this has probably been discussed, but I cant seem to find any info on how to properly wire a 4-20mA 2 wire sensor to a T3-OEM. I am guessing the wiring would be the same really as any other T3 controller. The pictures in the brochure show to connect sensors between the ground terminal and input. That however does not work for a 4-20mA sensor. Not being sure of the voltage that the input can handle, I set one of the analog outputs to manual and 100% to use it as a 10v source, my question is does it need to be 10v for the input or can I use the 24vdc input for the source of the 4-20mA sensor.
You should use 24VDC power supply for the 4-20mA sensor instead of 10V. Using one of the T3 analog outputs (AO) as a 10V source may not provide enough current, which can cause issues with the sensor’s operation.
The T3-OEM may be missing the 4-20ma option becuase its a cut down version of the T3 controllers. Hopefully when you are maneuvering in the T3 user interface it will not let you select that range setting.
The work around will be to use a 500 ohm resistor wired across the input terminal from plus to GND. Then wire in your signal to the plus, the 4-20ma current going through the resistor will be converted to a 2-10V signal. Once you have the signal working you can then head over to ‘custom analog inputs’ to configure the engieering units and scaling.
Here’s the math behind the current to voltage conversion thanks to GPT:
In the brochure for the T3-OEM it is advertised as being able to handle 4-20mA signals and you can select them as option. I suppose I should have said that I already connected it the way I described in my first post on this subject. The brochure also advertises the 10v output to be capable of 100mA, which is plenty for a 2 wire 4-20mA sensor as supply (the sensors i have are rated at 9-30vdc). But I am confused now, I have read in other posts that any of the T3 universal inputs are only good for 10V, but Lijun’s post above suggests that they are at least good for 24v at 20mA. Is that the case for all T3’s or just the OEM or?
There’s a lot to unpack here so I have broken it down to three sections:
4-20MA RANGE SETTING
Lijun will check into the brochure and confirm back on this in more detail.
As I understand it you have wired it as a normal 4-20ma transducer and configured the signal type as 4-20ma at Tab3.
If you dont see the 4-20ma option at Tab3 that means the T3-OEM doesnt have this option.
POWERING 4-20ma SENSORS
The 0-10V outputs are indeed capable of powering miscellaneous transducers in your project up to roughly 100ma per output. On some hardware you can use the H-O-A switch set to manual to get in fact, 12V out of them. If your transducer is capable of operating at 12V is should give a decent signal in your application.
T3 INPUTS MAX VOLTAGE
T3 inputs are indeed rated up to 10VDC. On board the T3 is a 500 ohm resistor which gets muxed into the circuit during the a/d conversion intermittently so its not something you can measure with a multimeter on the field wiring. As mentioned above, the 4-20ma current is only going to generate 10V max across that resistor.
CONCLUSION
The big takeaway is most of our gear does support 4-20ma input signals but they are a lot of work to commission & configure. When there’s an option to select either a 0-10V versus a 4-20ma transducer in your projects I will always recommend the 10V sensor option.
