Change sensor's common to low!!!

[auto_mitch]

Hi.
i have a problem with a pressure sensor. The sensor output is 4-20mA.
The common of the output is at high (+24V). I want to change the common to low (0V, GND) to plug the sensor to plc.
Could u help me???

 

[zachtheterrible]

a schematic would be very helpful.

 

[williB]

Hi.
i have a problem with a pressure sensor. The sensor output is 4-20mA.
The common of the output is at high (+24V). I want to change the common to low (0V, GND) to plug the sensor to plc.
Could u help me???

i have a similiar problem.
I have a micorcontroller although i am not going to say which one, or provide a link.
anyway one pin seems to be stuck , when i put a voltage into the pin nothing comes out..??
maybe you can help????

 

[Oznog]

Ah well you've got several problems.
The term for the sensor output is "differential".
The voltage is so low you must pay close attention to your circuit's offset error unless you only care about signal changes.

A normal differential op-amp won't do this job. You can use 3 op-amps in a truly differential circuit, but the offset error is a sum of resistor mismatching plus 3 op amps.
https://www.electro-tech-online.com/custompdfs/2004/10/sloa064.pdf

There are plenty of external ADCs which can do this, but not with an absolute voltage of 24V.

You can potentially tie Vin on the sensor to +5v and somehow generate a negative voltage to get the desired output level at the PIC. You need a guarantee that the common mode voltage doesn't float lower.

So, what I'm getting at is you need something called a "intrumentation amplifier". It creates the differential op-amp circuit mentioned previously inside a single chip, and matches resistors for lower offset. They usually have a fixed gain since they do their own resistors. You can get 30V versions, but probably not rail-to-rail inputs, make sure the level of the input is within the specified range.

Also, does your sensor NEED to be at 24V? This sounds like a strain gauge, and these work just as well on 5V as 24V. The signal level is lower, but you'd be better off frankly. Small signals (and unless you have an instrumentation amp, this is probably already too small) can be handled well by an external ADC with more bits. It avoids offset errors from amps which are pretty serious when you're talking about small signal levels.

 

[JimB]

What sort of sensor/transducer/transmitter is this?

You say it has a 4-20mA output, OK that fine, but does it have a two wire connection ie it is powered from the loop, or does it have a separate supply to energise it?

JimB

 

[jrz126]

Hi.
i have a problem with a pressure sensor. The sensor output is 4-20mA.
The common of the output is at high (+24V). I want to change the common to low (0V, GND) to plug the sensor to plc.
Could u help me???

i have a similiar problem.
I have a micorcontroller although i am not going to say which one, or provide a link.
anyway one pin seems to be stuck , when i put a voltage into the pin nothing comes out..??
maybe you can help????

Why dont you want to say which one it is? I'll bet you dont want to say because it's some new secret microcontroller made for the government or something. And if you did tells then you would have to kill us all to keep it a secret. :lol:

 

[Russlk]

I think WilliB was being sarcastic. The solution to the problem would no doubt be obvious if there were a schematic. But, if Auto_Mitch could draw the schematic, he would not be asking the question.

 

[williB]

I think WilliB was being sarcastic. The solution to the problem would no doubt be obvious if there were a schematic. But, if Auto_Mitch could draw the schematic, he would not be asking the question.

cant help me? huh :wink:

 

[panic mode]

JimB is right. what is the model and make of yout transmitter and PLC input card? If it's two-wire loop powered device, solution is very simple.
If the setup you use lacks optical isolation you can always use signal converters like from Weidmuller or Phoenix Contact. It's interesting that common is positive. Sounds like you are dealing with Japaneese standards.