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Current limiter circuit design

Myhay

May 18, 2015
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May 18, 2015
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Hello to all,

I am looking to build a current limiter for a DC motor.
The DC motor gets the command to spin more that it is necessary. I would like to build a circuit based on LM317 (any variants) or on a basic approach with transistors.

The motor should not spin more than 7 seconds. In the real scenario it spins around 15 seconds, so it is twice more that i need.
I want to build the circuit to cut off the power when the motor reaches to maximum and stops. At maximum position it is still powered but it is not useful anymore.
How can i calculate the needed R for LM317?
 

poor mystic

Apr 8, 2011
1,074
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Apr 8, 2011
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1,074
Hi :)
I'm sorry, I haven't been able to work out what you want.
If you want to time the motor's ON period, I believe you need a timing circuit.
If you want to limit current, you'll need to measure current somehow, and control it using some kind of regulator. However the LM317 is not a good choice - it's a voltage regulator, and what you need is a switch.
If you want to stop feeding the motor current once it reaches its maximum speed, you'll need to measure how fast the motor is spinning, and once again switch it off, probably using transistors of some kind.
 

Myhay

May 18, 2015
7
Joined
May 18, 2015
Messages
7
The situation is like this:
The DC motor gets the command from a button.
1.When the button is pressed the, DC motor spins forward. The motor reaches maximum position in 5-7 seconds. After 5-7 seconds, even if the motor is at maximum position it still gets the spin command. Basically i want to cut off the power after these 5-7 seconds so the motor won't force the set of gears from where he belongs. At this moment it forces them for 10 more seconds.

While the button is pressed, if i unplug the power and plug it back the command is resent and the motor forces to maximum point if he is already there.

2. When the button is released, DC motor spins backwards. The motor reaches minimum position in 5-7 seconds. At this moment, if it reaches minimum, it still tries to go further another 10 seconds. So in this case i would want to cut off the power again.
If the button is release, there will be no forcing here. It stays in this state even if in unplug the power and plug it back.

I was considering a 555 timer circuit but the polarities are reversed each time when i press/ un-press the button so i would need to ensure the right polarities in this case...

The main issue is that the motors reaches the min and max position and it still gets power to spin more even if physically it can't.

Is there any free software that will allow me to simulate with LM317 ?
 

davenn

Moderator
Sep 5, 2009
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14,304
you want to look at the use of limit switches at the max and min positions so as to switch off power to the motor
 

Myhay

May 18, 2015
7
Joined
May 18, 2015
Messages
7
I already considered that but i would not want to go into adding parts on the piece. This means drilling out holes and adding those switches in a place where i would not want to get. I would consider adding an electronic device (made by me) on the electrical part even if that element will dissipate heat.

There are two wires long enough to be cut to add this device.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
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25,510
Presumably the motor stalls when it gets too the limit of motion. This will cause the current to increase significantly. If you detect that rose in current and shut off the power you may have the beginnings of something.

You could use a fuse, but there are a number of reasons that may be impractical :)
 

Myhay

May 18, 2015
7
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May 18, 2015
Messages
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The motor take like 450mA when it is at min or max and it and still tries to push.
While is working between min and max it take around 75mA (+/- something).

So basically i would need to cut off the power if the current exceeds let's say 100mA.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
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25,510
What you need to realize is that the motor will draw the stall current as it starts, although the current falls as the motor speeds up.

Current is often measured by looking at the voltage drop across a current sense resistor.

So what you would be looking for is a circuit that triggers the motor on with a pulse and it is turned off by a persistent current above a set limit (I.e. A short duration pulse won't turn it off.

One method would be to use a 555 as a bistable. A capacitor can convert an on signal to a pulse and a current sense resistor, a few transistors and a capacitor can be used to turn it back off again.

I may be able to draw something for you later.
 
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