IP transducer on 48 VDC 1

[ALEXFV]

Hi everybody:

I'm looking for a special electropneumatic IP tranducer on 48 VDC for a pneumatic positioner.

Input signal: 4-20 mA
Output signal: 3-15 PSI
NEMA 4, IP65 or equivalent.

If someone would advise for a manufacturer and its web page or Rep. I will really apreciate.

Have an excellent day.

 

[Ashereng]

Sorry. I have never come across a 4-20 mA 48 VDC device.

Have you talked to any of your current Electro-Pneumatic Transducer vendors?

Can you conver your 4-20 mA 48 VDC to a 4-20 mA 24 VDC signal?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[dcasto]

Repost on the cuircut engineering, but if I remember right, there really is no 4 to 20 mA, it's actually 1 to 5 volts with a percision resistor in parallel.

 

[ALEXFV]

Hello all.

I apreciate your valuable comments.

Yes, I checked with vendors, and there's one positioner of Foxboro-Eckardt that handles 48 VDC as maximum, but it's only available with bus option. Plant is so old, then, there's no way to intoduce a field bus.

I don't need a positioner but an IP transducer. Regarding above, I'm trying to locate this IP transducer.

My problem is that I'm dealing with an old facility made by Alshtom, and user has some "ancient" equipment at this rare voltage.

Thanks for suggestion on repost to circuit engineering, but my application is not for a customized circuit: what I need to solve is a problem with old french valves which has not spares. So that I have to suggest a commercial an available option, in order to improve operation of the control valves.

Have an excellent day.

 

[dcasto]

Look at european suppliers. they use a lot of 48 Vdc and 48 Vac.

 

[JLSeagull]

Many models are suitable for use with much higher voltage than 24 Vdc. I have not looked at this detail in a long time. Contact most typical suppliers, Fisher (include them for Rosemount), Foxboro, Honeywell, ABB, Yokogawa, Toshiba, whomever Moore Instruments is this year, Brandt, ...

 

[JimCasey]

Voltage is not important to the transducer. The transducer will never see it, because it has a fixed impedance. The only voltage the transducer will see is the voltage necessary to force 20ma thru its internal impedance. With 250 ohm impedance (reasonably typical) , 4-20 ma drops 1-5 volts. (Ohm's Law)

Worry about the OTHER end of the loop. Can your controller output circuit absorb 48V? The controller acts as a variable impedance device to change its output signal, and that is where the energy is primarily dissipated. Then there is the resistance in the wire but most users install 14AWG wire if for no other reason than it is not too fragile to pull thru the cable trays. 14AWG cable will allow a 20ma signal to go for MILES before there is significant voltage loss.

THere is almost invariably information in the controller literature about loop impedance vs voltage.

 

[ALEXFV]

I'm still looking for the IP at 48 VDC, but all manufacturers that I reviewed has equipment with 40VDC as maximmum. As large as I could found, Conoflow has the maximum DC current (40 VDC). Fisher, Brandt, Fairchild, etc has 32 VDC as maximmum.

Maybe best option would be to install a voltage regulator together an IP tranducer at 24 VDC, so I can solve quickly the requirement of the plant.

If IP transducer on 48 VDC is not a commercial equipment, it would be quite difficult to get spares in case of an emergency.

Apreciate all your comments and support.

Thanks again everybody. Have a great day.

 

[JLSeagull]

Consider using a lower voltage source; or use a voltage divider circuit to only use 24 V of the 48 V.

 

[JimCasey]

I looked up the specs on the Masoneilan 7000.
There is NO mention of the loop voltage, Except that the electronics are isolated from the housing and that is tested to 800V. The teminal voltage is specified as 5.5 to 6.5 V. There IS a spec that the maximum allowable current is 100ma. I selected the MN7000 because it is all electronic and if something was going to be damaged by overvoltage, that would be a likely candidate.

First time I ever disagreed with JLSeagull: If you put a voltage divider on the circuit, you WON'T get 4-20 ma through the transducer. Part of the CURRENT goes thru the voltage splitter, too, so the Transducer output would be low since it monitors the current, not the voltage.

But really, there could be 100V on the circuit and as long as the controller only puts out 20 ma, the voltage drop across the transmitter will only be about 5V. THE CONTROLLER LIMITS THE CURRENT THRU THE LOOP. There is a small voltage drop thru the transducer, but the REST of the voltage drop is across the controller output circuit.

 

[JLSeagull]

My issue is the source of the voltage. We normally use a 24 Vdc power supply. These often put out up to 28 volts. I suggest that they buy a 24 Vdc power supply. However, if the source is 48 Vdc just apply the equal resistors between the 48 Vdc source and ground, then take the loop power between the resistors. I have not thought it through well but don't see why it couldn't work.

 

[JimCasey]

There will be three components to the loop.

A power supply. In this case: 48VDC
Series wired to a controller.
The controller acts as a variable impedance device to limit the current through the loop to between 4 and 20 milliamps.
Series wired to a transducer.
Transducer impedance can be assumed to be low, and fixed.
There will be a small voltage drop across the low, fixed impedance. Typically the voltage drop across the transducer is 1-5 V. 250 Ohms (typical) at 20 milliamps the Voltage drop will be 5 volts. The maximum power dissipated will be 0.1 watt. Regardless of the power supply voltage. The wire from the transducer returns back to the power supply to complete the circuit.

If the power supply is 48 v the transducer will never know it. BUT the variable impedance output circuit will have to provide an impedance of 2150 Ohms to drop 43 (remaining) volts at 20 ma. Energy dissipated will be 0.86 watt. Or at the other end of the span it will have to drop 47 volts and pass 4 ma. 11750 Ohms.

I understand more about JLSeagull's suggestion to use a voltage divider now. Wiring it like a center-tap transformer, It >Might< be a practical idea if the voltage matters to the controller. If the impedance of the Transducer is 250 Ohms, and you want to limit the voltage across the LOOP to 24V, then the impedance of the controller will range between 5750 to 950 ohms. As the impedance swings in the control loop, the balance of the voltage divider will change. The amount of the voltage swing will depend upon the shunt resistance, and that will be governed by the current rating of the 48V power supply. If it's rated for 1A continuous, then the voltage splitter could consist of 2 25-Ohm resistors and the voltage would not swing noticeably. However if the 48V power supply is only rated for 50 MA, you'd have to string a couple of 500-ohm resistors resistors together and the loop voltage would swing noticeably between 4 ma and 20 ma.

 

[ALEXFV]

Thanks everybody for valuable discussion.

It's incredible how difficult is to solve problem regarding european specs., but now I can see how to deal with problem.

Kind regards.