Has anyone got this universal analog output circuit to work?

[leeb_965]

Hi I am new to this forum and I'm struggling to find a 4-20mA voltage to current converter. I was recommended this circuit:

http://www.piclist.com/techref/microchip/aout.htm

But I can't get it to work.

Has anyone ever used this or can recommend a circuit for me please?

 

[Harald Kapp]

I'm not familiar with the circuit you reference, but it looks reasonable to me.
Can you specify more detailed what you "can't get it to work"?
Do you get any output at all?
If yes: what is the expected output vs. the measured output?
In which environment is the circuit expected to operate (what supply voltages, what load resistance/burden)?

See also this thread: https://www.electronicspoint.com/0-5v-4-20ma-t243919.html

Regards,
Harald

 

[leeb_965]

Thing is I dont know the typical changes in resistance so I randomly altered the gauge resistances on the simulation. I dont really know much about this stuff and Im not really sure what to expect at the output. Should there be only a current at the output or will there be a voltage as well?

I also found:

https://www.electronicspoint.com/atta...ge-current.gif

but Im not sure what the wires are for going through the center of the 1st op-amp.

 

[Harald Kapp]

O.K.
first: randomly altering values seldom does any good. Try to understand the circuit first. And on basic thing: Except under very rare circumstances (like superconductors) whenever there is a current, there is also a voltage. A current at the output can onlky flow if you add apath (ressitor) between the two output pins. Without load, there is no way for a current to flow and the circuit will just output max. voltage.

So which resistor did you change? In the schematic there is no resistor labeled gauge.

Also which wires "going through the center of the 1st OpAmp" are you talking about? I can't see any wires going through the OpAmp. Are we talking about the same schematic?

I recommedn you build the circuit on a breadboard, then you can experiment.

Harald

 

[leeb_965]

Yeah I found another but Im wondering if the vertical wire through the middle of the 1st opamp is part of it. What kind of voltages am I expecting at the output. I put in a volt at the input and got out 4mA but about 11 volts also.

 

[jackorocko]

It is not just a random vertical wire. It is the power to the IC containing the op-amps. It is a given to know that an op-amp needs power to work.


edit: look at the schematic you posted a link to. It states "1/2 lm358". This is a double op-amp IC package, so they only show power connecting to one half of the LM358, the other one half of the IC is powered by from the same source. Checkout the datahseet if you need a clearer understanding of the pin-out.

 

[rashid_s]

Hi all.

Tell me, is it possible to put MOSFET transistors instead of jumpers to switch operating modes?

 

[Alec_t]

Welcome to maker.pro, rashid_s!
Did you notice this thread is 11 years old?? Better to start your own thread if you expect answers.

 

[danadak]

Circuit looks OK. Here I change Rloop from 10 ohms to 1K ohm, two case, V4 = 0 and
V4 =5







In First case, V4 = 0, 4 mA stays well regulated over large Rloop change from 10 to 1000 ohms.

Second case V4 = 5V, at Rloop ~= 380 ohms the 20 mA falls out of regulation.

Note both cases Vsupply opamps = 12V. When OpAmp power = 7V circuit unusable for 20 mA
case, but 4 mA case looks good. However the circuit not usable if you wanted that high an Rloop,
1K (wiring long run resistance from sensor to destination, eg. processor or whatever) and 20 mA
for the max sensor V.

Yeah I found another but Im wondering if the vertical wire through the middle of the 1st opamp is part of it. What kind of voltages am I expecting at the output. I put in a volt at the input and got out 4mA but about 11 volts also.

This is a current loop, so changes in Rload should keep the current the same, but as
Rload changes the V also changes. Thats to be expected. V = I (constant) x Rload(variable).

Note I made an assumption your sensor input, V4, ranges from 0 - 5V to generate 4 - 20 mA.


Regards, Dana.