Limiting current WITHOUT dropping voltage - Page 1
Forum section: electronics / beginners

author: khatus
author_rank: Regular Contributor
timestamp: November 09, 2021, 05:13:05 pm
content: I'm trying to limit a current of a 12V power supply to 1A. I am using this schematic from an online source. While simulating it on proteus simulation software the current in the resistor R4 does not give 1A output current. What is the problem with my circuit?Here is the online schematic that I have followed[url=https://imgbb.com/]

author: TimFox
author_rank: Super Contributor
timestamp: November 09, 2021, 05:22:18 pm
content: You show two different "ground" symbols at the two power pins of the op amp.What is actually connected to the power pins? You need several volts of positive voltage at the gate of Q1 with respect to its source to get it to conduct any current at all. The 5.5 uA in your simulation indicates that the MOSFET is turned off. Also, your "measured" voltages of +12 and +2 at the input pins are consistent with a "dead" op amp, i.e. one not seeing power.Even so, if the amp were powered, the voltage at pin 2 would still be positive with respect to that on pin 3, which would drive the output at pin 1 to the negative rail, were that possible, and that would certainly shut off Q1.The other circuit has the larger and smaller resistors at R1 and R2 interchanged, so that the op amp input voltages are derived from the voltage drop in R3, and the voltage on the op amp power is +5 V.

author: rfclown
author_rank: Frequent Contributor
timestamp: November 09, 2021, 06:06:56 pm
content: Besides what TimFox mentioned, you didn't follow the online schematic. It has a P channel MOSFET and you have an N channel. Even so, any circuit like this isn't going to be a current limiter but a current source. Your post title says "Limiting current WITHOUT dropping voltage", which in itself doesn't make sense. The way a current limiting circuit works is to drop the voltage if you go over the limit. I would Google again to look for a circuit for a 12v supply with 1A current limit to see how it's done. Usually it is a circuit which will steal current (or voltage) from the pass device control (base or gate). The circuit shown is a feedback circuit which will always try to drive the output to a certain level.

author: TimFox
author_rank: Super Contributor
timestamp: November 09, 2021, 06:15:20 pm
content: One thing I left out (besides the polarity of the MOSFET mentioned above):The original circuit compares the voltage across the drain resistor (R3 in your drawing, R1 in the original) to that across the upper resistor (R1 in your drawing, R2 in the original), which are both with respect to the positive supply (+12 in your drawing, +5 in the original), and the voltage at that node must be within the common mode range of the op amp. The overall negative feedback is a function of the MOSFET connection and the op amp inputs, which must be overall negative. As "rfclown" said, analog current sources/regulators/limiters act by reducing the output voltage to obtain the desired load current.

author: Vtile
author_rank: Super Contributor
timestamp: November 10, 2021, 04:23:53 pm
content: I think you could think of it like follows.You have voltage supply of 12V and three resistors in series.R4 is load and have output jacks on both side of it and it is connected to circuit to with those. R4 for is adjustable resistor, which you are at times are adjusting.R3 is fixed with value of your desire. Your assistant have put Voltage meter parallel to R3 to calculate current flowing through all thee resistors.Q1 is variable resistor which is varied by your assistant to keep the current always in 1A based on voltage measured over R3.So when you change the load resistance, your assistance will counter correct the resistor Q1 resistance so the current stays constant.In following you can calculate by ohms law how much voltage is over load.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 12, 2021, 04:54:41 pm
content: Any pass element, let alone the resitors will cause a voltage drop. A good soultion is to use a current shunt to monitor the current and use the ouput to drive down the output voltage. There are hundreds of current monitors to choose from- your psu probably as sense terminals. If not, you can figure it out by looking at the point where the pass element connects to the error amp. When the voltage from the current monitor excedes the comparator threshold, current limiting kicks in lowering Vset which leads to current limiting. . This scheme is typical of bench supplies.See the good old LM723 datasheet for some basic implementations.D1,D2 and R1 protect against excessive voltages going up your sense line, dont forget them.

author: george.b
author_rank: Frequent Contributor
timestamp: November 12, 2021, 06:55:01 pm
content: Quote from: TimFox on November 09, 2021, 05:22:18 pmNot really. Those are the Proteus symbols for ground and a power rail (default VCC). They are, however, swapped: LM358 has GND on pin 4 and V+ on pin 8.

author: TimFox
author_rank: Super Contributor
timestamp: November 12, 2021, 08:27:44 pm
content: Use of such a symbol when presenting a diagram is not good practice, unless the voltage or "Vcc" is next to it.There are standard symbols for drawing electronic diagrams, and it avoids confusion to follow them.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 13, 2021, 03:59:34 pm
content: Set your phasor to stun or about 45'.