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Cap's Across Diodes on Rectifier Board???

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Old 10-24-2011, 07:07 PM
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Default Cap's Across Diodes on Rectifier Board???


First most of the home brew rectifier boards have small 3kv Orange Ceramic Disc caps across each diode I assume as a form of surge protection. I have been looking at OEM rectifier boards and most of them skip the caps across each diode. Now keeping in mind that I will not use this as my sole means of surge protection what is considered "best practice"??? I will have some form of surge protection on the power supply input side to guard against surges from the power company before my 110/120V makes it to the primary side of the transformer.I have not decided which type of components I am going to use for this just yet.

So as a back up does it do any good to put caps across each diode? If yes would 222pf 3KV disc caps be sufcient to act as back up surge protection? I have probably 100 of theses collecting dust from a project I did a few years back. If not what level of capacitance would you recommend to act as back up surge protection?


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Old 10-25-2011, 04:08 AM
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In the old days, and sometimes today also, the .01 ceramic caps placed across the filter caps are to shunt any RF to ground per say. You will notice the string of caps folllows the filter caps.

Of course there is also a cap placed at the Rf choke to Take any RF to ground that may find it's way through the Plate choke.

Surge protection? That is a new one on me for using ceramic disc caps in the HV PS.

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Old 10-25-2011, 05:52 AM
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Caps were necessary years ago. They were not there for rf bypassing.

Resistors and caps across diodes will not buy anything you need unless you desire to use ancient diodes. Just put plenty in series for more PIV.

Many textbooks show different rectifier configurations and their requirement for rectifier PIV. Those ratings can be a lot higher than one might think.

MOVs are nice for surge protection in homebrew equipment but IMO the easiest way to deal with spikes is to overbuild the rectifier stack with a lot of PIV.
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Last edited by Kamikaze; 10-25-2011 at 05:53 AM. Reason: Tuesday

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Old 10-25-2011, 07:21 AM
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Resistors and caps across diodes were an equalizing method. Todays diodes are consistent enough that most skip that step.
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From milliwatts to kilowatts its all cool to me .... but life is to short for QRP !!

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Old 10-26-2011, 02:43 PM
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Yes I know the combo of resistor,cap and diode but I had seen a bunch of recent home brew projects with just the classic orange dick caps across the diodes in the diode string on rectifier board. Instead of resistors one each part I am guessing the resistor string between the diode and the larger filter caps where tied in with traces that did not show in the photo. A lot of the older stuff I have read on this topic talked about the caps being used to prevent spikes ie surges in the rectifier section and they where not talking about after the filter caps either. My Library only has old old books on this topic most of them are from the 1950. I understand a lot has changed since then so that is why I wanted to ask. I have noticed that most modern OEM rectifier boards do no not have caps or resistors attached on top or bellow each diode. In fact most of them have the caps grouped together, all the resistors in one area and all the diodes in another and use through board construction to make all the needed connections to all the parts with out having them all touching each other directly or stacked on top of each other.

Most Americans are not very forth coming with photo's or design details. So I am forced to read web sites from Europe and review their projects to get insight into what has changed since the books I am reading where published and what is the norm today! So when I read something that is different from my ancient books or I see conflicting designs in modern home brew projects I pop in here and get the explanation as to what is the norm or best practice for today!

So as if I read things correctly while it would not hurt anything to put caps across the diodes it also will not do you any good. Why? Today's diodes do not need this in order to do their job just use plenty of them and keep PIV in mind?!

I am using 1000V 10a rectifier diodes( I got them for less then the cost of 1amp 1000V Diodes) and I was going to use 12 in each string I figured that will more then get the job done. I was going to put my resistors in between each string and feed through the resistors to the diodes strings then out to the filter caps. I am using (12) 450V 100uF Teflon caps. I have them wired so it is like I am using (6) 450V 200uF caps again should be gross overkill for what I need.

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Old 11-24-2011, 07:10 PM
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Quote:
Originally Posted by ElectronTubesRule View Post
Yes I know the combo of resistor,cap and diode but I had seen a bunch of recent home brew projects with just the classic orange dick caps across the diodes in the diode string on rectifier board. Instead of resistors one each part I am guessing the resistor string between the diode and the larger filter caps where tied in with traces that did not show in the photo. A lot of the older stuff I have read on this topic talked about the caps being used to prevent spikes ie surges in the rectifier section and they where not talking about after the filter caps either. My Library only has old old books on this topic most of them are from the 1950. I understand a lot has changed since then so that is why I wanted to ask. I have noticed that most modern OEM rectifier boards do no not have caps or resistors attached on top or bellow each diode. In fact most of them have the caps grouped together, all the resistors in one area and all the diodes in another and use through board construction to make all the needed connections to all the parts with out having them all touching each other directly or stacked on top of each other.

Most Americans are not very forth coming with photo's or design details. So I am forced to read web sites from Europe and review their projects to get insight into what has changed since the books I am reading where published and what is the norm today! So when I read something that is different from my ancient books or I see conflicting designs in modern home brew projects I pop in here and get the explanation as to what is the norm or best practice for today!

So as if I read things correctly while it would not hurt anything to put caps across the diodes it also will not do you any good. Why? Today's diodes do not need this in order to do their job just use plenty of them and keep PIV in mind?!

I am using 1000V 10a rectifier diodes( I got them for less then the cost of 1amp 1000V Diodes) and I was going to use 12 in each string I figured that will more then get the job done. I was going to put my resistors in between each string and feed through the resistors to the diodes strings then out to the filter caps. I am using (12) 450V 100uF Teflon caps. I have them wired so it is like I am using (6) 450V 200uF caps again should be gross overkill for what I need.
Waste of time with ceramic caps, as for 33uF @2700V your capacitance will be barely enough to keep ripple within acceptable parameters for a tube (depending on the current of course). See attached picture if it ever will get through.
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File Type: jpg HV PS.jpg (61.0 KB, 8 views)

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Old 02-03-2012, 09:11 PM
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Thanks for the photo. I am not dead or anything.....LOL I have 3 boys, am a single Dad and have also been looking for work. So I have been too busy to work on this stuff. Well technically I have the time but not the money and after looking for work in real life and on the internet my appetite for more time on the computer is just not their.

I will check in when I have anything worth while to report.

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Old 03-29-2012, 03:20 AM
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I have a few hundred 470 microfarad x 450v electrolytic caps which measure 2" tall x 1.375" dia that I'm thinking about putting on the used market. They came from prototype lasers and have been used for about 20 minutes - 2 hours.
They are 105° and have
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|M| and CE written on them.

Last edited by Needle Bender; 03-29-2012 at 04:55 AM.

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