The + loop voltage of 4-20mA output

[VictorPS]

What is the typical +loop voltage for a 4-20mA transmitter?
Is there any standard electrical specification about 4-20mA current loop?

Thank you,

 

[Dean Huster]

This isn't a bad spot to read up on 4-20mA current loops: **broken link removed**

I don't know that there's a maximum open-loop voltage specification, but you sure don't want a constant current source to be able to work into any load resistance presented or you could get lethal! 40mA available with a 400V source would be nasty, reminiscent of a 1955 television set! Most peak out at around 24 volts or so, good for about 950 ohms of line resistance figuring a 5v output at the load at 20mA. 950 ohms represents a heck of a lot of wire!

Dean

 

[mechie]

Current loop Transmitters

The industry "de-facto" standard is a 24v supply for 2-wire 4 to 20mA transmitters. I have seen odd ones that state maximum voltage drops of 20v (EX rated - maybe a co-incidence, maybe part of the EX design ??).

Some transmitters insist on a minimum line resistance - we actually put resistors in series with the loop to provide this :shock: , most transmitters are happy with any loop resistance/supply voltage combination capable of providing 5v to the transmitter at full load (20mA).

If you measure the voltage across the transmitter (2-wire type) the voltage will be a function of the current - Ohm's law. Opening the loop will show the open-circuit supply voltage.

 

[VictorPS]

I want to monitor the current in an existing 4-20mA loop, which already have one transmitter and one receiver.
1. Can I put in series 250 ohm to get 1-5V drop, by doing so, will it affect the current loop & accuracy?
2. A 4-20mA transmitter can drive many receiver , if the +loop voltage is sufficient, am I right?
3. Is there any isolated 4-20mA receiver?
4. I need a circuit for "high side" current sensing, can anybody show me?

Thank you,

 

[mechie]

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.

 

[VictorPS]

I plan to convert the 4-20mA to 1-5V, using OPAMP and ADC on PIC12f675. Because I am taping the current in between existing transmitter & receiver, so I prefer not to ground it at the middle of the current loop.

Someone tell me, I need "high side current sensing", it may be just differential OPAMP as I think, I can build a simple one, but the linearity, gain , temperature , accuracy is a big question to me as I am poor in analog circuit design. So if someone have already design the circuit for 4-20mA (which for my case) will save me a lot of time.
May be I shall try first before asking for it.

 

[mechie]

Microcontroller A to D ?

If the A to D on the microcontroller will accept 5v then can you not wire up as per the attached diagram ?
This would allow a common negative between 24v supply and microcontroller 5v supply.

 

[VictorPS]

Will the setup work for me? With condition, I must have good separate ground to the existing system ground.
Thank you,

 

[Phasor]

Figure 1 requires all 3 grounds to be completely seperate.

Figure 2 does not require separation of the transmitter and micro grounds, only seperation of the receiver ground, thus I would prefer Figure 2.