additional load resistor on
the signal sensor.
Load resistors may not be fitted inside the IC 40 for reasons
relating to heat dissipation.
Example for 24 V DC and 10 mA:

Thanks
 
I think they're asking for load resistors on discrete inputs to pull down leakage current through certain sensors that might otherwise cause a false "on" state at the input.
 
Yes, It's for a butterfly valve controller.



I like the current leakage idea Bit_Bucket_07

Is this right? Secondly, the controller we're using has a relay output so I'm assuming it doesn't matter???

Thoughts?



Cheers
 
Last edited:
Yes, It's for a butterfly valve controller.



I like the current leakage idea Bit_Bucket_07

Is this right? Secondly, the controller we're using has a relay output so I'm assuming it doesn't matter???

Thoughts?



Cheers

Even relay outputs typically employ surge suppression across the output contacts, thereby allowing a very small amount of leakage current.
 
In the example above, is the 10ma current something that was measured? I don't recall ever seeing this as a specification in the PLC documentation.

Thanks for the assistance
 
You are correct, the load determines the current draw.

If you are using a PLC relay output for control, then the relay needs to switch a voltage, either 100-230Vac or 24Vdc (polarity is important for a DC connection).

The reason the resistor is mentioned is that solid state Digital Outputs need a minimum load in order to function properly. If the actuator's Digital Input load current is lower than the minimum PLC Digital Output current requirement, then a resistor is added to the field circuit to increase the load current so that the PLC DO operates properly.

The actuator requires permanent AC power on the N, L1 terminals, 100-230Vac 50/60Hz.

The digital inputs do NOT drive the actuator motor directly, they are merely logic inputs. The actuator's motor is powered from the line.
IC40-internal-schematic-wiring-connections.jpg
 
Last edited:
Thanks everyone

I guess I have been lucky enough to never have had outputs not function due to the load being less than the minimum switching current of the device.

Ken
 
It is not a problem of minimum required current in the IC40 input, it anyways will continue to absorb 3 mA ±1.5 mA. The possible problem is interference in the cable.

What happens is that this small current of 1.5 mA is likely to be generated by interference and then the I40 module will receive a false signal.

Adding the resistance increases the threshold to 8.5 mA and is more difficult to happen this false signal.

But the resistance must be added in the terminals of IC40, not in the PLC
 
lfe said:
It is not a problem of minimum required current in the IC40 input, it anyways will continue to absorb 3 mA ±1.5 mA. The possible problem is interference in the cable.

What happens is that this small current of 1.5 mA is likely to be generated by interference and then the I40 module will receive a false signal.

Adding the resistance increases the threshold to 8.5 mA and is more difficult to happen this false signal.

But the resistance must be added in the terminals of IC40, not in the PLC

I agree with this analysis. However, that last part I "think" may need to be flipped around?...

...Load resistors may not be fitted inside the IC 40 for reasons
relating to heat dissipation.

This appears to state that load resistors must not be fitted inside the IC 40 for risk of overheating and would instead have to be fitted at the output end (PLC)?

G.
 
7mA x 24V = 168mW

I do not think that power will generate much heat.

I installed hundreds of Kromschoreder servos in the past and the primary source of heat is not going to be that resistance but the proximity of burners boilers and kilns, which is where they are usually mounted.
 
lfe said:
7mA x 24V = 168mW

I do not think that power will generate much heat.

I installed hundreds of Kromschoreder servos in the past and the primary source of heat is not going to be that resistance but the proximity of burners boilers and kilns, which is where they are usually mounted.

The above reason might explain to us why a load resistor "could" be fitted in the actuator without causing any overheating issues, and that may be fine.

But you had written...

ife said:
...the resistance must be added in the terminals of IC40, not in the PLC

If you are standing over this statement, why do you say that they "must" be fitted in the actuator? And why do you say that they must "not" be fitted at the PLC? I'm just wondering what you are basing your advice on which is apparently the complete opposite to the manufacturer's advice?

Also, one might imagine that, if the heat were not an issue, it should not matter where they are fitted?

However, I too have experience using these actuators and have my own opinions on why the manufacturer might advise us not to fit resistors in the actuators. I also have a good reason why I adhere to the manufacturer's specifications for such devices, irrespective of whether I might think it critical or not. But those opinions and reasons might be for later.

Regards,
George
 
if the resistance is installed on the PLC side then if an interference is able to generate a 1.5 mA current in the long cable run then a false signal will still be generated, instead if it is installed in the servo, the threshold will be increased to 8.5 mA .

There is no doubt that the long cable path is the most susceptible part to interference and that is where the minimum current must be increased.

The resistor also has some effect installed in the PLC since it reduces the impedance but for me there is no doubt that it is much more effective in the servo.

For similar reason as for example in an RS-485 loop the resistors are installed at the ends.

And I know very well that I disagree with the Kromschroeder manual, but I have several times installed resistors inside, resistors and especially many Varistors to reduce the noise generated by the motors.
 

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