a basic electronics related question 1

[shafeequemk]

Consider we are Using 24V dc field instrumentation system. If we are measuring the voltage using a multimeter, it will not show 24V, if the control valve connected in the line is at 50% action state. Why?

 

[Skogsgurra]

Could be just about any reason for that.
If the 50% is duty cycle, the answer is obvious - that is how PWM works.
It could also be a simple question of voltage drop, bad supply or even using the multimeter on AC instead of DC. Have you checked the multimeter on a known DC voltage?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[danw2]

>If we are measuring the voltage using a multimeter

You do not specify between which points the voltage is measured.

Voltage across a properly functioning/sized power supply will be 24Vdc.

The voltage drop across the dropping resistor R of the analog input (DCS/PLC/RTU/controller/indicator/recorder AI) will be I*R; 1-5V for a 250 ohm resistor.

The voltage drop across the transmitter's (+) and (-) terminals will be power supply voltage minus loop's IR drop.

 

[shafeequemk]

The voltage drop across the transmitter's (+) and (-) terminals will be power supply voltage minus loop's IR drop.
good reply..
usually when meassuring voltage between two +&- terminations at the marshalling cabinet will be always very below 24V, wont it?

 

[ScottyUK]

Most, but not all, I/O cards use a standard 250[Ω] resistance so the volt-drop should normally be in the range of 1V to 5V for a loop current of 4-20mA. Check the spec for your specific card.

 

[waross]

Are you using a " 24V dc field instrumentation system" or a 4-20 ma current loop fed by a 24 Volt supply?
Big difference.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[shafeequemk]

(4-20)mA based with 24 suply

 

[waross]

This is a current based control system. With the valve at 50% the current in a simple control loop should be 12 ma. With a 24 Volt supply the sum of the voltages across each device in the loop will be 24 Volts. At zero percent valve opening (on a direct acting valve) the voltage across the controller will be whatever it takes to drive 4 ma through the loop. As the valve opening is increased, the voltage across the controller will drop and the voltage across the valve will increase.
These loops are more accurately tested with a milli-ammeter rather than a Volt-meter.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[danw2]

>usually when meassuring voltage between two +&- terminations at the marshalling cabinet will be always very below 24V, wont it?

Yes, if there's any load at all in the loop the voltage across the transmitter terminals will always be less than loop supply voltage.

If you connect a 2 wire, loop powered transmitter up to a 24Vdc power supply on the bench and put a 250 ohm load resistor in the loop, you'll get readings like those in the table.

 

[shafeequemk]

mr warros, thank you very much...

 

[waross]

Nice presentation, danw2. May I add that I have seen nominal 250 Ohm resistors so far of spec as to cause problems.
May I suggest that measuring the voltage drop across a KNOWN resistance is a good way to infer the current in the loop. That is usually adequate for trouble shooting. However if you need very accurate current measurment, the accuracy will be limited by the tolerance of the resistance. Once you have checked the actual resistance of a particular device, the accuracy of your measurement method will improve to near the accuracy of the meter used to check the resistance.
In the real world a loop may contain more than two devices.
eg: A transmitter, an indicator, an alarm relay, an I transducer, a controller. If the voltage measurements across devices times the input resistance of the devices agree, then, for trouble shooting purposes, you may often assume that all the input resistances are close to spec. If the current through one device is off by a noticeable percentage, you probably have an off spec resistor.
I hope that this helps with your testing.


Bill
--------------------
"Why not the best?"
Jimmy Carter