O - 10 volt controls to 4 - 20 ma controls

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JdoubleU

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We just purchased a actuator valve that can operate on 0-10 volt control or 4-20 ma control. The instructions say to install a certain size resistor in parallel at the valve to make it a 4-20ma control. How is the resister making this happen?
 
Besoeker

Besoeker

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Jakewhis said:
We just purchased a actuator valve that can operate on 0-10 volt control or 4-20 ma control. The instructions say to install a certain size resistor in parallel at the valve to make it a 4-20ma control. How is the resister making this happen?
Click to expand...
A 500 ohm resistor will give 10V at 20mA.
 
don_resqcapt19

don_resqcapt19

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retired electrician
The valve must have a built in "zero" offset if you can convert 0-10 volts to 4-20mA. The resistor on a 4-20mA loop will result in a voltage signal of 2-10 volts.
 
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hurk27

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Portage, Indiana NEC: 2008
Jakewhis said:
We just purchased a actuator valve that can operate on 0-10 volt control or 4-20 ma control. The instructions say to install a certain size resistor in parallel at the valve to make it a 4-20ma control. How is the resister making this happen?
Click to expand...

To answer the question you had (in red above):

The controller controls the current in the series loop from 4-20ma, since current is always equal in all parts of a series circuit it is easy to control one or several devices with this same current signal, each device will operate on a voltage range, most are 1-5 vdc for a 0-100% span set to what ever units of measurements your process may be using, to get this voltage you must create a voltage drop, you do this by inserting a resistor that will give you the voltage range of the device you are controlling whether it be a valve or a chart recorder or a feed forward to an adjoining system, by taking the voltage rating of the device and dividing it by the current will give you the needed resistor that will give you the current to voltage you need for the device, but as others said, normally a valve set to fully stroke from 0 to 100% in a 4-20ma system it has a 4 volt span of 1-5vdc, and has a 250 ohm resistor parallel across its terminals, to change it to a 2-10vdc device you would just change the resistor to a 500 ohm resistor, but heres the problem if the valve was set to operate on a 0-10vdc controller your zero% point of the valve stroke is below the 2 volts you will get at 4ma, most valves will have a way to reset this zero% stroke point, some by loosing the two locknut's and lengthening the actuator rod (Fisher valves) or as was said by changing a pivot point so that at 2 volts the valve will stroke all the way to zero (closed in a fail shut valve or 100% open in a fail open valve)

But one thing that is very important, you must know the total output voltage that your controller can output before it looses the ability of reaching 20ma in the series circuit, this very very important because if you go beyond the max amount of total loop resistance you will not be able to reach the 20 ma setpoint and a fail open valve will not fully close, and using 500 ohm resistors to allow a 0-10 volt device to operate on 4-20ma loop can eat this up fast.
Moore brand controllers that use a 24vdc power supply input will have a max 22vdc of output so you are limited to 1100 ohms total circuit resistance so you have to watch out that your total series loop including the wire doesn't exceed this, Honeywell's that have a built in 120vac power supply have an output that can go as high as 32vdc so you have a little more head room at 1600 ohms, but some controllers will allow separate loop excitation where you can use a 48 volt power supply or more that will allow 2400 ohms or more of headroom.

Most transmitters will be separately excited so your max loop resistance will be the max output voltage of the transmitter.

To test the max output voltage of a controller or self excited transmitter just open the circuit and measure the voltage when the controller/transmitter is driving at 100% out, in most cases it will try to go to full voltage because it will try to reach the 20ma when you open the circuit but keep in mind some smart controllers will detect this open and shut down so you might have to already have the volt meter connected before you open the circuit.

Also they do make 4-20ma loop extenders that can extend the amount of devices that can be installed on a loop by extracting the 1-5vdc signal through a 250 ohm resistor and turning it into another 4-20ma circuit that is excited by its own power supply, this keeps the loop voltage down to the max voltage ratings that some devices might be limited to.
 
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