Electronics > Projects, Designs, and Technical Stuff
4-20mA current loop
dagobah700:
Dear Members!
I have desingned this simple 4-20mA current loop transmitter based on some ideas on the Web. I have simulated it in different programmes like Tina and EveryCircuit and it works without any problem.
Despite that when I built it on a breadboard the TL072 op amp just refused to work. I known this IC isn't a rail-to-rail one, but according tho simulations it should work. I had problems with this op amp before this project too.
I am really confused now. I hope somebody can help me to solve that.
Thank you all.
(PS: Schematic attached, I've verified the connections on the breadboard and included bypass caps too)
OM222O:
this looks way more complicated than it needs to be ??? please provide a "theory of operation" as I can't follow what each section is doing ( I think the second op amp is acting like a difference amplifier for the driving stage?
I can recommend replacing the op amp by the MCP6002, it has worked in almost all current loop designs that I created in the past. they're fairly cheap and available from any local supplier.
Hiemal:
That and the TL072 isn't specced for single rail operation either.
An LM358 would work, or as OM222O suggested an MCP6002 works too. Just be sure not to exceed the MCP6002's max voltage allowed.
dagobah700:
Thank you for your fast answer!
First of all the goal was to create a two wired current loop so the power and the signal can travel on the same loop. For this reason I should measure the transmitter circuit's current "consumption" too. When current is flowing through the R3 it drops a negative voltage on it (compared to GND). The IC1B part of the TL072 amplifies it by the gain of minus two. After that the IC1A part of the TL072 compares this voltage with it's non-inveting input voltage and drives the IRF530 what acts like a voltage controlled current-source. V2 simulates the sensor's output voltage. R1 simulates the cable resistance and the detector circuit.
The problem is, when I turn my irl circuit on, the opamps output voltages immediately reach +23V. At first I thought it is a gain issue, but after I tested the op amps on symmetric supply voltage they actually amplified.
I will try the two suggested op amps tomorrow.
OM222O:
it's not the symmetry of the supply rails. op amps don't care about that at all, however if you're powering the op amp from ground and a positive voltage source (let's say 24v) and your inputs are going negative (below ground) there is a very good chance the op amp is damaged. I'm not sure what you mean by carrying "signal" and "power" on the same line. the current is what actually drives the actuator (you mentioned a sensor instead of actuator ... if it's a sensor, you shouldn't be trying to back drive it with a current loop! you should be converting that current into a voltage and reading it!). a lot of industrial appliances use the 4-20mA standard as their input / output.
A much more simplified version of this circuit would be this:
If your control voltage (Vset) is 0 to 4v, use a 180 ohm resistor for Rset, that will give you 0-22mA range (which gives you some head room, but you can drop the resistor to 150 ohm to gain back more headroom).
Also make sure you have sufficient V for the load to drive 20mA. Assuming 24v again, that's about 1.2k ohm, which about 200 of it, is used in Rset, so your load shouldn't exceed 1k ohm.
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