Film capacitor across secundaries - good or bad? - diyAudio
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Old 13th July 2014, 11:03 PM   #1
PauloPT is online now PauloPT  Portugal
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Default Film capacitor across secundaries - good or bad?

I know paralleling small film capacitors with big electrolytics usualy causes more harm than good. My goal is not to reduce the impedance of the electros at HF but to suppress RF noise from getting into the power supply. Does a small film cap - say 100nF - across the secundaries before the rectifiers accomplishes this?

My PSU is built with 4 x ultra fast LQA30T300 rectifiers and 3 x 4700uF electrolytics per rail. Currently I have no film cap at the PSU and also no film cap bypassing the decoupling electros at the amp board. So, no film caps anywhere. The reason for this is my fear of causing unwanted ringing. But I need to suppress RF noise coming from the exterior (cell towers, wireless router, power lines, and all sorts of EMI interference). What would be the best way to accomplish this?

Your help is greatly appreciated.

Thank you,
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Last edited by PauloPT; 13th July 2014 at 11:18 PM.
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Old 13th July 2014, 11:13 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

Its standard practice to wire small caps across the rectifier legs
to reduce RF, a better place than across the secondaries.

rgds, sreten.
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Old 13th July 2014, 11:43 PM   #3
rayma is offline rayma  United States
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Quote:
Originally Posted by PauloPT View Post
I know paralleling small film capacitors with big electrolytics usualy causes more harm than good. My goal is not to reduce the impedance of the electros at HF but to suppress RF noise from getting into the power supply. Does a small film cap - say 100nF - across the secundaries before the rectifiers accomplishes this?

My PSU is built with 4 x ultra fast LQA30T300 rectifiers and 3 x 4700uF electrolytics per rail. Currently I have no film cap at the PSU and also no film cap bypassing the decoupling electros at the amp board. So, no film caps anywhere. The reason for this is my fear of causing unwanted ringing. But I need to suppress RF noise coming from the exterior (cell towers, wireless router, power lines, and all sorts of EMI interference). What would be the best way to accomplish this?

Your help is greatly appreciated.

Thank you,
Probably an input EMI filter module. The diodes certainly look good.
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Old 13th July 2014, 11:51 PM   #4
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A proper mains filter might be a better idea.
http://www.ckp-railways.talktalk.net/filter.jpg
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Old 13th July 2014, 11:53 PM   #5
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A 1uf film cap works very well. Also put a resistor that provides a 1% load helps. Some put a small resistor in series I prefer them all in parallel. Caps across the diodes let in noise.
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Old 13th July 2014, 11:55 PM   #6
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I thought caps across diodes were to stop the switching noise from the diodes.
This is usually a problem in high voltage circuits if you don't use fast diodes.
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Old 14th July 2014, 12:03 AM   #7
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Quote:
Originally Posted by nigelwright7557 View Post
I thought caps across diodes were to stop the switching noise from the diodes.
This is usually a problem in high voltage circuits if you don't use fast diodes.
That's the idea but if you let in more line noise than switching creates you lose. Since only a few diodes do that the single cap gets the switching noise and damps the line.
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Old 14th July 2014, 09:41 AM   #8
AndrewT is offline AndrewT  Scotland
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Sudden impulses (starting or stopping a current is an impulse) can create ringing in a system that is unstable.
Stability can be improved by adding compensation.
This compensation can be a simple resistor. It damps LC systems.

A PSU is a system that can ring.
Adding capacitors either across the secondary, or across the rectifier, or across the diodes in a rectifier can CHANGE the frequency of the ringing.
Capacitors rarely stop ringing.
Often they make the system more unstable and thus make the little bit of ringing that was there, much worse.

Adding an R+C snubber across the secondary is one way of attenuating the ringing when impulses are applied to the system.
Or you can add 4 R+C snubbers across the diodes of the rectifier.
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Old 14th July 2014, 09:46 AM   #9
PauloPT is online now PauloPT  Portugal
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Quote:
Originally Posted by AndrewT View Post
Sudden impulses (starting or stopping a current is an impulse) can create ringing in a system that is unstable.
Stability can be improved by adding compensation.
This compensation can be a simple resistor. It damps LC systems.

A PSU is a system that can ring.
Adding capacitors either across the secondary, or across the rectifier, or across the diodes in a rectifier can CHANGE the frequency of the ringing.
Capacitors rarely stop ringing.
Often they make the system more unstable and thus make the little bit of ringing that was there, much worse.

Adding an R+C snubber across the secondary is one way of attenuating the ringing when impulses are applied to the system.
Or you can add 4 R+C snubbers across the diodes of the rectifier.
Yes, I understand that, that's why I'm avoiding small caps as much as I can. But how can I filter noise (not switch noise from the diodes) coming from the exterior? How do I optimize the PSU for noise suppression?
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Old 14th July 2014, 09:54 AM   #10
AndrewT is offline AndrewT  Scotland
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add a mains RF filter to the equipment input.
add RF filters to all the other cables entering, or leaving your equipment.

An example of how I attenuate interference of MY power amplifier.
I add a 47pF across the input socket.
I add a 680pF across the PCB input
I add a 1nF across the speaker output to Chassis.
I add a 1nF across speaker return to Chassis.
I add a 1nF across the signal return (RCA barrel) to Chassis.
I add an IEC filter socket as the mains power input.

I have no trouble with RF entering my power amplifiers even when they are strung out across the floor with NO CHASSIS
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Last edited by AndrewT; 14th July 2014 at 09:59 AM.
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