constant current constant voltage power supply circuit - Page 1
Forum section: electronics / beginners

author: franksterb92
author_rank: Contributor
timestamp: September 20, 2023, 07:56:57 pm
content: im messing around with a circuit based on the HP E3631A Power supply circuit and had a question about the current limit part i understand essentially its taking the voltage drop across R66 but when i build the circuit it doesn't seem to work the voltage part works but not the current could anyone point me to maybe a formula to calculate the current out from the voltage drop and more info on the topology of this current limit circuit or maybe explain more about how it works thanks

author: TimFox
author_rank: Super Contributor
timestamp: September 20, 2023, 08:01:35 pm
content: As posted, I can't read what is on the far left and the right of that resistor.The current through R66 can be calculated directly from Ohm's law and the (relatively small) voltage measured across it:I=V/R.In use, the feedback voltage from R66 only controls the output voltage when the current through your external load exceeds the current setpoint; otherwise, the voltage setpoint controls the output voltage.

author: franksterb92
author_rank: Contributor
timestamp: September 20, 2023, 08:15:21 pm
content: here is what i built up

author: TimFox
author_rank: Super Contributor
timestamp: September 20, 2023, 08:23:27 pm
content: No filter capacitor after the rectifier bridge?Where is your current sense resistor, equivalent to R66?

author: franksterb92
author_rank: Contributor
timestamp: September 20, 2023, 08:49:46 pm
content: No I didn't put the caps on cause of safety as I have this hooked to a mains transformer but it's the low side 0.2ohm one R1 it is

author: TimFox
author_rank: Super Contributor
timestamp: September 20, 2023, 08:55:05 pm
content: Please re-write that sentence (I cannot understand the last 10 words) and use at least a small electrolytic after the bridge.

author: franksterb92
author_rank: Contributor
timestamp: September 20, 2023, 11:22:31 pm
content: The sense resistor is R1

author: TimFox
author_rank: Super Contributor
timestamp: September 21, 2023, 03:28:42 am
content: Why don't you have a filter capacitor (typically an electrolytic) after the bridge?

author: xavier60
author_rank: Super Contributor
timestamp: September 21, 2023, 08:55:04 am
content: Quote from: franksterb92 on September 20, 2023, 07:56:57 pmPlease post the whole schematic if you would.

author: Marian_elf
author_rank: Contributor
timestamp: September 21, 2023, 11:17:26 am
content: This is what you should have posted ( see the attachment ).It's the complete schematic, and it reveals what happens.In the right side on the output there is a ground point noted as "A", that is where some parts of the circuit is connected.The current limit part is done by U26A ( 074 opamp ).It's noninv input goes to that A point by R97 ( basically on the output side of the current sensing resistor R66 ).It's inv input goes to the left side of R66 by means of R93.Which basically makes U26A a noninv error amplifier, followed by Q8 which controls the series pass transistor's base.When the output current reaches some point, pin 3 goes more positive then pin 2, the EA's output drives more current in Q8's base, turning it on, which turns off the series pass power PNP, thus reducing the output.Your tested circuit cannot work because Q2 can only supply current in Q1's base, and since Q1 it's a PNP, it only turns on when you draw current out of it's base ( meaning the PNP base needs to be more negative then the emitter in order for the device to turn on ).The E3631 has U27 regulator configured as a constant current sink which draws current from the series pass PNP's base.You can use in your circuit some resistor connected across Q1's base and collector.That resistor needs to be calculated so it can draw enough current from the base ( divide the maximum output current to Q1's hfe at that maximum current ), making sure to give it enough headroom, and also that Q2 can safely handle that current.PS: The main filter cap after the bridge rectifier is mandatory!

author: franksterb92
author_rank: Contributor
timestamp: September 22, 2023, 08:18:53 am
content: oh so thats what they are using that lm317 constant current circuit for to pull current from the pnp driver ok thats makes since now thank you its obvious now about that ground label and the current error amp and i understand the cap thing after the rectifier im poking around this circuit on a breadboard and just learning more about op amps and dont want to get shocked with all the energy a cap would supply

author: Marian_elf
author_rank: Contributor
timestamp: September 22, 2023, 08:49:28 am
content: You are welcome.PS: You shouldn't even try to start experiments at this level on such a high voltage ( it's lethal you know... ).Do start with safe voltages, up to 30V or so, for example a 24Vac transformer.Then after gaining the proper knowledge and experience you can start going higher in voltage.But first you really need to learn good basics, how each component type work, what it it's purpose and drawbacks, etc...I strongly suggest reading at least the first chapters of "The Art of Electronics" book, it is really a legend amongst electronics engineering

author: EPAIII
author_rank: Super Contributor
timestamp: September 22, 2023, 10:15:53 am
content: Leaving a filter capacitor out does not make that circuit any less dangerous. In fact, one could argue that since without one the Voltage after the rectifier is going to have a large AC component, making it actually more dangerous. Read about Edison vs. Westinghouse/Tesla and the war of the currents.https://www.history.com/news/what-was-the-war-of-the-currentsIn any case, the AC outlet you are plugging this into can supply more than enough energy to kill. So DO be careful.Quote from: franksterb92 on September 22, 2023, 08:18:53 am

author: franksterb92
author_rank: Contributor
timestamp: September 23, 2023, 02:56:11 am
content: ive ben doing electronics for like 10 years alot more microcontroller stuff i know basic analog electronics how the op amps work ect but im not the best with transistors other than using them as switches and ive ordered the nat semi LINEAR APPLICATIONS HANDBOOK i do want the art of electronics tho

author: franksterb92
author_rank: Contributor
timestamp: October 02, 2023, 01:01:53 am
content: i found a e3630a on ebay and ive ben probing different spots on the circuit i understand more about this now but im still stumped how they configure the current limit to limit a 550ma ohms law says the drop across the 1.78r resistor should be 980mv wen i measure the non inverting input to the op amp its 1.01v