Hello,

I'm currently doing a project on PID controller using 8051.

Here i'm using a pressure process station. I have a pressure sensor (piezo resistive) and a i/p converter both uses a 4 -20 ma current loop. This Pressure will control a globe pneumatic valve to the Pressure tank.

My questions are,

1) How to read the signal from the transmitter?

So far i've searched internet and found that a 24VDC is supplied to the transmitter and it is read as voltage through a 250 ohm resistor. Hope this will work fine.

2) How to give output to the i/p converter?

I've no idea on how to make this 4 - 20 ma output current loop? The i/p converter I've is loop powered.

Please help me.

 

Question 1. Yes, the 250 ohm resistor will work fine. Make sure you have a precision resistor, check temperature coeficient as well.

Question 2.

See the AD420 from analog. Serial input 4-20 mA DAC. The 4-20 mA current to the I/P converter can be sset through the serial interface which can be connected to the 8051. Isolated through optocouplesr if required, which I would advise.
http://www.analog.com/en/digital-to-analog-converters/da-converters/ad420/products/product.html
Check the AD420 datasheet how to connect it to the I/P converter.

Regards,

Heinrich