Simple Current Limiting

Yes, that will work. Please share your circuit with us if it's unusual or particularly elegant.

I am with you on this. Most of the systems that I have had to work on had limit switches to interrupt current after travel limit has been reached. The only exception that I can think of is the power window motors. They are momentary switches that have circuit breakers that auto reset.

I'm with Del.

A GA for good & simple solution.

I got to your reply after I had posted one in the same sense.

It is the most used way even by major manufacturers like Siemens [I had been working on their large X-Ray plants & Tables like tables for Tomograhy which tilts on different planes & positions; all points being controlled by End / Position points breaking.

End point changes over the switch(s) to Off the running circuit & changes it to the next [mostly REVERSE ].

Which is normally required as in this post.

Opened louvers ' next move will be to close; unless you have a plan multi-positions movements.

Even here it will be much simple than Electronically sensed project.

Have a fine day

If cheap is good then use a automatic door closer for home garages. It has switches that stop the motor in the open and closed positions. They are relatively inexpensive. Or look at their design. Your sensing device of current will have to compensate for surge at motor start up. Would be simpler just to use switches.

Ah yes, another "I've already designed a system by buying these parts but now I want to use the same parts and have it work just a little bit differently. But this should be simple for you, because I don't understand this stuff." Ok I've finished my little rant, I apologise to anyone offended.

The inclusion of limit switches will certainly be a better method with what you already know. But you are correct that using hard stops will be a cheaper final assembly if you knew a host of other information. To rely on the hard stop approach you must know the following:

1. The force required to close & open the louvre when new and clean.
2. The force required to close & open the louvre when old and dirty.
3. How fast the louvre will be opened & closed.
4. The gear reduction and therefore the force to torque and speed conversion.
5. The torque required to damage any of the mechanical parts.

Now if you knew these critical items before you selected a motor, you may have been able to select or fabricate a motor meeting all of these requirements. You may have found that no motor can be made to meet these requirements. This would lead to either abandoning the hard stop approach, modifying the requirements or redesigning the louvre.

"But I already have these parts. The motor fits the mounting bracket. Can't you brilliant engineers figure out how I can do my job." (Note: When begging it is wise to flatter. It doesn't always work but it does help. ) Sorry , but from your information alone I cannot give you a definitive answer. But I do have a path that you can follow to find out what may work. That is if you were lucky enough to select suitable parts in the first place.

By putting a resistor to limit the current between the supply and motor you will limit the current going into the motor. Likely you will not require full current while moving but this is one of the unknowns you've given us to work from. If the winding resistance of the motor measures 12 ohms then with a 12V supply the motor will draw nothing more than 1 ampere from a 12V supply. So adding a 12 ohm power resistor in series will mean that the running current will be only a half ampere. If this resistance means that you can only get the motor to run but not start then you are on a good path. By adding a capacitor that can charge up to the full 12V between the motor and limiting resistor you will get a full surge of current when starting.

Now hopefully you've purchased a motor that can be reversed. Oh, you didn't know that not all DC motors are reversible? Well that's not a problem I was going to address. Depending on how you wire this up you will need five times the resistance selected multiplied by the capacitor to build up a suitable charge before starting the motor shaft. You will have the motor resistance times this capacitor for the amount of time the capacitor will be providing full charge. After that time you will be drawing the current through the resistor.

"But, but you're giving me so many things that aren't known for me to find out. I wanted something simple and cheap to do my job." Well I hate to burst your bubble but this is as simple as I can make it. You've also not paid anyone here for there opinions so this is certainly cheap.

Good Luck

Hi all

Thank you for your replies:

• I'm not a guest, but I wasn't logged in, sorry
• My roller door has current limiting, I know because I had to adjust it as it's got old and stiff
• The motor is adequate for the mechanical torque
• It can be reversed
• I don't want to use limit switches because of the environment they're in, wet and cobwebby
• My power supply which has adjustable current limiting works fine on cutting out, so start up current is not problem
• The switch is a momentary on - off - momentary on, so generally the the power will stop when it's adjusted to position, this is only to stop the louvres at their limits
• I'll try using a variable resistor, another suggestion was to use a higher supply voltage and a LM317 or LM350

Thanks Tony

Although it is possible to sense current etc. of a stalled motor, often used in robotics when powering up to define the full angle of movement, chips are available also.

A far simpler method for a dirty environment are magnet(s) on the moving part (painted ones to stop rusting) and hall effect switches for sensing, on the fixed structure.

They are sealed semiconductors and can easily be cabled in a waterproof manner. There are no moving parts in such switches, they sense the magnet and require no physical contact.

Here is how it works:-

Hall effect switches

Searching on the web with "hall effect switch" will give you dozens of companies that produce them.

They have been around I believe since the late 60's or early 70's, I have forgotten exactly when I first saw and used one....maybe the article will give more detail on that score if required.....

Thats the simple way to go.

Regards.

Why not to try End limit switches operated by a Cam on the motor or any other place you think simple to be.

Any Micro-switch [SPCO] with a lever fitted to operate the switch by push to one side & release to go on the other.

SPCO =Single pole change-over. Even DOCO can be used & are mostly available in market.

It is very simple & most of travelling Mechs [side to side, revolving or tilting ] use this simple & more reliable

Limit switches may be a simpler and safer option.

OP has already said its a dirty and wet environment, reliability of such switches questionable.....

actually the simplest method would be limit switches that cut the power when you reach the end of travel. Wet and dirty is no excuse in my opinion. I've used mechanical and reed limit switches on deck mounted level winds for subsea umbilical reels without problems, it is just a matter of engineering it a bit.

While this is not a dramatic problem to overcome, how does one move off of the limit switch position and guarantee no movement in the wrong direction? This is why one must know how the motor is controlled.

I spoke (wrote) imprecisely. what I meant was it sent a signal back to the motor controller which in turn cut the power.

Again thank you all for your comments:

• As I said in #7 the motor controller is a switch, sprung centre off, (push the switch one way and the unit opens, push it the other and it closes, let it go and does nothing) the motor only operates when the user holds the switch
• The current limit is for when an idiot operator deliberately holds the switch when the unit is either totally open or closed
• I could use some kind of limit switch or sensor but it would increase the cost of the product
• I could also use a small micro but would prefer not too, because of cost
• I know from my tests with my bench power supply that current limiting works fine
• I am only after a few simple components to achieve this

Thanks Tony

OK...I have to chime in here.

First off, Current sensing is not that tough. Current sense transformer, Op amp, small microcontroller. I do it here on motors all the time. But.....

If you don't have the skills or if it would not be cost effective. Then Magnets are the best solution. I we use them here in VERY harsh environments. Hall Effects are great. But more complicated. They make magnetic sensors that are simple reed switches encapsulated in plastic. They require no power to run, the magnet just closes the switch. So its exactly like using limit switches with armatures or cams on the motor etc..

Best of all they are CHEAP! the ones we use here (Now remember we buy something like 10K of them a year) are less then a dollar. So I would bet you could get them for about 2 or 3 bucks.

I think they are made by Hamlin Sensors. Google them.

Good luck with your project!

Bill12780

PS Del is right...He is always right! Follow the KISS system "K"eep "I"t "S"imple "S"tupid