How this current is controlled in the circuit??? - Page 1
Forum section: electronics / beginners

author: omsiva
author_rank: Newbie
timestamp: November 18, 2023, 07:35:00 am
content: Hello all,After bridge rectification, they are d5,d6,R2,R1,DZ1,D7,R3,R9,D8,R5,R4,T1,T2,T3,R6,R7,R10,D9 which controls current. .I am an undergraduate student, Cant able to understand the operation of current regulation. help me to understand the circuit.

author: MrAl
author_rank: Super Contributor
timestamp: November 18, 2023, 07:36:26 am
content: Quote from: omsiva on November 18, 2023, 07:35:00 amPost the circuit so we don't have to guess.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 18, 2023, 07:54:10 am
content: Always start looking for a shunt resistor in the current path. In this case it's R3, a 0.27 ohm resistor.Then start looking what is around it. In this case the Base-Emitter junction of T1, which is balanced with D8.T1 starts conducting when it's B-E voltage gets over around 600mV, and the current (though R3) with which this happens is dependent on the divider with R5 and R4.When T1 starts conducting, the voltage over R7 rises, this opens T3 and that will pull the adjust pin of the LM350 to GND.T2 and LED D9 are an indication for when the current limit is active.The circuitry round C3, C4, D5 and D6 form a charge pump to generate an auxillary negative voltage, that is limited by DZ1 and D7 to be approximately -6V (This also means you have to feed it from a transformer. DC input won't work with this circuit) The negative voltage is used to be able to lower the LM350 voltage to 0V. Do note that with the charge pump, the output voltage will not be very accurate (But adequate for lot's of simple applications) R14 is used as a minimum load to prevent the output voltage from drifting upward when there is no other load on the circuit.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 18, 2023, 08:47:34 am
content: Here's simplfied schematic with the adjustment components omitted.This circuit provides "foldback limiting". ie as the current is exceeded, the output voltage is reduced.It is not a "constant current limting" circuit as found in most bench PSUs.

author: macboy
author_rank: Super Contributor
timestamp: November 19, 2023, 06:06:20 pm
content: Quote from: Terry Bites on November 18, 2023, 08:47:34 amAll current limiting will reduce the output voltage when the current is exceeded. How else would the current be reduced?Research what foldback current limiting is. It isn't as you describe. With foldback limiting, the short-circuit current will be much less than the set current limit.This circuit as I see it will not implement foldback current limiting.

author: TimFox
author_rank: Super Contributor
timestamp: November 19, 2023, 06:22:43 pm
content: "Foldback" means that when the current limit kicks in, and the output voltage falls to keep the current below the limit, the current limit value falls to reduce the power dissipated in the regulator (which has a higher voltage difference across it with the reduced output voltage).That may require a re-start after the output load problem is removed.

author: soldar
author_rank: Super Contributor
timestamp: November 19, 2023, 11:54:16 pm
content: I have re-arranged the schematic which I think makes it easier to interpret and understand.

author: coromonadalix
author_rank: Super Contributor
timestamp: November 19, 2023, 11:57:56 pm
content: same explanation for the current limitting, foldback ...the diode in serial with the shunt already give an 0.6 volt drop, if you play with the current potentiometer to the minimum, it should trig the limitterwich means upon parts values / calculations, you can have a very low output current, i use the same circuit for lm723 based psu's, it's enough to lit a led without blowing it while having no resistorthere is a negative volatge applied to the voltage adjustment potentiometer, normally it means you can or could go down zero volts at the outputand yes you can not work it from a simple dc source unless you use some small dc-dc converter to create a negative voltagesay icl7660 tcm 7660 etc ... and you can do more stable negative voltage if you use some tl431 ...t2 r8 r9 d9 are to lit the red led under current limitting