current limiting

[jebediah]

Hi,I am building a 4-20mA circuit. I built it by getting 4-20V and throwing a 1K resistor in. What is the cons to this circuit as opposed to building a 4-20mA with an XTR116 Burrbrown?

Also, my min(4ma) is good but the 20mA drifts, how do I limit the current to 20mA using transistors?

Thank you.

 

[BrianG]

4-20mA current loops don't work that way. The transmitter must employ current feedback to an amplifier so that it maintains the particular signal current constant over a wide range of variation in loop energising voltage and cabling resistance. (That's why it's good for monitoring signals from some distance, or adding extra receivers.)

You could build your own transmitter, but why bother if you have the XTR116. Incidentally, Burr-Brown were taken over by Texas Instrument, and they still make this device.

 

[OperaHouse]

A cheap and easy 4-20 ma can be made from a three terminal LM7805 5V voltage regulator in series with a power source. Place a resistor from the output term to the usually grounded terminal determine the current. Example 5V/500 ohms = 10ma. Use a pot instead and it is adjustable, two nine volt batteries in series and it is portable. The output is now the usually grounded pin. Because you never know what the voltage burden is, using just a resistor isn't very predictable.

 

[buzzp]

I would not expect to get any accurate signals from your set-up. Look at some app notes for op-amps. There is some real simple circuits that will give you much better results than counting on a resistor for stability and no feedback.

To answer your question, the Burr-brown part is way more accurate. The resistor will change value as it heats up (the type of resistor will determine how bad). Now you have issues with the voltage source accuracy and the resistor accuracy affecting your circuit. In addition, you have no feedback so the circuit can correct itself.

Try going to onsemi.com and look at app notes. You will find an easier, more accurate circuit using a single op-amp. Throw your circuit in the trash unless you want to learn more about electronics. Good luck.