Electronics > Projects, Designs, and Technical Stuff
User defined current limiter
RawCode:
Hello guys.
I would like to design a used defined current limiter for my thesis work.
In my design i have a DAC output amplified to obtain +/-10V. I would like to add a current limiter to it, but I'm a bit confused about how i can do it.
The user should be able to set a maximum current, and a microcontroller should set the maximum current setting for example the gate voltage of a mosfet.
The maximum current should be from microampere range to hundreds of milliampere. Is it doable?
I tryed to simulate a small circuit using a mosfet in series to the opamp output and tying the feedback path after the mosfet.
It seems to work, but i wonder if this is a reliable simulation and if the final circuit would be stable.
Considering that the circuit is able to output a +/-10V swing, is it sufficient a nMOS? is it necessary a pMOS?
Should the mosfet(s) have a separate body connection?
How can i calculate a relation between the gate voltage and the maximum output current?
Can you help me?
Thanks in advance :)
David Hess:
Pin drivers for automatic test equipment usually have a digitally programmable current limit. This it is implemented as part of a class-AB output stage like you would find in a 2-quadrant regulator or power supply but with an added current control loop like you would find in a constant voltage and constant current power supply.
The difficulty here is that what you probably want is an absolute current limit instead of a bipolar current limit but that can be accommodated with some difficulty.
What you are suggesting in your example will not work because with the current limiting element within the voltage feedback loop, so the voltage feedback loop will try to ignore the current limit.
coromonadalix:
I'm trying INA226 INA219 from texas instruments
I want to implement a minimum curent sense and a maximum limit ...
you have acs7xx hall current sensors too
David Hess:
Check out the Linear Technology LT1970A which does what you want. At the very least it will give you some ideas about how to proceed.
RawCode:
Thank you for your replies :)
--- Quote from: David Hess on July 14, 2020, 01:41:56 pm ---What you are suggesting in your example will not work because with the current limiting element within the voltage feedback loop, so the voltage feedback loop will try to ignore the current limit.
--- End quote ---
Probably i'm wrong, but is this kind of current limiter in the feedback path inside an UA723? If yes, why does it work?
The full circuit is the following
There is a transimpedance amplifier, which depending on the load's current, produces a voltage.
This voltage is compared with a fixed reference with a difference amplifier wich drives the mosfet gate.
In this picture the reference is 5V, which means 5uA that are flowing through the load. As you can see looking at the plot, it doesn't limiting the current.
It seems that the transimpedance amplifier does not provide a virtual ground to the load.
--- Quote from: David Hess on July 14, 2020, 01:41:56 pm ---What you are suggesting in your example will not work because with the current limiting element within the voltage feedback loop, so the voltage feedback loop will try to ignore the current limit.
--- End quote ---
Is this a completely wrong idea/approach?
I put the mosfet in the feedback path since i would like to have the same voltage at the input, but current limited.
How can i achieve that?
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