Motor over current sensor

Thread Starter

Matt D.

Joined Nov 10, 2013
65
Hello,
I found this circuit and wondered if this can also do the opposite. Instead of sensing over current, can it sense under current?

Would this work with batteries or would there have to be a voltage regulator with batteries to keep things consistent when the battery voltage drops over time?

Thanks,
Matt
http://www.romanblack.com/current.htm
 

MaxHeadRoom

Joined Jul 18, 2013
19,697
Just use the info in reverse, set so that it switches on at normal current level, not over current.
The other way is to monitor in a analogue fashion.
Max.
 

Thread Starter

Matt D.

Joined Nov 10, 2013
65
Just use the info in reverse, set so that it switches on at normal current level, not over current.
The other way is to monitor in a analogue fashion.
Max.
I want it to stop the motor when the current drops. So the motor will be under a load and then when the load is removed but the motor will still be turning, but at a reduced current. Then cutoff the motor.
 

MikeML

Joined Oct 2, 2009
5,444
That is a really crappy current sensor. Look at what it does as a function of ambient temperature. Roman should be ashamed...

If you are worried about over-discharging a battery, that is usually done by detecting a drop-off of battery voltage; not load current.
 

Thread Starter

Matt D.

Joined Nov 10, 2013
65
I have the Pic Motor Development board and have used their similar method, Check out the schematic and see how it is done. (r26) 505HR100 current sense resistor.
Max.
Thanks. I was not planning on using the board, but rather use a latching relay to turn off the motor?
I don't understand too much of this stuff, but could I set up a sense resistor with the motor and go to the input side of a Schmitt trigger?
 

MaxHeadRoom

Joined Jul 18, 2013
19,697
I don't understand too much of this stuff, but could I set up a sense resistor with the motor and go to the input side of a Schmitt trigger?
That was going to be another suggestion, if you don't want to go with the analogue approach, a comparitor circuit also could be used with the current sense resistor.
You can also get mosfets with extra pins with current sensor internal.
The development board is just for design purposes and testing.
Max.
 

Thread Starter

Matt D.

Joined Nov 10, 2013
65
Thanks. I'll probably need a bit of hand holding to get through this. And to be able to pick out a sense resistor.
I'm planning on using TI Schmitt SN74AC14 with 4 AA alkaline so about 6v.
I don't quite understand at which voltage it switches. On the top of the 3rd page of the datasheet it has positive going threshold voltage and negative going. For 5.5V and positive going it lists min 1.6V and max 3.9V. For 5.5V and negative going it lists min 1.1V and max 2.3V. I don't understand why they overlap. Is this kind of a floating imprecise range?
The hysteresis is min 0.5V and max 1.6V.
The switch could be anywhere in the min through the max voltage ranges for either positive or negative going?
 

BR-549

Joined Sep 22, 2013
4,938
Hey Max, can you give me a pointer to those mosfets with extra pins with current sensor internal. I would like to learn about those.
 
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