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MOSFET or Relay, which to use

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ComSciGuy
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 I will be using a stepper motor to raise and lower a table and I want it to hold without the need to constantly drive the coils for the necessary torque and I’m thinking a stepper motor with an electromagnet brake would work well in this application.  The driver diagram for the brake says to use a relay and a diode.  After watching Bill’s video on MOSFETs and Transistors, it seems this would be a good application for a logic level MOSFET or a power MOSFET and a transistor (I saw this in a different video) to drive the MOSFET gate with a lower logic voltage? 

Which option would be better?  I plan to make a custom PCB, so either option be equal in implementation.  So I’m wanting to know about other qualities of one vs the other, i.e., will the magnetic field of the relay cause interference, is one more reliable than the other…  Thanks.

 


   
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Will
 Will
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@comsciguy

First, welcome to the forum.

Are you thinking that the power required for maintaining the table's position will be less for the brake than the holding current for the stepper ? That is, are you looking for a way to reduce or eliminate the need for power once the table is at the desired height ?

If that's the case, you may be better off using a threaded rod which is driven by the stepper(s) to raise and lower the table. That way you can maintain the elevation with no power draw at all since the screw won't "unwind" under the weight of the table after the stepper stops. Further, the threaded rod will increase the leverage and thus reduce the force required from the stepper to lift the table (albeit requiring a longer time to move the table).

Experience is what you get when you don't get what you want.


   
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Inst-Tech
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Posted by: @comsciguy

 I will be using a stepper motor to raise and lower a table and I want it to hold without the need to constantly drive the coils for the necessary torque and I’m thinking a stepper motor with an electromagnet brake would work well in this application.  The driver diagram for the brake says to use a relay and a diode.  After watching Bill’s video on MOSFETs and Transistors, it seems this would be a good application for a logic level MOSFET or a power MOSFET and a transistor (I saw this in a different video) to drive the MOSFET gate with a lower logic voltage? 

Which option would be better?  I plan to make a custom PCB, so either option be equal in implementation.  So I’m wanting to know about other qualities of one vs the other, i.e., will the magnetic field of the relay cause interference, is one more reliable than the other…  Thanks.

 

Hi @comSciGuy,

Yes, your assumption is correct..you can use a power Mosfet in this application..The benefits of of the Mosfet are small circuit footprint, very fast switching time, and no counter emf to deal with as with the counter emf generated  by a electro-mechanical relay, hence, the "flyback" diode used to bypass the relay coil when the magnetic field of the relay collapses. There are also Solid state relays that can also be used https://www.celduc-relais.com/en/dc-solid-state-relays/

Check out the numerous ways you can do what you intend to do..Without seeing exactly what your operating parameters are for your design..it would be difficult to advise you on what to pick..

At any rate, good luck on developing your project. I'm interested in seeing what you come up with..

Regards,

LouisR

 

 

LouisR


   
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Ron
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@comsciguy If the e-brake is only powered while the table is moving and then held in the braked position via springs then you draw NO power when stopped. I think @will has an even better solution but without knowing weight of table it's impossible to be sure.

Arduino says and I agree, in general, the const keyword is preferred for defining constants and should be used instead of #define
"Never wrestle with a pig....the pig loves it and you end up covered in mud..." anon
My experience hours are >75,000 and I stopped counting in 2004.
Major Languages - 360 Macro Assembler, Intel Assembler, PLI/1, Pascal, C plus numerous job control and scripting


   
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ComSciGuy
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Thanks for the info and suggestions.  The specific application is a tool that already has a threaded rod for raising an lowering the bed.  The tool can be configured into a jointer or planer mode.  To do the reconfiguring the planed bed must be lowered to about 7" to get enough clearance for the top to come down.  However, when you use it as a planer, most of the time you'll be planning something closer to 1".  This requires about 80 turns of the threaded rod.  Which as you can imagine, gets tedious especially if you need to switch between the two modes fairly often. 

In my intended solution, the stepper motor will turn the threaded rod to raise and lower the bed.  In addition, the precise height of the bed can be determined and displayed in the software and it will have other convenient features.

  The bed has some backlash that is normally corrected with a lever that is tighten to lock the bed into place.  Obviously, for an automated setup, you don't want to manually engage and disengage this lock and forgetting to release it when attempting to move the bed would lead to missed steps.  I first was looking at a motor with a newton meter or two of torque, but this would require the motor to be constantly energized to hold the bed to prevent the backlash.  

  The only drawback I can think of is the inability to turn the threaded rod when power to the motor is off which you could do with a standard stepper motor; however, I don't plan to have the handle on it, instead the belt sprocket will take its place.

I hope that brings more clarity to the challenge.

Walter


   
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ComSciGuy
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Posted by: @will

@comsciguy

First, welcome to the forum.

Are you thinking that the power required for maintaining the table's position will be less for the brake than the holding current for the stepper ? That is, are you looking for a way to reduce or eliminate the need for power once the table is at the desired height ?

If that's the case, you may be better off using a threaded rod which is driven by the stepper(s) to raise and lower the table. That way you can maintain the elevation with no power draw at all since the screw won't "unwind" under the weight of the table after the stepper stops. Further, the threaded rod will increase the leverage and thus reduce the force required from the stepper to lift the table (albeit requiring a longer time to move the table).

Thanks for the warm welcome.  From my understanding the e-brake is engaged when power is removed from the brake control line and you apply 24v to release the brake to perform the movement.  So yes I would assume the power required to maintain the table position would be less for the stepper motor with the e-brake than a standard stepper motor.

As I stated in my previous reply, the planner table (or bed, as I've also called it), does have a threaded rod.  I don't have the machine yet, so I don't know the exact mechanics of the lifting mechanism.  The threaded rod is parallel with the bed, so there must be some gearing involved.

 


   
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Will
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@comsciguy 

I was actually talking about not requiring any brake at all. Threaded rods require a sideways torque to move them, most will just stay where they are when a load is applied along the axis of the thread.

Consider, as an example, a old-style piano stool with a twist-to-adjust height. You raise or lower the seat by applying a very mild horizontal twist but, once the seat is at the required height, one can sit on it without requring any force applied to hold it at that height.

That's why I suggested that if you're using  threaded rod you may not need ANY braking at all (and hence require no power at all to maintain the table's position.

If you're too young to remember piano stools, then substitute a vice into the same analogy. You can tighten it by rotating the handle attached to the screw drive but it doesn't flop open when you stop turning.

 

Experience is what you get when you don't get what you want.


   
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Ron
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@will I am familiar with machines of the type @comsciguy is talking about. The reason they have a locking lever is they are either planing or jointing wood. This creates a lot of vibration and some back pressure. Now that I know the machinery involved I am thinking a mag brake is insufficient. Instead, simply add a solenoid to the locking handle.

Arduino says and I agree, in general, the const keyword is preferred for defining constants and should be used instead of #define
"Never wrestle with a pig....the pig loves it and you end up covered in mud..." anon
My experience hours are >75,000 and I stopped counting in 2004.
Major Languages - 360 Macro Assembler, Intel Assembler, PLI/1, Pascal, C plus numerous job control and scripting


   
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ComSciGuy
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Posted by: @will

@comsciguy 

I was actually talking about not requiring any brake at all. Threaded rods require a sideways torque to move them, most will just stay where they are when a load is applied along the axis of the thread.

Consider, as an example, a old-style piano stool with a twist-to-adjust height. You raise or lower the seat by applying a very mild horizontal twist but, once the seat is at the required height, one can sit on it without requring any force applied to hold it at that height.

That's why I suggested that if you're using  threaded rod you may not need ANY braking at all (and hence require no power at all to maintain the table's position.

If you're too young to remember piano stools, then substitute a vice into the same analogy. You can tighten it by rotating the handle attached to the screw drive but it doesn't flop open when you stop turning.

 

https://sawmillcreek.org/showthread.php?297320-Hammer-A3-31-converted-to-quot-CNC-quot

image

The link above is similar to what I had in mind.  I'm assuming you are referring to a vertical threaded rod.  In this application a vertical rod would not work since the threaded rod in the middle of the table is what will raise and lower the bed.  

 

 


   
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