RC snubber network design - Page 1
Forum section: electronics / projects

author: LondonNight
author_rank: Contributor
timestamp: January 19, 2022, 05:12:44 pm
content: Lets assume we live in an ideal world and we have an ideal copper coil with inductance H and resistance R. We ignore the capacitive component of the coil. Lets assume an RC series snubber network is placed in parallel with the coil. The snubber network is made up of a resistor Rs and capacitor Cs. We apply the DC voltage U on the coil, and when the circuit reaches DC steady state, we instantly disconnect the ideal power supply from the circuit. How do we dimension Rs and Cs in order for the inductive spike to not go past 0.1U?

author: JohanH
author_rank: Frequent Contributor
timestamp: January 19, 2022, 05:21:26 pm
content: It's relatively easy to figure out Rs and Cs experimentally. This is what I do, works perfectly:https://www.diyaudio.com/community/threads/simple-no-math-transformer-snubber-using-quasimodo-test-jig.243100/

author: LondonNight
author_rank: Contributor
timestamp: January 19, 2022, 05:45:54 pm
content: Thanks, but i need a formula to give to my teacher as an answer. Besides, thats for AC transformers, i got a DC coil

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 19, 2022, 10:32:00 pm
content: Ah, this is a school thing?Solve the RLC system for near-critical damping. R and C drop out in the end. Probably you'll use a lookup table, because calculating overshoot analytically is a pain (solving a system of exponentials), and maybe the result will be a little off due to the configuration, but that is also the methodology I would expect to see in a class.Speaking of class, what methods did they cover, anyway? If your instructor did not introduce the subject, clearly you have no hope of completing it yourself, and your failure is their fault. Is this actually the case? Did you miss a few classes, have you read others' notes, have you read the textbook, etc.?AC is irrelevant; it's the easier case actually, in the sense that, sometimes when the switch opens, the instantaneous voltage/current is near zero and therefore little overshoot occurs. It's luck whether the switch opens during a peak. Of course, we design to the worst case, so this doesn't really mean anything.Tim

author: LondonNight
author_rank: Contributor
timestamp: January 20, 2022, 11:42:05 am
content: I missed almost all the classes, but thats in the past. How do i start at solving the RLC system for near-critical damping?

author: jonpaul
author_rank: Super Contributor
timestamp: January 20, 2022, 12:51:15 pm
content: LondonNightTypical of the current generation of our time.The purpose of a test, problem or exercise in school is for the student to LEARN how to do a problem by THINKING, READING and perhaps RESEARCHING.Asking a forum to do your homework is a shortcut, but you will not learn.You should decide if you proceed to solve a problem BY YOURSELF or just ask others and take the lazy way out.After you have your degree, will you still run to the forum to do your work?Bon Chance,Jon

author: fbpicsous
author_rank: Newbie
timestamp: January 20, 2022, 01:06:43 pm
content: It's DC. I think:You know U RL L -> can calculate IYou can calculate Energy (J) into the inductance(and Csnubber is charged at Vdc)When you disconnect the supply, L works as constant current supply anddischargethe capacitor.You know I, RL and Ucapacitor, -> you can calculate Rsnubber to limit voltage across inductance.With Energy, you can estimate absolute maximum voltage across capacitor (It is theoretical, but imagine if all energy is transferred into capacitor)E(J) = 1/2 CU²And at last, all Energy is dissiped into the 2 resistors. (-> calculate power of R)I hope I didn't make any mistakes

author: LondonNight
author_rank: Contributor
timestamp: January 20, 2022, 01:13:12 pm
content: Ok, I lied, Im at a mechanical faculty and this is not for school, its for a personal project. And no, in this semester I have't yet missed any lessons at all, and I plan to keep it that way.Electronics is just one of my hobbies. I'm building an electromechanical computer (a second one) and I just wanted to know exactly how to dimension the two components there, since I have to repeat that snubber circuit about 4000 times, if not more, in the computer, so I can't have the capacitor be any size bigger or smaller than required, for that reason... And sir, please dont generalize and accuse a whole generation, because its not at all fair to do so. There's good apples and bad apples everywhere.As for my snubber circuit, I do know how it works roughly, and that's all I need for now, but I don't wanna waste all my time doing research on such a circuit, when there are people here who already know the answer and could just tell me the way the capacitor and resistor are dimensioned.

author: LondonNight
author_rank: Contributor
timestamp: January 20, 2022, 01:17:03 pm
content: Thank you so much! Its such a simple explanation, now I get it! My only question is, wouldnt the resistor affect how much energy is being transfered to the capacitor? Do I have to adjust my capacitor accordingly?

author: fbpicsous
author_rank: Newbie
timestamp: January 20, 2022, 03:44:49 pm
content: It was only some considerations to understand how overvoltage come ... (not talk about oscillating frequency, time to damping, capacitor characteristic request, etc)For extra fine tuning, I think it would be more complicated... (And yes, have a look at RLC damping =1/R* sqr(L/C).I suggest you to make some tests instead of calculate exactly all interactions.(Sometimes, a 10nF capacitor is not bigger than 1nF capacitor ...)Interesting ideas about how energy is transferred :https://www.edaboard.com/threads/where-does-the-1-2-come-from-in-the-1-2cv2-capacitor-energy-calculation.378670/