Problem with designing over current sensing

[CMOS]

lm324 current sensing

I am working on a microcontroller based project in which I need to sense current flowing through a load. If the current increases beyond 30mA, I need to interrupt the uC using high to low transistion. I tried making difference amplifier using LM324 to amplify the drop created across 1Ω resistor inserted series with the load. The difference amp. is followed by a comparator. But it does not work as the maximum output LM324 can give is near 3.5V at Vcc=5V. I need atleast 4V output so as to be on safe side and avoid false interrupt caused due to logic 1 being recognised at logic 0 near thresholds.

I know I can do i using rail-to-rail output op-amp like TLC2272 but those are costly and difficult to find in the local market.

Can anyone help me in designing this using low cost methods?

 

[mropz]

current sensing using lm324

If you are using a discrete comparator, you can try to change the voltage of comparison (one of the inputs of the comparator).
If your comparator is the own I/O input port of the microcontroller, a possible solution could be to use a pull up resistor (or a resistive divider with a pull up resistor) in the output of the LM324, in such way that you change the voltage window position of the amplifier voltage. For example, you can make the output vary from 1V to 3V, or 2-4V or 3-5V... so, you only must use the range that allows the correct operation of the comparator.

I hope it can help you... if does, please click helped to give me some points!

 

[sunking]

lm324 as current sensing

there are many kinds of rail-to-rail output op-amp.
go to
https://www.google.com/search?hl=zh-CN&newwindow=1&q="rail-to-rail"+pdf&lr=

 

[artem]

lm324 as current sensing

If LM324 rise time is enough for you - make a comparator on that opamp and avoid unnecessary components(oiffset voltage has to be checked).
Can also introduce slight hystheresis for proper detection.

 

[IanP]

how op amp can sense the overcurrent

For just over-current sensing any opamp will be OK ..
Have a look at the attached picture ..
As the reference voltage (in this case) this circuit uses 1.5V, and to provide 0V or 5V output it has added a BJT ..
Close to 30mA (1.5V) the comparator will oscillate, so you can add 1kΩ resistors in series with both inputs, and 1MΩ resistor between its outpit and (+) input (hysteresis) ..
Regards,
IanP

 

[CMOS]

amplifier using lm324

Hi Ian,
I'll implement that circuit and let you know the results.