Capacitors across rectifiers;why?

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

johnheath

Well-known member
Joined
Jul 31, 2014
Messages
890
Location
Sweden
Hi all...

While looking at different schematics I sometimes see this capacitor (C1) across the rectifier bridge and sometimes not.

Since the Circuit works in both cases I just would like to know why it sometmes is used and what the purpose is?

Kind regards

/John
 

Attachments

  • Filterteori.png
    Filterteori.png
    3.4 KB · Views: 137
Thanks…
But while still at it… "to damp HF noise" what values would be preferable? I guess that it depends on the frequency that you would like to dampen?

/John
 
I use a 100nF film cap suitably rated in all my designs. The reason I use it is that in the early days I did not have one and I got this very short 100Hz spike sitting on the top of my HT supply. The capacitor suppresses the small spike you get whenever the diodes turn off. In many cases you can get away without the cap if you use fast turn off diodes.

Cheers

Ian
 
Thanks Ian

I have seen that cap in your designs… and now I have the explanation why as well :)

Regards

/John
 
Hi John

In the late 70 when I was @ BGW some of my models had problem that went to Germany. They did not pass VDE (Line Harmonic levels). The 50Hz Harmonics on the AC Power line were out of the limits. I placed a 0.01uf disk cap on the bridge rectifier and it reduced the harmonics. In the late 70 Rectifiers became much faster and the AC Line harmonics became a problem that needed to be controlled. Today the IN4xxx diodes and similar are very fast devices.  As Ian is saying now all the rectifiers need some type of snubber. I use my LISN and the spectrum analyzer and verify my designs. Be a Boy Scout and be prepared.

I hope this helps.  Duke
:)
 
Hi Duke

Yes, thank you, all information about this is very much welcome… actually I will test my own builds to see if I have a need for it or not… all improvements are good I guess

Thanks

/John
 
Hi John

My best and favorite tool is my $3.00 spectrum analyzer that I bought about 1960 . It is an all plastic AM transistor radio with a loop stick antenna. This is a GREAT SNIFFER, you can hear the diode noise of if the circuit is in oscillation, the radio goes silent.
Duke
:)
 
Audio1Man said:
Hi John

My best and favorite tool is my $3.00 spectrum analyzer that I bought about 1960 . It is an all plastic AM transistor radio with a loop stick antenna. This is a GREAT SNIFFER, you can hear the diode noise of if the circuit is in oscillation, the radio goes silent.
Duke
:)

Hmm … you have my attention :) Is it possible to see a picture and perhaps a description on how to use it?

Kind regards

/John
 
they use caps and resistors across the HV stuff too, mostly to keep the voltage distributed evenly among the diodes,

don't drop your screwdriver across these stacks,

 

Attachments

  • rhv.jpg
    rhv.jpg
    74.8 KB · Views: 124
Samuel Groner said:
Proper damping in fact requires an RC snubber, not just a capacitor
Correct. A cap by itself only shifts the ringing to a lower frequency (where it may be less of a problem). A suitable snubbing network is usually a 10nF cap in series with a 1k to 5k resistor. You can of course tune the exact resistor value if you have the equipment and patience.
 
I've seen this with 4 caps (one across each diode in the bridge) also. 10nF or 100nF, I forget which. Plus a proper snubber across primary or secondary. It is more about stopping crap getting back on the mains than stopping crap getting onto your rectified DC, I think. Using Schottky or other very fast diodes for your rectifiers is an alternative approach.

This paper is useful - http://www.hagtech.com/pdf/snubber.pdf
 
A lot of interesting inputs here :)

Matt Nolan said:
I've seen this with 4 caps (one across each diode in the bridge) also. 10nF or 100nF, I forget which. Plus a proper snubber across primary or secondary. It is more about stopping crap getting back on the mains than stopping crap getting onto your rectified DC, I think. Using Schottky or other very fast diodes for your rectifiers is an alternative approach.

This paper is useful - http://www.hagtech.com/pdf/snubber.pdf

So… "very fast diodes"? I use rectifier bridges like this or similar depending on need for voltages and current
https://www.elfa.se/sv/likriktarbrygga-1000-quad-15-diotec-kbpc610/p/17000175?q=*&filter_Category3=Diskreta+halvledare+%2F+rör&filter_Category5=Likriktarbrygga&filter_Genomsnittlig+framström%2C+max=6+A&filter_Category4=Likriktarbryggor&filter_Buyable=1&page=3&origPageSize=50&simi=99.5

Regards

/John
 
Hi John

Here is my SNIFFER, It is a all plastic AM Radio. The LOOP ANTENNA is the directional pickup coil. Tune to a blank station, turn on your product and run the antennal loop up and down the AC Line cord and inside the product looking for noises. The small cable is connected to the loudspeaker and can be connected to an ANALOG type meter as the an DVM without a bar graph may not pick up the hottest place. Good hunting.

Duke :)

BTW Fast rectifier tend to make more noise.
 

Attachments

  • AM-RADIO- 002.jpg
    AM-RADIO- 002.jpg
    650.2 KB · Views: 89
Audio1Man said:
Hi John

Here is my SNIFFER, It is a all plastic AM Radio. The LOOP ANTENNA is the directional pickup coil. Tune to a blank station, turn on your product and run the antennal loop up and down the AC Line cord and inside the product looking for noises. The small cable is connected to the loudspeaker and can be connected to an ANALOG type meter as the an DVM without a bar graph may not pick up the hottest place. Good hunting.

Duke :)

Thank you so very much Duke... I'll go hunting for a a cheap radio first but I like the idea a lot.... simple but clever :)

Regards
/John

BTW Fast rectifier tend to make more noise.
 
Detailed info about power supply snubbering on this thread in post #1 attachments.
http://www.diyaudio.com/forums/power-supplies/243100-simple-no-math-transformer-snubber-using-quasimodo-test-jig.html

With careful layout and wiring, and use of smallest acceptable diode such as UF4007 or a valve diode, then in high voltage designs there is typically no need for transformer snubbering, and certainly no need  for snubbers paralleling diodes.  But low voltage supplies, especially for high current loads with huge filter capacitance, are a different matter.
 
Audio1Man said:
Hi John

Here is my SNIFFER, It is a all plastic AM Radio. The LOOP ANTENNA is the directional pickup coil. Tune to a blank station, turn on your product and run the antennal loop up and down the AC Line cord and inside the product looking for noises. The small cable is connected to the loudspeaker and can be connected to an ANALOG type meter as the an DVM without a bar graph may not pick up the hottest place. Good hunting.

Duke :)

BTW Fast rectifier tend to make more noise.

So basically it's just a small Radio thats it?

Can I use any type of small radio to do the same thing, any requirements at all?

Found this also:
http://www.diyrecordingequipment.com/blogs/news/15851868-5-minute-project-how-to-build-a-circuit-sniffer


 
Back
Top