Voltage Output 2V to 5V with 5K pot with 3.5V output in the middle

[SeaMaster]

Hi,
I'm trying to design a circuit to control hydraulic valve that requires voltage (not the common PWM found in most other hydraulic valves) to control.
I have joystick with 5K pot that would like to utilize for this project.
The valve will stay in the centre (neutral) at 3.5V and swings of 1.5 volt up or down will make it open in the appropriate direction. I'm not sure what is the current draw requirement, but i thin 1A will be sufficient. So in brief:
2V max open one way
3.5V neutral position
5V max open the other way
My initial idea was to do the with LM317 adjustable regulator control, but I'm ready to aboundon it as can't make the resistors values to adjust the output as per the valve requirements with the 2.5V output being provided when the wiper is in the middle 2.5K on each side

Any ideas how or what schematic can meet the above requirements?

[sleemanj]

https://docs.google.com/spreadsheets/d/1HD0tzSoef0jmiv7g1oEDHRcbc_XZmmBFHqzxtJ3-pqA/edit#gid=0

File > Make A Copy and change the VIn as you desire.

Havn't used this in years so you might want to check the calculations. (Checked, they look ok to me).

[freda]

get 1 of two more pots on either side, and adjust. did you try that?

[John B]

A 5k pot in series with a 3.33k resistor will give a range of 2-5V if powered with a 5V reference. It would be a good idea to then buffer the output voltage with an opamp.

Edit: Err, with that output current requirement, I would probably feed that reference voltage into a switchmode controller to then provide the desired output voltage.

[Benta]

What's the supply voltage?

[sleemanj]

Oh I didn't notice the current requirement. 

You'll need to take a different approach than just a 5k pot unless your source voltage is very considerable indeed (and the pot very special!)

[SeaMaster]

"What's the supply voltage?"



I can make it anything i want from 28V DC input with switching PS

[ledtester]

I'm not sure what is the current draw requirement, but i thin 1A will be sufficient.

Can you measure the required current? Usually a control voltage signal isn't required to source a lot of current.

Or, do you have a datasheet (or even model number) of the valve?

2V max open one way
3.5V neutral position
5V max open the other way

What happens when the control voltage is at 0 volts? Is it the same as when it is at 2V?

[SeaMaster]

It is Italian proportional valve made by Brevini. unfortunately the boat that it is on is in new Zealand and Im back home in Vancouver
All I know from the Brevini North American dealer is the information I supplied before:
2V opens it oneway 3.5V both sides closed (spool in the centre) and 5 volt opens it the other way with proportional output for everything in-between
In the past I drove them with little linear regulator and a pot, but it was very non symmetrical  as I couldn't  get the linear regulator to adjust the output voltage as per the spec required with linear 5K pot. So always I had one side perform way better than the other, because I had no choice but had to just the trim to the middle where the valve will be off, but then in this case one of the sides always suffered with insufficient voltage to fully open

[ledtester]

I found this catalog at Brevini's web site:

https://www.brevinifluidpower.com/products-family/proportional-control-valves/

catalog link:

https://medialibrary.dana-industrial.com/wp-content/uploads/Technical-catalogue-Valves-and-Electronics.pdf

In the catalog there are various circuit diagrams for "proportional control" valves such as this one:



So it is possible you could just use a pot.

In any case, if you can identify the actual product the catalog might have useful information with regard to interfacing the control signal.

[SeaMaster]

Thank you for the links. It appears that I found the valve type I have (see the attached image)
From what I could understand the one that i have has input impedance of 12K, but regardless, i still created a buffered schematic that can handle some current.
Please review and comment. the schematic was tested in LTSpice and appear to work well.

[ledtester]

Are you intending to use an actual OP113 or is that just representing a generic op-amp?

It looks like you're going to have a higher voltage available (either 12V or 24V).

I would simplify your op-amp circuit with just an op-amp voltage follower. R6 and the 5K joystick gives an input voltage range of 2V to 5V. Feed that into an op-amp follower but use 12V (or 24V) as the positive supply rail for the op-amp. Then pick a modern op-amp that is unity gain stable and you're done.

The OP113 is unity gain stable but with a 5V supply it is only guaranteed to output a max. of 4V  (see page 5 of the datasheet).

OP113 datasheet:

https://www.analog.com/media/en/technical-documentation/data-sheets/OP113_213_413.pdf

Please review and comment. the schematic was tested in LTSpice and appear to work well.

It would be helpful to have the .asc file.

[magic]

The thing you have drawn in SPICE is not going to be stable. And the PNP is upside down.

Don't overthink it.
Generate 5V with LM78L05.
Divide it down with the pot and R6.
Buffer with LM358 voltage follower powered from the 28V input.

[SeaMaster]

Sorry that was just random opamp i picked as the one that i was planning to use NCS20071 i did not have spice model for nor i know how to get one...

[Zero999]

The thing you have drawn in SPICE is not going to be stable. And the PNP is upside down.

Don't overthink it.
Generate 5V with LM78L05.
Divide it down with the pot and R6.
Buffer with LM358 voltage follower powered from the 28V input.

Exactly.

Another option is to regulate the 28V down to a lower voltage using the 317. This might be a good idea if there's a risk of voltage spikes on the DC input

[John B]

I think the conversation really needs to go back to the first post. What exactly are you driving? Does the valve have some active electronics of it's own that accepts a control voltage signal?

You mention around 1A of supply current. Are you sure it isn't just a solenoid? In which case why couldn't PWM control work? If it's an issue of PWM frequency being too high/low etc you can always add additional L/C filtering to get something approximating a DC signal.

All the solutions so far seem to be along the lines of a linear voltage reg which seems wasteful and unusual to control a valve.

[magic]

This has been answered: 12kΩ input impedance.

[Zero999]

One disadvantage of the circuits posted so far is, they all rely on the potentiometer's value being fairly accurate. The problem is potentiometers typically have a poor tolerance of 20%. A solution to this is to bypass the potentometer with a resistor and adjust the value of the other resistor to compensate. The disadvantage of this is it uses an extra resistor and more current.


EDIT:
If you're using the  78L05 anyway, add a potential divider to the output to get 2V and buffer it with the spare LM358, which is free and connect the potentiometer between 2V and 5V.

[SeaMaster]

Thank you all for the replies. I'm not sure yet which one will be the best solution for this case. I will have to think about that and decide what should be implemented. The solution with OpAmp are simple, but can't drive much current. To be on the safe side, I would like the design to be able to drive about 1A of output current. 
Linear regulator will not work for that.
So I'm still quite not sure what will work....

[John B]

This is why I was a little confused. 12k input impedance with 5V max input voltage is less than a milliamp. How many valves do you intend to drive, where are you getting the 1A figure from?

[Zero999]

Thank you all for the replies. I'm not sure yet which one will be the best solution for this case. I will have to think about that and decide what should be implemented. The solution with OpAmp are simple, but can't drive much current. To be on the safe side, I would like the design to be able to drive about 1A of output current. 
Linear regulator will not work for that.
So I'm still quite not sure what will work....

You said in your other post it has an input impedance of 12k, which means the most it can draw is 417µA. Where did you get 1A from? I can tell you, it won't draw 1A from the 2V to 5V control voltage connection.