Displaying a 4-20mA temperature transmitter output on my PC

[Fish51]

I wonder if someone can help with the following problem.

I have a temperature controller which has a 4-20ma transmitter built into it. The loop supply is supplied externally to the
controller at 24volt dc. After reading one of the forum questions about “I want to log 4-20ma signals to a PC inexpensively.” It would seem
that a USB-6008 DAQ device will do the job for me. However am not an electronics engineer or experienced in this matter and have read on other web sites that it can be easy to blow up your DAQ device as well as your laptop from common mode voltage CMV.  I believe this device is not isolated.

It would be very helpful if someone could provide some advice on how to wire up my system and check that I am not going to blow up anything. I need very clear / simple instructions, wiring diagrams if poosile to help me in this task.

Many thanks

[Matt.S]

Hi Fish51

The USB device you want to use should be suitable for this application however I would recommend looking through the spec sheet for the device.  This will give you all the information about inputs and outputs, such as Max voltages and currents along with input inpedeances etc.  It can be found here.  Hope this helps if you have further questions then post back and I will look into it for you.

Regards 

[K C]

It's bit tricky to answer this. You already know that is east to blowup your device, so you are already warned.

Your USB DAQ device is not floating. It is connected to your laptop/PC. Unless your laptop is running without mains connected.

But is your 4-20mA device is floating then it should be no problem.

Just connect the 4-20mA output to a 500Ohm resistor and measure the voltage across the resistor. The maximum will be 10V.

Although the DAQ spec. says that there is an overvoltage protection of 35V think twice before you do this.

Kees

[Fish51]

Hi Kees

Thanks for your reply. You ask if my 4-20mA transmitter is floating or not. This 4-20mA transmitter is located in a temp controller with is powered from a 230VAC supply. Does that mean its not floating? How do i establish if its floating or not?

You mention about connecting across a 500 ohm resistor to give a 20mAx500ohms=10volt signal. Is there a reason for not connecting across a 250 ohm resistor (5v)? Better sensitivity/range?

Fish51

[K C]

Floating means that there is no connection between the two devices other then the current loop.

Since your transmitter is powered by a mains power supply I assume that the supply contains a transformer for galvanic isolation.

A load resistor of 500 or 250 Ohm will work both fine.

Kees

[Robiejo32]

hi,

I understand this post if old

But i am in similar situation. i have a few 4-20mA sensors which we want to using a pc and take data to lab view
i am thinking about a usb 4-20mA but if there are any better solution please suggest.

[K C]

Hi,

If you have a USB 4-20mA input device than you can connect it directly to your source. 

Kees

[omega_io]

there are so many 4-20mA board out in the market.

Here is a I2C 4-20mA current loop receiver board and you can use this with a usb to i2c converter.

that will give you a USB interface. all you will need to do is send commands and it will respond with the 4-20mA values. This board also has 16V power supply. so no need of external power.

Cheers!!!! 

[Robiejo32]

Thank you so much for providing me the correct receiver to use it with these sensor i still need to know about power supply connection with suggested 4-20mA current loop receiver while using 2 wire and 3 wire system can you provide me the schematic