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19th December 2013, 10:19 AM  #1 
diyAudio Member
Join Date: Oct 2012

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 = (2.8E5)*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 quasiunaudible 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. 
19th December 2013, 10:52 AM  #2 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

You have lots of decimal places.
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regards Andrew T. 
19th December 2013, 11:48 AM  #3 
diyAudio Member

As the value of the damping resistor Rs increases, the damping factor zeta decreases:
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. . . 
19th December 2013, 12:49 PM  #4  
diyAudio Member
Join Date: Oct 2012

Quote:
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? 

19th December 2013, 01:19 PM  #5 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

have you checked your "lots of decimal places" yet?
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regards Andrew T. 
19th December 2013, 01:31 PM  #6 
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Join Date: Oct 2012


19th December 2013, 01:49 PM  #7  
diyAudio Member
Join Date: Oct 2012

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


19th December 2013, 01:58 PM  #8 
diyAudio Member

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:
Maybe the appendices of (the Quasimodo design note) might shed some additional light. 
19th December 2013, 02:56 PM  #9  
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

Quote:
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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regards Andrew T. 

21st December 2013, 01:59 AM  #10 
diyAudio Member
Join Date: Jun 2008
Location: Sarasota, FL

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 .52 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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