More on 4mA to 20mA loops
1. Can I put in series 250 Ohm to get 1-5V drop, by doing so, will it affect the current loop & accuracy?
This is normally OK - assuming the total loop impedance isn't too high
(250R is a common value because it gives a convenient output voltage).
If the loop impedance is too high the transmitter will probably be fine at low currents but "run out of headroom" for higher currents, a quick check-cal will show this.
2. A 4-20mA transmitter can drive many receiver , if the +loop voltage is sufficient, am I right?
Within reason. You need to get the transmitter's data sheet to be sure but as a guideline ...
If you assume the transmitter is designed for a 24v loop with no minimum impedance then :-
For one extreme, 24v across the transmitter at 4mA ... 24/4 = 6kOhm internal impedance;
For the other extreme, assuming at least 5v is required for the transmitter at 20mA ... 5/20 = 250Ohms internal impedance;
The difference is 5k75; this will be the ABSOLUTE MAXIMUM additional loop impedance you can have.
3. Is there any isolated 4-20mA receiver?
Yes. If the indicator/controller needs this then an EX barrier (used for explosive area installations) gives the input complete galvanic isolation, these devices need a power supply though!
4. I need a circuit for "high side" current sensing, can anybody show me?
At its simplest a 4 to 20mA moving coil meter in the positive rail?
I assume the negative side is earthed or has some special condition to preclude measuring it. I don't see the problem here - more info needed.