ohm to 4-20mA converter circuit

Use a transconductance amplifier/attenuator.

You don't say what the ohm range is, so little can be said about it specifically.

What is an LDR anyway? I assume that the "R" stands for resistance.

LDR - light-dependent resistor?

Oh! A photocell! Of course!

In that case, since the LDR needs a supply and a sense resistor connected in a divider circuit, if the "dark" resistance is too high to squeeze 4 mA through it practically, then a few electronic components may be needed to properly condition the signal.

Ideally, the "darkest" resistance and the "lightest" resistance would be such that a divider resistor could be used which would cover the 4 - 20 mA range just perfectly. The odds of that happening are slim, at best.

Nevertheless, if LDR is indeed a "light dependent resistor" or photocell, and a few operating parameters can be given, then it's just a matter of scratching out a simple circuit to get what's needed.

PW, please note that I'm replying to you as well as the original poster, whom I haven't heard from yet.

Bill

Have a look at XTR101 (or others of this series) from Texas Instruments, it`s a reliable circuit for your application requiring only some passive Rs and Cs for a complete design.

You can have voltage-, current- and resistive input for a broad range of sensors.

Regards Uwe

Use this circuit as a starter. If your LDR is different than one in this circuit, change R1 (possibly upwards), R31 and R41.

Note that the span adj. has effect on zero adj. VDD=5V, VCC=12 to 24V.