How to calculate RC across high voltage windings? - Page 1
Forum section: electronics / projects

author: 001
author_rank: Super Contributor
timestamp: October 09, 2019, 04:31:55 pm
content: Hi!I see RC across high voltage windings (240-400V) at many old gear transformersIs it sort of "snubber"? How to calculate it?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 10, 2019, 01:23:53 am
content: R = sqrt(L / Co)C >= 2.5 CoTim

author: 001
author_rank: Super Contributor
timestamp: October 10, 2019, 03:50:30 pm
content: Quote from: T3sl4co1l on October 10, 2019, 01:23:53 amThanx a lot!L and C0 are inductance and capasitance of transformer windings, aren`t they?Can You tell right words to google more about this?

author: MagicSmoker
author_rank: Super Contributor
timestamp: October 10, 2019, 05:21:11 pm
content: Co is the stray capacitance of the resonant network you want to dampen and is often difficult to determine, but this article explains a common trick:https://e2e.ti.com/blogs_/b/powerhouse/archive/2016/05/05/calculate-an-r-c-snubber-in-seven-stepsL is the stray inductance which is the leakage inductance of the transformer, in this case. Anticipating your next question: you can measure leakage inductance on the winding of interest by shorting out all other windings.Note this approach to RC snubber design is for damping ringing from stray resonant networks. Another common use of an RC snubber across a transformer or a switch - particularly at mains frequency - is to absorb the energy stored in the magnetizing (and leakage) inductance during turn-off. In this case the capacitor is sized so that LI^2 = CV^2 where I is the peak magnetizing current (for the magnetizing inductance) or total current (for leakage inductance) and V is the peak voltage it is permissible to allow the capacitor to charge to.

author: 001
author_rank: Super Contributor
timestamp: October 10, 2019, 05:44:11 pm
content: Quote from: MagicSmoker on October 10, 2019, 05:21:11 pmThanx a lotIt is that I means

author: floobydust
author_rank: Super Contributor
timestamp: October 10, 2019, 07:12:34 pm
content: For snubbing (plate) transformer leakage inductance, I connect 1kHz square-wave to primary at a few volts peak-peak, and then scope the secondary with no load. You can see overshoot or undershoot and adjust the snubber values.Measuring inductance, resistance and using math did not work I found because the winding capacitance is coupled to both the windings, and core, and is not easily known. It also does not tell you power dissipation in the snubber resistor.Example 120VAC to 1,250VCT 300mA transformer (Hammond 720X) which is about 1:10 turns ratio.I found 0.01uF 3kV and series 1k ohm across each secondary winding works best, I used two sets total four parts.This is to lower stress on rectifiers and lessen switching EMI.For audio transformers, the technique also works.

author: 001
author_rank: Super Contributor
timestamp: October 10, 2019, 07:46:23 pm
content: Quote from: floobydust on October 10, 2019, 07:12:34 pmThanx a lot! Is itratedfo 3kV?Why not to use single capacitor across windings instead RC?

author: floobydust
author_rank: Super Contributor
timestamp: October 10, 2019, 08:10:28 pm
content: Using no resistor gives too much ringing and danger of making a Tesla coilYou don't want a resonance because it will switch the rectifier on and off many times every cycle, they need reverse-recovery time.Using only one capacitor works but I found I got a spike with common-mode voltage. Each secondary leg is not always out-of-phase for fast transients. Perhaps the capacitance is not identical.First I used two 0.01uF 1kV disc capacitors but they are noisy (loud buzz) and then one exploded, 625VRMS was too much so I upped to 3kV rated parts.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 10, 2019, 11:42:36 pm
content: You often see a too-large cap used with no resistors. This doesn't dampen the ringing much, but keeps the amplitude low because the resonant impedance is Zo = sqrt(L/C). Large C gives small Z, which means smaller Vpk for a given Ipk.There's also the matter of different inductances in play, depending on the situation. A primary side switch may need to be damped for magnetizing inductance, while a secondary side rectifier may need to be damped for leakage inductance. You can cover each with a separate RC network, if you're really keen.Tim