DCmotor current limit and switch off

Skalman

Joined Feb 19, 2022
6
Hi Folks!
Looking for a simple current limiter for a 12V dc gear motor. Normal current ca 40 mA. When current rises the circuit should shut off and remain off.

The motor drives a grain feeder, a spiral rotating inside a tube, rotating 2-4 revolutions every 20 seconds. But it can be stopped by unwanted things passing through and stop it, the current rises and motor stalls and finally it burns or cracks. Like putting a bar in a bicycle wheel.

Some transistors and current raising over a resistor, voltage falls?, and the transistsors switches of somehow? I also need a signal going out to a Raspbarry Pi computer giving it a High status on a GPIO pin telling the motor stoped.
Some have an idea how to disign this circuit?
Thank You

(English is not my first language, retired technichs and math and electronics teacher in Sweden)

KeithWalker

Joined Jul 10, 2017
2,611
Hi Folks!
Looking for a simple current limiter for a 12V dc gear motor. Normal current ca 40 mA. When current rises the circuit should shut off and remain off.

The motor drives a grain feeder, a spiral rotating inside a tube, rotating 2-4 revolutions every 20 seconds. But it can be stopped by unwanted things passing through and stop it, the current rises and motor stalls and finally it burns or cracks. Like putting a bar in a bicycle wheel.

Some transistors and current raising over a resistor, voltage falls?, and the transistsors switches of somehow? I also need a signal going out to a Raspbarry Pi computer giving it a High status on a GPIO pin telling the motor stoped.
Some have an idea how to disign this circuit?
Thank You

(English is not my first language, retired technichs and math and electronics teacher in Sweden)
Put a current shunt in series with the motor and feed the voltage across it to an amplifier/comparator circuit. Set the comparator voltage so that the motor will be switched off if the maximum current is exceeded.
You may need to disable the comparator briefly on startup if the motor inrush current exceeds the set point.

crutschow

Joined Mar 14, 2008
31,112
As Keith stated, a comparator and a relay, plus a few passive parts should be all you need.

Do you have ready access to electronic parts such as a LM339 or LM393 comparator, a small 12V relay, as well as diodes, resistors, and capacitors?

Skalman

Joined Feb 19, 2022
6
Thank You both this far. I understand the idea, and yes, I can fix the components, have a smal stock left over before retired and there are shops as well.

But first, may I ask you to take a look at this two attached circuits and give some advice.

The first one, how and where to take out a signal to my Raspberry Pie? I can not figure out myselfe. This circuit works nice, remote turning the handle on a ball-valve. At the end positions the current rises and motor swithes off. The lines in the upper right corner where the power is connected. Capacitors elyt, + facing to each other

The other curcuit uses a "rectifiere"? (name?) This might also work but where to put out the signal telling my RPI the motor has shutted down. I also need a "opto" or a relay from the Raspberry Pi GPIO pin to connect, not good idea to connect direct to the Pie.

The solution from Keith is good but needs some more work, "etch" a printed board and more soldering, okey for me if those first two doesn 't work. Do You have some schema over it? I would appreciate it.

Thank You!

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LowQCab

Joined Nov 6, 2012
2,628
Here's how I would do it .............
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Skalman

Joined Feb 19, 2022
6
Thank You LowQCab!
Very nice indeed. What value for the Zenerdiod? Volts?
I guess when current rises over the 20 Ohm 0,5 w resistor the P- Fet will shut down, right?

To handle it from the Pi and a relay, I think I could use the SPDT contact and the relay is holding all the time the motor is running. Release the relay will "push" the "stop" button, is that okey? I wonder , can the "start" button be"pushed" all time the motor is running? No? Override the Zener function?

I have only one (1) free GPIO pin from the Pie, so maybe need some modification for the "start" button. For now the GPIO pin is "high" all the time motor is running. Internal timer gives the amount of turns, that giving the right amount of grain transported.
That is the primary goal.

I also think the outgoing signal could take place on the anod side of the LED. Using a 2N7000 can connect direct to the Pi, bi directionell, and limit current to the Pi. See the schema attached. Increasing the R2 to fit the 12 V instead the 5V.

Hope you will correct my thoughts and answer.
Thank You.

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crutschow

Joined Mar 14, 2008
31,112
Here's how I would do it .............
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Interesting circuit.
One concern.
The trip voltage to the TL431 consists of the +12V plus the voltage across the 10R shunt resistor.
That would make the trip point sensitive to the +12V supply voltage, which does not seem to be something you want.

Might be better to put the shunt resistor in the ground side of the motor, and detect the voltage across that for the current sense.

LowQCab

Joined Nov 6, 2012
2,628
"" What value for the Zenerdiod? Volts?
I guess when current rises over the 20 Ohm 0,5 w resistor the P- Fet will shut down, right? ""

That's not a Zener, but that's the accepted Schematic-Symbol for an Adjustable-Shunt-Regulator,
which is totally different than a Zener.
It "acts-like" a Comparitor with a permanent 2.5-Volt Bias-Voltage,
which means that it conducts to Ground any time it's "Reference" Pin exceeds 2.5-Volts.

The Shunt-Regulator watches the Voltage going into the Motor,
that Voltage has to pass through a 10-Ohm-Resistor before it gets to the Motor,
so if the Voltage drops below your pre-set level because of a Motor-Overload,
and stays that way for more than ~3-Seconds or so,
the P-FET will then turn Off, and stay Off.

If the "START" button is held down, the P-FET will never turn "Off".

If You want to control the Motor with a Micro-Controller,
simply replace the "START" push-button with the usual "Open-Collector" Transistor.
The pulse to the Transistor must be long enough for the Current drawn by the Motor to
stabilize after it's normal start-up Current-Spike.

The same thing can be done with the "Off" button to stop the Motor with a Micro-Controller.

No Relays are needed or wanted.

Find the Data-Sheet for the Shunt-Regulator attached ......
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LowQCab

Joined Nov 6, 2012
2,628
Might be better to put the shunt resistor in the ground side of the motor,
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If his Power-Supply sags, it will be due to Motor-Overload,
so the function remains valid and accurate.
Also, he gave a very-wide Current-swing specification,
and this Circuit is easily adjusted for a precise Current-Trip-Level, and is quite accurate and repeatable.
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crutschow

Joined Mar 14, 2008
31,112
this Circuit is easily adjusted for a precise Current-Trip-Level, and is quite accurate and repeatable.
Still depends upon the characteristics of the supply, which have not been stated.

LowQCab

Joined Nov 6, 2012
2,628
There's almost a 1.5-Volt swing between "normal" and "overloaded" Current levels,
(~40ma to ~120ma ),
that would even allow for a Li-Po-Battery running down,
without affecting the intended function of the Circuit.
And if it is, in fact, Battery-Powered,
additional sensitivity when the Battery is getting low would be a bonus for Battery-Life.
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Skalman

Joined Feb 19, 2022
6
Thank You LowQCab for the explanation. I have not learned that symbol thought it was a Zener, now I understand.
I see the "chat-arguing" between You and Mr Crutschow and understand, and it also gave me more insight/understanding.
Let me say, the Power to the motor and the whole Control-thing is a stable 12 Vdc- "thing" (Sorry my english) with very little "ripple" and lots of Amperes if needed.

Just for further more understanding, the "thing" I have is a burner, burning grain and that way heating up my house, in Sweden we have "winter" in summertime mostly
For now I control this "machine" by a programmable Zen relay system so called ladder graphical program and that controls a built in PLC circuit.
Unfortunately it can not take all the necessary inputs and have run out of available contacts. The solution was to use the Raspberry Pie computer and learn the Python language and create a program controlling the burner, the temperature of the house and all that stuff. It also controls how to get the grain from outside the house from a big container and in to the basment/boiler room.
For an old dog like me it was a big deal learn the Python, since I have programmed only in the old Pascal language. The electronics is much easier but it was some years ago I was active and things developed.

So, there we are now and sorry for this little digress.

Now I must evaluate all these circuits and make some tests on a breadboard. Please give me some time and I shall come back later, it can take weeks.... old boy....I check this site if something new come up. I have also some ideas about the first circuit I attached using only the upper half of it, still not know how and where to get out a signal back to the Raspberry Pieindicatong tha motor stopped. Any suggstions for that? Please?

Thank You all.

MisterBill2

Joined Jan 23, 2018
13,722
Another possible option is to use a constant current regulator to power the motor, set to regulate at the normal running current. The monitor the motor voltage.When the motor stalls the voltage will drop, and if the processor board has an analog input you have the whole system, with the added benefit of ajustable motor protection. Use the lowest voltage 3-terminal regulator that you can find so that the most voltage is available for the motor, because the CC package will always drop it's rated voltage. Just the 3 terminal regulator and one resistor between regulator common and the output terminal, with the load tied to the common terminal. The resistor sized to drop the regulated voltage at the desired load current. You do still need the two 0.1 mfd bypass capacitors for the regulator, though.

Skalman

Joined Feb 19, 2022
6
Well MrBill2
Thank You.
Seems simple. One drawback, the RPi has no analog input. Only Low/high digital.
I still thinking to try get out a signal from the first circuit. You see that circuit has been proved out and it functions very well. Not many componets either.

The program should be something like this:
while:
....."Motor is on"
........If GPIO xyx = = high (or low)
............"shut of motor"
..........."do something"
.....
end
The dots mark the indent spaces.

LowQCab

Joined Nov 6, 2012
2,628
A full explanation of how this whole system is supposed to work may get You
Trying to "fix-what-You-already-have" may not be the best plan.
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MisterBill2

Joined Jan 23, 2018
13,722
OK, no analog inputs does change the game a bit. So use comparators to indicate the crossing of different voltage levels. An LM339 is a quad comparator, with just a bit of external logic you can convert the 4 outputs into 2 binary bits, and save I/O. And I am slightly familiar with pellet burning stoves, but they burn wood pellets.

crutschow

Joined Mar 14, 2008
31,112
There's almost a 1.5-Volt swing between "normal" and "overloaded" Current levels,
(~40ma to ~120ma ),
that would even allow for a Li-Po-Battery running down,
without affecting the intended function of the Circuit.
Don't see how.
Any drop in battery voltage reduces the current limit value.
If the battery voltage drops by 1V, for example, then you only have a half volt left for the current limit (giving a limit of about 40mA in this case) .
The certainly would seem to affect. the desired circuit function unless you want the circuit trip on a normal current when the battery voltage drops more than a volt.

Skalman

Joined Feb 19, 2022
6
Well........ now it is done. See the attached schema/circuit.

The simplest solution was the best off all. It is functioning. When braking the sprocket wheel the mA rises and about 100mA and the circuit breaks and the motor stops and stay stopped until the power line switches off and put on again. Just as wanted. Squeezing the wheel hard enough the motor stops immediately. The Opto gives the signal back to the Raspberry Pie telling engine is running.Trans are BC337 0,8 A the shunt resistor 5,6 Ohm and Elyt 200 myF. They limit the startup current.

So... now thank You all for the afford and help. If someone wants to use this solution be my guest but tell from who yo take it.

Greetings from Sweden.

Thank You.
PS
Posted this on a wrong link first, sorry.

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MisterBill2

Joined Jan 23, 2018
13,722
YES!! A "bi-stable" circuit that changes state when the current limit is exceeded. Rather brilliant. Thanks for sharing it.