User defined current limiter - Page 1
Forum section: electronics / projects

author: RawCode
author_rank: Regular Contributor
timestamp: July 14, 2020, 08:53:06 am
content: 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

author: David Hess
author_rank: Super Contributor
timestamp: July 14, 2020, 01:41:56 pm
content: 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.

author: coromonadalix
author_rank: Super Contributor
timestamp: July 14, 2020, 02:21:31 pm
content: I'm trying INA226 INA219 from texas instrumentsI want to implement a minimum curent sense and a maximum limit ...you have acs7xx hall current sensors too

author: David Hess
author_rank: Super Contributor
timestamp: July 14, 2020, 03:01:10 pm
content: Check out theLinear Technology LT1970Awhich does what you want. At the very least it will give you some ideas about how to proceed.

author: RawCode
author_rank: Regular Contributor
timestamp: July 14, 2020, 04:49:04 pm
content: Thank you for your repliesQuote from: David Hess on July 14, 2020, 01:41:56 pmProbably 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 followingThere 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 pmIs 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?

author: RawCode
author_rank: Regular Contributor
timestamp: July 15, 2020, 02:10:25 pm
content: I'm trying different to find the best way to limit the current.This circuit is by far the most close to the thing to what i want, but there is a problem.As you can see in the circuit, the transimpedance amplifier's(X1) output is firstly inverted(or buffered depending on the current flowing direction) and then feed to two comparators(X3, X4) which drive the MOSFETs, comparing the inverted(buffered) transimpedance amplifier's output with a reference voltage.As you can see in the plot, the load's current(red line) is limited as expected at 5 nA (that's because 2.5V across the 500MOhm generates a 5nA current, which is converted in 5V by the transimpedance amplifier).As you can see though, the output voltage(green line) is also limited to 2.5V even if the input voltage(blue line) rises to 10V. I don't expect this since there is an opamp that should maintain the voltage equal to the input due to feedback. I really need that the voltage rises even if the current is limited.Can you help me understand why this happens? Is this current limiting circuit bad designed? If yes, can you suggest me how can i improve it?I know about the Ohm's law. But what i want to design is aimed to recreate an usual experiment that scientist do in paper about memristors. They limit the current to prevent the memristor damage, but they increase the electric field, i.e the voltage, on the device. That's is why i'm trying to force the Ohm's law.That is one of the classic memristor characteristic. They limit the current to prevent device damages.

author: Zero999
author_rank: Super Contributor
timestamp: July 15, 2020, 08:12:44 pm
content: What sort of response are you after? Do you want a current source or a device to shut off the power, when a certain current limit has been exceeded?1µA to 100mA, is a huge dynamic range. You'll definitely need to change the sense resistor, to switch ranges.

author: David Hess
author_rank: Super Contributor
timestamp: July 15, 2020, 11:31:16 pm
content: Quote from: RawCode on July 14, 2020, 04:49:04 pmIt works for the 723, and operational amplifiers which often use the same method, because the current limiter diverts base drive from the output transistors. Some power supplies work the same way.

author: RawCode
author_rank: Regular Contributor
timestamp: July 16, 2020, 08:30:15 am
content: Quote from: David Hess on July 15, 2020, 11:31:16 pmI see. Thanks for the explanation, very appreciatedQuote from: Zero999 on July 15, 2020, 08:12:44 pmI managed to obtain whis response with this circuit. The JFET behaves like a variable resistor and when the current limit is reached, the JFET increase its resistance and limits the current, but it allows the voltage to rise, as you can see in the plot.The problem is that this circuit uses JFETs, which are not meant to high currents(as far as i know), so probably at 100mA they can't be used. I tried with standard ehnancement MOSFET, but in simulation they doesn't behave like a variable resistor.I tried with a depletion NMOS and it works as expected when the voltage rises from 0 to 10V. When there is a negative sweep(from 0 to -10V), since there are no depletion PMOS, i cant limit the current. So i would like that the NMOS behaves like a very low resistance during this sweep, but at approximately 7V probably the channel pinches off even with the gate at -15V and the characteristic is not a straight line as you can see.Also as you can see, the characteristic doesn't start from 0, but starts at approximately 200mV. That's strange.Do you have any suggestions how can i improve this circuit?Quote from: Zero999 on July 15, 2020, 08:12:44 pmYep, this schematic is simplified, but the real circuit has some relays that switch the feedback resistor.