Converting a 2...10VDC signal to 4...20mA

I have an actuator, Belimo cat.# AFB24-SR, that has 2...10VDC control input and feedback output signals. However, in their spec sheet, they say you can add a 500ohm resistor to the control signal to convert it to a 4...20mA control input signal. Now I understand with Ohm Law V=IR so

I = V/R = 2/500 = 0.004A = 4mA
I = V/R = 10/500 = 0.02A = 20mA

I got that part, but their spec sheet doesn't explain why I can't do the same to the feedback output signal and their tech support just said "it doesn't work, others have tried, you need a signal converter."

Can anyone explain this to me? I was always under the impression you didn't convert V to A with just resistors to begin with, it was always A to V. Thanks.

It would work if your 4-20 mA analog input has no input impedance. But in a real world, there will be an input impedance, so there will a Voltage drop across the analog input. Therefore the current through your resistor will not be for example 5V/500Ohm = 10 mA, instead maybe 5V/(500+250)Ohm = 6.6 mA.

The actuator operates in response to a 2 to 10 VDC or, with
the addition of a 500Ω resistor, a 4 to 20 mA control input from an electronic
controller or positioner. A 2 to 10 VDC feedback signal is provided for position
indication. Not to be used for a master-slave application.

Click to expand...

I think you have read the actuator's manual incorrectly. See above, it says if you have a 4 to 20ma signal coming FROM a controller or ??? you can use a 500 ohm resistor to create the 2 to 10 v signal to the actuator.

Or see link if you want voltage to current signal conditioning..for the feedback signal.

https://www.automationdirect.com/ad...Voltage,_Current_or_Potentiometer_Input/FC-33

Pressure independent valve and variable speed pump

Hello all,

I saw that thread and I though I give it a shoot and ask a general control strategy question about energy valves or ePIVs.
I am not an HVAC expert obviously but I am trying to understand the correlation of the pump control and an energy valve or ePIV.
Lets say we have a heating-cooling circuit that feeds FCUs. In the feed pipe there is a VSD pump (grundfos) that works in DP mode. As more load is added (FCU valves open) the DP will drop and the pump will accelerate to maintain the DP constant at the selected setpoint. In the return pipe there is an electronic pressure independent valve fitted that will modulate its actuator to keep a constant flow regardless of the pressure variation.
This is an actual setup I came across and I cannot understand it. I think they should either use DP control in the pump OR the ePIV. I dont see how the two can work together as in a load change both devices in their own way will try to compensate for that. In case of demand increase, the pump will speed up to maintain DP and the ePIV will regulate the flow. How will that work?
How is the pump of the system related to the ePIV? If only one is necessary then what is the best? Fixed speed pump+ePIV controlling the flow or variable speed pump with constat valve position?

Maybe I talk nonsense but can an HVAC engineer enlighten me.

Thanks.

The input is simple a 2-10 Volt input, wih high impedance 100 kilo ohm.
you can use this input also with a 4/20 mA signal, by using a resistor of 500 ohms.
oke sp far it is easy.
the feedback output is a 2-10 Volt with max 0.5 mA and it is NOT possible with a resistor to get 20 mA out of this, you are correct.

A solution can be to remove the resistor on your PLCinput so the input will be from 2-10 Volt.
if input is 1-5 volt 2 resistors of 10 kohm can be used.

shooter said:

A solution can be to remove the resistor on your PLCinput so the input will be from 2-10 Volt.
if input is 1-5 volt 2 resistors of 10 kohm can be used.

Click to expand...

If there is need to take soldering iron, I would instead buy couple resistors and OP-amplifier (OP324) before soldering of PLC.

https://www.allaboutcircuits.com/te.../chpt-8/voltage-to-current-signal-conversion/

On industry use buy instead signal converter from stock.