4-20 current limiting

[cjd1965]

Hi I have a temperature transmitter which has a PT100 to 4-20mA converter built into it

The chart recoder was replaced and this channel is broken/not indicating.

The recorder vendor is saying that it has a resistor across the terminals and that i should limit the current.....shouldnt the head unit limit the loop current to 4-20 ?

I think they are talking nonsense

 

[daba]

Hi I have a temperature transmitter which has a PT100 to 4-20mA converter built into it

The chart recoder was replaced and this channel is broken/not indicating.

The recorder vendor is saying that it has a resistor across the terminals and that i should limit the current.....shouldnt the head unit limit the loop current to 4-20 ?

I think they are talking nonsense

Well yes, the transmitter, assuming it is functional, should be providing enough voltage to the receiver's input shunt resistor (it is most common for 4-20mA inputs to measure voltage, which is why there is a shunt resistor on the input). The input shunt resistor is very often 250 ohms, which implies 1-5V for 4-20mA

Most 4-20mA transmitters can only raise their output voltage high enough to drive n receivers in series, n usually being 2.

And yes, you are right, a 4-20mA transmitter will typically limit the current to 0-22 mA

It may be that the reason the recorder channel is "broken/not indicating" is that there is no current, possibly a break in the wiring somewhere. Have you measured volts across the recorder's input ??

 

[danw]

It is quite common for people to not understand why a resistor is used or not used. I had a conversation about this with one of my co-workers yesterday. I suspect that the person recommending a resistor to limit the current was not clear about the purpose of a resistor in a current loop.

The purpose of a resistor in a current loop is NOT to limit current. In fact, limiting the current in a current loop defeats the purpose of a current loop, which is to provide a signal that accurately reflects the variable.

The purpose of a resistor in a current loop is to create a voltage drop that an analog input can 'see' or 'read'. The nature of a current loop (which is why it is so popular), is that the device sourcing the current will automatically compensate for inclusion of a resistor in the loop, maintaining the required loop current and therefore maintain the accuracy of the variable. So, within limits, adding a resistor will not limit the current.

Analog inputs ALWAYS 'see' or 'read' voltage. Analog inputs can not, by themselves, 'see' or 'read' electrical current.

By the laws of physics, a current passing through a reistance drops some amount of voltage. The exact voltage drop can be determined by Ohms Law. E = I*R.

Hence a resistor is used to create a voltage from a current that the AI can 'see' or 'read'.

Some receivers (the device with the analog input) have a resistor built in for current measurement.

Other receivers require that a resistor be attached across the (+) and (-) AI terminals.

The normal resistance of a voltage input is probably 1M ohm (meg ohm). That amount of resistance will limit the current, appearing to have no signal at all.

A lower value, on the order of 100 to 250 ohms is tyically used to create a voltage drop for an AI. That value, 250 ohms, in parallel with 1 Mohm will not load the analog input.

You need to check the manual for the recorder and see whether or not a resistor is needed for a current signal and what size resistor is needed. Honeywell Trendviews use a precision 10 ohm resistor.

 

[cjd1965]

Hi The recorder doesn't need an additional resistor. The vendor is implying that the internal resistor is burnt out because my circuit is not currently limited

Thanks

 

[shooter]

yes vendor can be correct.
however the solution is simple, put the resistor external.
Most recorders work with 1-5 Volts, and the input can have 6 Volts max, when there is a short in the sensor it could have had 24 Volts on the input, so the measuring resistance internal is broken, or replace it, however the 24 V can also damage the internal ADC.
Please tell type of recorder, maybe a diagram is possible, so repair can be done.
On newer recorders the resistance can be very low, and the vendor is correct to have a current limiter(fuse of 25 mA) in the 24 V supply, (for each channel)

 

[cjd1965]

Hi Shooter in my opinion a fuse is a protective device.not a limiter.

The unit is ABB RVG200

I am unable to investigate due.to customer access restrictions until tomorrow

I have a current splitter in my toolkit and am thinking of splitting the loop as an alternative to finding and buying a limiter at short notice

 

[danw]

A transmitter is a current regulator. It regulates, and at the same time, limits the current in the loop.

The RTD transmitter is probably a 2 wire, loop powered transmitter with a 24Vdc loop power supply.

What happens in 99.99% of the time when an analog input resistor burns out is that someone shorts the (+) / (-) terminals of the transmitter, like the red line shown in the diagram.

That connects the 24Vdc) power supply (+) directly to the (+) side of the analog input resistor. The loop current regulator, the transmitter is no longer in the circuit. The only thing limiting the loop current is the analog input's resistor. The current is nowhere near the level of tripping the power supply's crowbar.

A 250 ohm resistor would have to be able to dissipate 2.3W and most are 1/8W, not 3W, so are nowhere near capable of withstanding the heat and burn out.

Dan

 

[cjd1965]

Hi Dan
Thanks it is a 24V DC temp. Transmitter.

I will.inspect the instrument wiring etc tomorrow

The confusing part is I can measure 4 to 20 if I bridge the cable with a meter. I need to check voltages too which I didn't do
I agree the TT is a current limiting device it was confusing why I was being told it wasnt

 

[shooter]

Yes the TT is limiting, however when there is a short of somekinf anywhere you must have a limiter (costly) or a simple fuse as it normally is operating.

Do not forget **** happens at murphys place

 

[daba]

Hi Dan
Thanks it is a 24V DC temp. Transmitter.

I will.inspect the instrument wiring etc tomorrow

The confusing part is I can measure 4 to 20 if I bridge the cable with a meter. I need to check voltages too which I didn't do
I agree the TT is a current limiting device it was confusing why I was being told it wasnt

By "bridging" the output of the transmitter to measure the current, you are effectively shorting out the current limiter into the receiver, exactly as Danw has described. Probably the first time you measured loop current this way is when you burnt out the receiver's shunt resistor.

Now it is burnt out, the receiver input is now high impedance, and will not pass the 4-20mA process signal. Each time you connect your meter in this way, the transmitter will regulate the current through your meter, but take the meter away, and the loop resistance is too high for the transmitter to drive a 4-20mA signal through....

The ONLY way to measure the "loop current" is to break the loop somewhere, and put your current meter IN SERIES with the transmitter/receiver loop.

But the absolute safest way to measure that the loop is working correctly, is to measure Volts across the receiver's input terminals. Most 4-20mA input devices have an internal 250 ohm "shunt" resistor, (some have 500 ohm, check your documentation for the exact value) and the receiver measures the volts across that. To generate 4-20mA through a 250 ohm resistor will require 1-5V

V=IR
1 = .004 * 250
5 = .020 * 250

Put a 250 ohm 1% resistor across the receiver's input terminals and see if the receiver starts working again... there's a good chance it has survived, and only the internal resistor is damaged.

 

[danw]

The world if full of analog inputs that are not 250 ohms:

 

[danw]

For reasons I've never understood, industrial pressure transmitters typically have a pair of test terminals, behind which is a diode, but temperature transmitters do not. No idea why TT's suffer discrimination.

The test terminals are there so that a milliameter can be connected to get a mA reading without opening or breaking the current loop to insert a milliameter.

But that's the only time I know when using a milliameter without breaking or opening the circuit is safe to all components (and to the person making the test).

 

[daba]

The world if full of analog inputs that are not 250 ohms:

I haven't seen many that aren't, so made the statement in ignorance of the facts DOH !

Anyway, the maths still holds true, whatever the internal resistance of the receiver..

V = IR

 

[Lare]

For reasons I've never understood, industrial pressure transmitters typically have a pair of test terminals, behind which is a diode, but temperature transmitters do not. No idea why TT's suffer discrimination.

The test terminals are there so that a milliameter can be connected to get a mA reading without opening or breaking the current loop to insert a milliameter.

But that's the only time I know when using a milliameter without breaking or opening the circuit is safe to all components (and to the person making the test).

Diode can be inserted anywhere on the loop for quick testing without breaking loop for measurement.
It is pretty uncommon that there is diode installed across connection terminals on automation enclosure even that it won't cost much.

 

[danw]

Anyway, the maths still holds true, whatever the internal resistance of the receiver..
V = IR

Absolutely true.