I own a small handheld manual tool that can open and close the mechanical part of a linear expansion valve but how is this done electrically with a coil?
For reference, this is for an air conditioning system.
This is what the service manual gives me for troubleshooting one the coil.

 

This is a simple "Stepper-Motor",
it's the same type of Motor that controls the Idle-Speed of the Engine in your Car.

They are not necessarily designed for Speed, or Brute-Power,
but more for precise-positioning, usually via a Threaded-Rod, one step at a time.

There are plenty of specialized-Chips designed to control this type of Motor.

Note: The fact that the 2-Coils are "Center-Tapped" indicates a specific-type of Stepper-Motor,
requiring a Driver-Chip that is compatible with this particular Winding-Configuration.
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That is indeed the circuit of a "Unipolar" type stepper motor, that applies a (usually positive) voltage to the center connection of both windings and then controls which of the four are energized by four (open collector) NPN transistors.

 

This is a simple "Stepper-Motor",
it's the same type of Motor that controls the Idle-Speed of the Engine in your Car.

They are not necessarily designed for Speed, or Brute-Power,
but more for precise-positioning, usually via a Threaded-Rod, one step at a time.

There are plenty of specialized-Chips designed to control this type of Motor.

Note: The fact that the 2-Coils are "Center-Tapped" indicates a specific-type of Stepper-Motor,
requiring a Driver-Chip that is compatible with this particular Winding-Configuration.
I would like to use something that I could interface with an Arduino board.
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.
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Thank you for the reply.
Can you elaborate on what kind of driver-chip I would need to control this mechanism?

 

That is indeed the circuit of a "Unipolar" type stepper motor, that applies a (usually positive) voltage to the center connection of both windings and then controls which of the four are energized by four (open collector) NPN transistors.

Thank you for the reply,
Can you elaborate more on exactly how this works? and what is needed to control this mechanism?
How is the direction controlled by energizing the transistors?

 

Not "just any Controller" will work.
The Motor is designed to operate with a Controller that supplies a specific, and limited, amount of Current.
The Controller is not only a "Polarity-Switcher", but also a "Current-Regulator".

Unless the specifications of the original Stepper-Motor-Controller can be obtained,
this Motor, or Valve, can not be used without the possibility of accidentally burning-up the Motor-Windings.
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there are many online resources explaining stepper operation.
here is one
https://blog.poscope.com/stepper-motor-driver/

 

Thank you for the reply,
Can you elaborate more on exactly how this works? and what is needed to control this mechanism?
How is the direction controlled by energizing the transistors?

Yes, I could go on in a lot more detail, but NO, I will not. Stepper motors are a fully mature technology and not only the basics but most of the insights are published and presented very clearly.

 

@lsheaf can you identify the motor by any markings?

 

You can check the article below to see how a Stepper Motor works.
Stepper Motor Forward And Reverse Testing circuit.

 

You can check the article below to see how a Stepper Motor works.
Stepper Motor Forward And Reverse Testing circuit.

Thats OK for testing, but in practice the +ve would be much higher than the motor rated, and current monitoring would take place.
So a different circuit would typically be used for an actual project.

 

Thats OK for testing, but in practice the +ve would be much higher than the motor rated, and current monitoring would take place.
So a different circuit would typically be used for an actual project.

Yes, and many applications may use some uC or Arduino to control.

 

Personally for any one getting in to steppers I would look at the Bi-polar versions and use IC's such as L297 - L298 combo.

 

If the need is for stepper control for any commercial project I seriously suggest using a driver made by the stepper manufacturer that is optimally matched to the motor. My reasoning is that if ever pieces need to be replaced, the client will be able to quickly know just what, and from where.

Of course, the only stepper motor in a machine that failed was one that the mechanical team mounted where it was under water during every machine cycle. It was not even a water resistant motor.

 

If the need is for stepper control for any commercial project I seriously suggest using a driver made by the stepper manufacturer that is optimally matched to the motor.

Personally I don't favor applying restrictions on my sphere of work, I would much rather have options.
When I would retro-fit a machine for CNC, I would mix and match makes of drives and servo motors at will. !

 

Back to the original question from post #1. The motor rotates in small steps as the drive power is switched in a repeating pattern among the four halves of those winding connections. The benefit of this scheme is that the motor can be driven any small distance quite accurately and as slow as is required for the adjustment.

 

@lsheaf can you identify the motor by any markings?

I've got lots of them laying around, I'll take a picture of it for you guys tomorrow.

 

I suggested matching the driver to the motor because it will then not take as much effort to produce a good match. NO OTHER REASON. Perhaps the TS is not a master servo tech? Not everybody has 30 years of stepper motor experience.

 

But never too late to learn !

 

I looked at all the LEVs I have and none of them have a wiring diagram.
Just to be clear, this is the type of valve I'm dealing with. My LEVs are also 5 wire as opposed to 6 wire shown in this image.