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I need an electronic switch. First post.

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DaleS

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Hi everyone,
I have spend a large part of this afternoon browsing through the posts on this site. I am very impressed with the diversity of problems and solutions that arrive here. I have built circuits from plans and kits in the past, but I have only limited understanding of the operation of transistors and logic chips.
The reason for the post: I have a 12V motor with a lever connected to it so that it will turn a partial turn in one direction and stop when a limit switch opens. A single pole switch energizes a DPDT relay to cause it to turn clockwise. When the single pole switch is opened the relay is de-energized and the contacts reverse the polarity to the motor and it turns counterclockwise until stopped by a limit switch. The motor draws 1.4 A at 12 V when stalled, less when operating for a maximum of 0.2s.
The problem: I want to build a small delay circuit to prevent motor burnout if there is an obstruction that prevents the motor from completing its movement to actuate the limit switch.
My idea was a simple FET (or MOSFET or 1.5A transistor), a capacitor and a resistor that would allow current to flow from the time the relay closes or opens for only about 0.5 s (not a critical time) and then would turn off to prevent motor burnout. 0.2 s is sufficient time to allow the normal movement of the motor. I hope that there is an elegant 'one wire in one out' design that could be put in series with the limit switch. If a problem prevents the limit switch from turning off the power the circuit will.
I haven't found anything on this site or others specifically like this where the on signal (12V) starts the timer that then opens that circuit.
Thank you in advance for your help.
Dale
 
Dale,

Please post you schematic...it will help with the solution. :)

Ken
 
Schematic

Hi Ken,
Thanks for your prompt response. I have a ratty pencilled schematic. I will clean it up and take a picture of it. I then need to figure out how to convert into a small but visible image and then import it to this site. May take a bit of time. The 'Insert Image' icon above asks for a url address for an image, which I don't have.
Dale
 
Dale, do you have a scanner? That would be the best way, scan it, convert to PDF, & attach...
 
Dale, do you have a scanner? That would be the best way, scan it, convert to PDF, & attach...

PDF? PNG would be better.

Dale, if you scroll down while you're editing your post, you'll see a button labelled "Manage Attachments". Click that when you're ready to attach an image (or document or whatever) to your post, and just follow the instructions.


Good luck!

Torben
 
Torben:
Converting it to a PDF would be a better solution than trying to take a picture of it. Some people don't know about png files. I don't want to get into a whole "which file format is better" thing with you, but is always nice to see that you are still around :)
 
Torben:
Converting it to a PDF would be a better solution than trying to take a picture of it. Some people don't know about png files. I don't want to get into a whole "which file format is better" thing with you, but is always nice to see that you are still around :)

Hi Mike,

I agree that taking a picture of it would be less optimal than scanning, but since the TWAIN scanner app is likely to offer the option to just save the file as an image there is no reason to convert that image format to a document format for upload.

Of course, particularly brain-damaged scanner programs might only save as TIFF or something but those are thankfully pretty rare these days.


Regards,

Torben
 
Torben:

Of course you agree with me, you offered an alternative format to use, other than a picture.

However, we don't know if Dale even has a scanner to begin with; now do we?
Maybe you just insulted his scanner, maybe his scanner is TWAIN compliant.

Dale, if you do have a scanner and it is TWAIN compliant, I don't think of it as brain damaged, like some people do, I prefer to say; coding challenged scanner.
 
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Torben:

Of course you agree with me, you offered an alternative format to use, other that a picture.

Where did I suggest he use a format other than an image format? I suggested that he use an image format (PNG) for the image, instead of a document format (PDF). It really doesn't matter how he gets the image into the computer to start with.

However, we don't know if Dale even has a scanner to begin with; now do we?

Nope.

Maybe you just insulted his scanner, maybe his scanner is TWAIN compliant.

His scanner--assuming it exists, of course--is unlikely to be non-TWAIN compliant. Not sure what that has to do with the scanner software he's using. It's just that some scanners come with over-cute and under-useful vendor apps. Those are brain-damaged. Or coding challenged for the politically correct. :)

Dale, if you do have a scanner and it is TWAIN compliant, I don't think of it as brain damaged, like some people do, I prefer to say; coding challenged scanner.

I didn't say TWAIN compliant scanners are brain-damaged, I said some of the apps that sometimes come with them are. Any scanner app which can't save to standard image formats I would consider fodder for the bit bucket. (Yes, TIFF is a standard, but not a useful one for use on the web).


Regards,

Torben
 
Now, now Torben, lets not go off the deep end here. Lets just help Dale with his original problem. Like I said before
I don't want to get into a whole "which file format is better" thing with you
 
Hi everyone,
Well, here goes. First of all, thank you again for your support.
Since my last post I have found some schematic software, learned it well enough to make my simple schematic, and attempted to export the file (not possible as a PNG or PDF) from ExpressSCH but as a BMP to Word Perfect graphic format and then from there to a PDF file. If this doesn't work, please be patient as I try something else. The one problem I had in some previous attempt was the coils in the relay missing. If they are still missing you could use a grease pencil to sketch them in on your monitor (just kidding).

I also had to put up with the distraction of the Canadian Ladies Curling final.

I am at about 12 hours on this project already. Once I have the circuit change it will take much less time to build the whole thing!
I have the file saved as: sch, emf, bmp and pdf.
Dale
 

Attachments

  • ActSch.pdf
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Schematic clarification

Hi again everyone.
I decided that some clarification of the schematic might be necessary.
The switch on the left is an SPST toggle that turns the motor on by energizing the relay. It rotates clockwise until the top NC limit switch opens and turns off the power to the motor. The relay is still energized.
When the user switch on the left is turned off the relay opens, reversing the polarity to the motor so it rotates counterclockwise and enabling the other limit switch. The other limit switch opens to cut power to the motor. The problem is that if the motor hangs up in the middle there will be continuous power to the motor and it will burn out. As I only need 0.5 s of power, the electronic off addition seemed like a good insurance strategy. The motor could be reset by just changing the position of the user switch to try again to unjam the motor.
The location marked on the schematic as "EL SW Here?" is the only place where the voltage doesn't reverse. I appreciate that there would have to be two small circuits, one to the right of each diode and before the limit switch.
Here I go repeating myself again. I hope this ends up in the right place on the page. I have not used a forum before, except to read as a guest.
Thanks again for taking time to respond.
Dale
 
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You should have a **broken link removed** across the motor to minimize inductive kick.
Here is something simple that should work. I haven't simmed or built this circuit so no warranty is expressed or implied:
 

Attachments

  • limits.PNG
    limits.PNG
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Power FET

Hi and thanks for your prompt response.

I didn't know there were power FETs that will handle current from either direction. If your circuit works, I don't think that I need the limit switches even. I just have to set the pulse length to match the movement of the motor -- a very elegant solution.
I have a small breadboarding apparatus that I will test this on shortly.
Thank you for your help.
Dale
PS. My daughter lives in Vancouver, we near Edmonton.
 
RC snubber

Hi again,

At this voltage and current, what would you suggest. Would 100 Ω and 1 µF be a good place to start. I gather that circuit performance stability is the issue here. With DC I would put a diode across the motor, but in this case it would have to be some other semiconductor that would handle current in both directions -- if there is one.

Dale
 
I didn't know there were power FETs that will handle current from either direction.
The FET is wired so it only handles the current in one direction. ie The reversing relay is between the FET and the motor so the FET only sees one polarity.
PS. My daughter lives in Vancouver, we near Edmonton.
We'll turn this into a Canadian board yet! :D
EDIT: For the snubber, you should use a 8.2Ω resistor if the max motor current is 1.4A The non-polarized capacitor you'll have to determine by experiment. You'd need a scope for that. You could start with a 1uF.
 
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Bidirectional timing

Hi again,
As I look at this schematic I can understand that the timing will work when SW is closed, but what about when SW is opened and the motor begins to reverse? I, of course, do not understand the 4070 circuitry either.
Thanks for your help.
Dale
 
kchriste,

By changing the arrangement of DaleS's limit switches, dropping the diodes and placing them in series, once a limit switch is opened, the motor will never move again. DaleS's arrangement stops the motor from moving in one direction, but allows it to start moving in the opposite direction when the polarity is reversed by the relay.

ken
 
Electronics 101 lesson 56

Hi both of you.

I have learned so much about this circuit since I started. I can deal with the limit switches so no problem. Thanks Ken. This circuit may make them unnecessary.

My big lesson for this morning was from kchriste's 4070 exclusive OR gate. I was originally puzzled about the circuit because I understood that when the user switch was engaged the delay would occur and stop the motor. It was only when I was in the store picking up components and looking at the data sheet for the gate that I understood that "H-L = on" produced the same result as "L-H = on" after the switch was opened, thus the circuit should provide approximately the same delay on opening the switch as closing it. I hope that kchriste will excuse my failure to grasp that concept originally.

I do have a question about the power FET. Is this an N FET or a P FET. (lesson 57). I found an entire page of power FETs in a manual, but no internal diagrams. I will have to look them up on line. I am gathering that a power FET of the proper polarity in a 2 or 3A size would work.

I am also having trouble finding the exclusive OR gate locally. May have to order some on line. Can't wait to start prototyping!

Any advice on the power FET would be appreciated and, thank you again for all your help.

Dale
 
kchriste,
By changing the arrangement of DaleS's limit switches, dropping the diodes and placing them in series, once a limit switch is opened, the motor will never move again. DaleS's arrangement stops the motor from moving in one direction, but allows it to start moving in the opposite direction when the polarity is reversed by the relay.
ken
OOpps you are right! What was I thinking? :eek:
DaleS, you need to keep your original limit switch wiring with the diodes. Sorry about that. Brain fart I guess.
I do have a question about the power FET. Is this an N FET or a P FET.
It is an N FET. Size it to handle apx 10A or more just to be safe. 10A N FETs are pretty cheap and you may even find one that handles more current cheaper. What suppliers, besides RadioShack, are convenient for you?
I am also having trouble finding the exclusive OR gate locally.
With some modifications, you could use a exclusive NOR gate (4077) instead or even a quad NAND gate (4093 or 4011) wired as exclusive OR.
 
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