Simple 4-20mA Generator

Hi what application would like to use this equipment. Regard's

Not too difficult, just make sure offset potentiometer is not part of the gain adjusting circuit and vice versa. If you have only one polarity supply voltage then this can be a little tricky because you will have to reference everything to the mid voltage instead of ground. Also when adjusting the offset, don't forget to give the circuitry time to come to a thermal equilibrium. Many people forget that the offset values of most op-amps are after five minutes of operation.

I believe this can be done with 2 simple resistors as it looks like you are converting a 0-5 vdc control signal to provide a 4-20ma signal. Is this for a Honeywell damper motor? See a Honeywell damper motor specifications sheet for details.

Norm

Try an XTR101 fro Texas Instruments.

OK, to followup on my post and specifically answer Bri4eng's question. Looking at the XTR115 design, we can see an easy way to use our own op-amp to achieve most of their result.

The 4-terminal voltage regulator needs to be a low-current, high-voltage type. The op-amp needs to be a single-supply type, such as an LT1013. You have to provide the offset for the 4mA zero offset.

I_O = V_IN (1 + R2/R3) / R3 is the current-loop operating formula.

Serious industrial 4-20mA applications have constraints that aren't easily met with a simple home-brew circuit. For example, if the sensor fails or becomes disconnected, perhaps the reading should be zero = 4mA, or perhaps 0mA, which in many systems is an even better fault indicator.

Or if it's a temperature sensor transmitter in a heating application, it should fail to above 20mA, to shut off the heating. Another example, the maximum current should be limited to 25mA, or similar. Sometimes one terminal of the sensor is at ground, and an isolated circuit is called for.

Transmitter voltage compliance should be from below 12V (even 8V), at full current, and up to above 60V in some systems.

I ran out of time to edit my post. For an op-amp, the LT1013, as mentioned, would draw 0.7mA power supply, plus a bit for BJT base current at high outputs.

For a 5V op-amp regulator, an LM2936-5 or LM2936HV-5 (40 or 60V) would work fine, and draws only 0.03mA quiescent + 0.01mA for its PNP pass-element base-current for the '1013 op-amp current.

This totals 0.8mA quiescent, which is a bit high if you hope to have a low-current "OFF" state for fault signaling. An LT1077A single op-amp has a low offset voltage and draws 0.05mA, for 0.08mA total quiescent, which should look like a disconnected state for most receivers. Both of these op-amp types work with up to 3.8V signal levels with a 5V supply.

"Both of these op-amp types work with up to 3.8V signal levels with a 5V supply."

That's relevant for the 2nd amplifier of an LT1013 dual, or for a second LT1077, but the inputs of the op-amp in the circuit always run at V_EE or 0V for the op-amp.

Try this:-http://circuitelec.blogspot.com/2009/04/4-20ma-current-loop-transmitter-circuit.html If you're not happy with it, Google "Current Loop Transmitter" Good luck.

Since you have already built and got working most of your project, you do not want to start again with a new IC!!

1. You can design a circuit, like yours, which will give the right offset and span, but adjusting it to the required value takes repeated moving of span and zero, in succession, to a closer approach to the ideal, as you swing input min to max.
2. The secret to stopping the zero being altered when you change the span is to take the signal from the zero pot wiper via a third op-amp configured as a "unity gain" buffer (- input connected direct to its own output).

Because the buffer has near zero output resistance, change of current as you change value of span pot does not move zero. Also zero pot is not in feedback loop, changing its value does not affect gain.There is an extra op-amp input offset error, but this is usually acceptable. Lots of commercial gear saves an op-amp, but it takes adjustment time if fixed resistors are not adequate for tolerance needed.

This is a good general-purpose solution, and hopefully Bri4eng had a compatible non-inverting amplifier configuration and was able to solve his problem using it. Here's a GA from me.

And also here's a drawing showing independent zero and span adjustments for the single-opamp circuit I posted.

Since its op-amp input node is a summing junction, always sitting at 0V, adding a zero current for 4mA, etc., has no effect on the gain. (Yes, we have a + input to the op-amp, but it's still an inverting input for the overall feedback loop, because the transistor acts as an inverter, through the power supply and bypass cap, to the load resistor R1.)

Go to Analog Devices web site & u will find what u need.

Why to invent the wheel?

Dov