Need help on understanding the circuit. - Page 1
Forum section: electronics / projects

author: Achu
author_rank: Contributor
timestamp: October 13, 2019, 04:39:50 am
content: Hi,Anybody know how the feedback loop in this circuit work?I am very unfamiliar with this configuration.The author claims it helps to connect transistors to the heat sink without insulation.

author: IanB
author_rank: Super Contributor
timestamp: October 13, 2019, 04:57:21 am
content: It looks like a regulated power supply circuit with over current protection. Can you ask a more specific question?

author: IanB
author_rank: Super Contributor
timestamp: October 13, 2019, 05:02:00 am
content: Also it would be more polite to link to the author's web site rather than copying an image without attribution.

author: Achu
author_rank: Contributor
timestamp: October 13, 2019, 05:58:28 am
content: Sorry for my "impoliteness".here's the linkhttps://ludens.cl/Electron/Ps20/Ps20.htmlBTW my question is SPECIFIC.Quote from: Achu on October 13, 2019, 04:39:50 am

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 13, 2019, 06:38:20 am
content: Feedback through the ground symbol. Note it's a low-side regulator. The lack of compensation is particularly concerning/disturbing, as the transistors have voltage gain on top of the op-amp's already high voltage gain. Perhaps this is why he needed to put a huge capacitor on the output.Tim

author: Achu
author_rank: Contributor
timestamp: October 13, 2019, 06:49:27 am
content: Quote from: T3sl4co1l on October 13, 2019, 06:38:20 amForgive my ignorance.I you were me what would you do to increase its stability?thanks

author: IanB
author_rank: Super Contributor
timestamp: October 13, 2019, 06:52:48 am
content: Quote from: T3sl4co1l on October 13, 2019, 06:38:20 amThe designer says this:Quote

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 13, 2019, 10:16:02 am
content: Yeah, that.Adjustable compensation would be provided by adding a series resistor between U1 and U2 -in, and an R+C across U2, output to -in. Typical values would be 1-10k in series, and the R+C being on the order of 10k and 1nF. Actual values for best results could be wildly different, it depends.Adjustment consists of reducing the value of C2 or eliminating it, and testing the output step load response. Increase or decrease the RC values as needed.Step response: use a power resistor to draw an idle current about 10 to 50% of full rated load, then use another resistor plus a switch to draw a momentary additional current of 10-50%. The switch can be a MOSFET driven by a frequency generator so you get a constant sequence of steps. Read the output voltage with an oscilloscope, typically with AC coupled input, and in the 1-100us/div range.It should be possible to obtain as good or better response compared to the 1000uF, without having to put the capacitor there. The benefit is not having obligatory energy storage directly at the output -- that is, if a short circuit occurs, the capacitor dumps all its charge (which can be a very destructive peak current of 100A+) before current limiting kicks in. With a smaller capacitor, less peak current is delivered, and the current limiting circuit acts more quickly.Not that it's all that much energy (to the casual human, it looks like a little spark and a pop sound), but also not that a load will necessarily survive even with the current limit acting instantly (a 10A supply shorted into, say, a 2N3904, won't save the 2N3904 regardless of how gently those 10A are applied!). The advantage is seen when a few power transistors start to cook off in your amplifier or whatever, and instead of all of them failing in cascade (from the extra energy dumped from the capacitor), the damage is more limited (all transistors might end up dead anyway, but, survival is at least more likely).Really, economically speaking, it's more a matter of how much time you spend replacing transistors when they die, times how often you need to replace them, versus the effort of improving the supply design. If your time is cheap and your parts are abundant, or your load is reliable (maybe because it already contains protections that this supply lacks), it might not be a priority.There are other possible tweaks, say to improve recovery from current limit (it will take some time for the voltage to recover, and it will overshoot in the process), or to make the current limit adjustable, or to make it fold back to reduce power dissipation safely under fault conditions. These functions make a great bench supply, but are probably not needed for a simple power supply.Tim

author: IanB
author_rank: Super Contributor
timestamp: October 13, 2019, 06:01:47 pm
content: Also, this is not meant to be a lab grade power supply for experimentation. It is rather a utility supply designed to power amateur radio communication equipment.