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Old 19th December 2013, 10:19 AM   #1
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Default Overdamping RC snubber network

Hi All,

I have a question on snubber network to damp ringing in a transformer. I've seen most of the threads and articles trying to find the "optimal" value for snubber capacitor and resistance to obtain a "optimal" damping factor of the ringing (with RC snubber or CRC snubber). Why not to use a brute force approach and, given a capacitor value, find the maximum value of resistance allowed by power dissipation on the resistor itself of the RC series snubber network? Is there any drawback in this approach?

As an example and given the power dissipation formula of the resistor in this great article (http://www.hagtech.com/pdf/snubber.pdf), with a 0.1 uF capacitor and 70V RMS@120Hz on the output of the transformer secondary, the power dissipated on the resistor is:

P= R*(Vrms*2*3.14*f*C)^2 = (2.8E-5)*R

which gives me, for a 0.8W resistor:

R < 28K Ohm

which is much much bigger of the optimal values usually calculated (in the range of 100 ohms), and that will make for sure an over damped system. Not using a so big resistor, what if I use a 5K/10K ohm resistor? (I'm actually using 470 ohm resistor with quasi-unaudible buzz…but still is there)

Thanks in advance for your responses and thanks to Mr.Pass for giving me the ideas and tools to enjoy diy audio. Should my question be stupid/non sense/obvious sorry for wasting your time.
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Old 19th December 2013, 10:52 AM   #2
AndrewT is offline AndrewT  Scotland
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You have lots of decimal places.
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Old 19th December 2013, 11:48 AM   #3
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As the value of the damping resistor Rs increases, the damping factor zeta decreases:
Click the image to open in full size.
Unfortunately it is easy to create an underdamped, oscillatory RLC circuit with (zeta < 1.0): simply make Rs too big.

If your goal is to make an overdamped RLC circuit, with (zeta > 1.0), the equation suggests decreasing the value of the damping resistor Rs. Just be sure that the capacitive reactance of the snubber's series capacitor Cs, is at least 10X lower than Rs, at the RLC's natural frequency omega_n. 25X lower would be even better.

. .
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Old 19th December 2013, 12:49 PM   #4
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Quote:
Originally Posted by Mark Johnson View Post
As the value of the damping resistor Rs increases, the damping factor zeta decreases:
Click the image to open in full size.
Unfortunately it is easy to create an underdamped, oscillatory RLC circuit with (zeta < 1.0): simply make Rs too big.

. .
Hi Mark,

thanks for you're answer. My idea is to have a very low damping factor by increasing Rs, so that we have no oscillation (= no buzz on the trasformer). Maximum value for Rs would be constrained only by power dissipation of Rs itself.
From your answer it seems that having a low damping factor is not good, but would you please explain me why? Is there any other effect on the power supply I'm not considering?
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Old 19th December 2013, 01:19 PM   #5
AndrewT is offline AndrewT  Scotland
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have you checked your "lots of decimal places" yet?
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Old 19th December 2013, 01:31 PM   #6
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Quote:
Originally Posted by AndrewT View Post
have you checked your "lots of decimal places" yet?
Hi Andrew,
yes I did it....did I make a mistake?
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Old 19th December 2013, 01:49 PM   #7
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Quote:
Originally Posted by Mark Johnson View Post
As the value of the damping resistor Rs increases, the damping factor zeta decreases:
Click the image to open in full size.
Unfortunately it is easy to create an underdamped, oscillatory RLC circuit with (zeta < 1.0): simply make Rs too big.

If your goal is to make an overdamped RLC circuit, with (zeta > 1.0), the equation suggests decreasing the value of the damping resistor Rs. Just be sure that the capacitive reactance of the snubber's series capacitor Cs, is at least 10X lower than Rs, at the RLC's natural frequency omega_n. 25X lower would be even better.

. .
Hi Mark,
sorry for the last answer...I see your point now. From the equation you posted:

  • by increasing Rs -> oscillation increase
  • by diminishing Rs -> oscillation decrease
Just still don't understand how your equation works...if I set Rs very low (as low as zero) I should have a very high (infinite) value for the damping factor with no oscillation at all, and this is not the case whit a pure LC circuit (R->0). Am I still missing something?
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Old 19th December 2013, 01:58 PM   #8
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It's been my experience that (Wikipedia's article about the damping ratio) has matched experimental results in the real world. Here's a small portion of that article:

Click the image to open in full size.

Maybe the appendices of (the Quasimodo design note) might shed some additional light.
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Old 19th December 2013, 02:56 PM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
P= R*(Vrms*2*3.14*f*C)^2 = (2.8E-5)*R
28000 * (70V * 2 * 3.142 * 50Hz * 0.0000001F)² = 0.135W (50Hz) or 0.2W (60Hz)
The harmonics produce more heating in the resistor relative to their level because the impedance of the cap is lower at the higher frequencies. Fortunately the harmonics of the fundamental are low in level relative to the fundamental.
All this falls apart because this simple calculation ignores the phase.
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Old 21st December 2013, 01:59 AM   #10
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If you want a simple way to reduce RFI from your power supply, just place a .1uf stacked film cap across the secondary of your transformer. You really don't need a resistor in series with it, you could use a small value like .5-2 ohms if you like.

To see the effect of the cap, place a battery powered AM radio next to your power transformer and tune it to the low end of the band where there is not a station, and listen without the cap, and then with the cap. You will hear the buzz go away with the cap.
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