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 Post subject: Re: Schematics for a Capacitor direct leakage tester
PostPosted: Jul Wed 20, 2011 3:54 pm 
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Id add a 100K across both of those electrolytics to equalize voltage otherwise you may have a surprise some day.


Has this been done yet or are you just going to live dangerously?


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 Post subject: Re: Schematics for a Capacitor direct leakage tester
PostPosted: Jul Wed 20, 2011 6:31 pm 
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Location: Minnesota
Equalizing resistors went out of style years ago, that was 1950's technology.

Dave


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 Post subject: Re: Schematics for a Capacitor direct leakage tester
PostPosted: Jul Thu 21, 2011 2:57 am 
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Joined: Nov Fri 26, 2010 3:44 am
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Location: Oak Hill, Virginia
Dave,

You could be correct regarding equalizing resistors however,

The Complete Guide to Electronic Troubleshooting, by James Perozzo, Delmar Publisher, Pub. 1994 recommends the practice. Please see page 119, in the chapter on Fixing The Power Supply.

In my opinion, it's an excellent book.

Possibly you can provide a published reference that it went out of style?

Tony


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 Post subject: Re: Schematics for a Capacitor direct leakage tester
PostPosted: Jul Thu 21, 2011 3:06 am 
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Location: Pocasset, Cape Cod, MA
I got around that problem by using a 1000V capacitor. To answer an earlier question, yes the printed schematic was missing the "k" on the bleeder resistor across the capacitor. I don't remember what I used, but ordinary carbon comps aren't rated for 1000V; if you're going to use two in series you might as well use them for equalizers also, with a pair of 450V electrolytics. My unit was built in the 1970s from available parts. Incidentally I wrote it up for the old Antique Radio Club of America Gazette and it was rejected; the editor felt it was too dangerous and might make them liable for accidents.

edit: "published reference"? A lot of authors kept parrotting obsolete information without ever thinking it through. Two electrolytics in series are self-equalizing.
http://antiqueradios.com/forums/viewtopic.php?t=153805


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 Post subject: Re: Schematics for a Capacitor direct leakage tester
PostPosted: Jul Thu 21, 2011 3:23 am 
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Tony L wrote:
Dave,

You could be correct regarding equalizing resistors however,

The Complete Guide to Electronic Troubleshooting, by James Perozzo, Delmar Publisher, Pub. 1994 recommends the practice. Please see page 119, in the chapter on Fixing The Power Supply.

In my opinion, it's an excellent book.

Possibly you can provide a published reference that it went out of style?

Tony


Tony

The topic has been bandied around before. In theory, the voltage on each cap should stabilize at a point that coincides with lowest possible leakage current for both caps in series. Leakage current increases rapidly as an electrolytic caps maximum operating voltage is reached or exceeded. If you search for posts by Alan Douglas, made in the past few years, you'll find quite a few pages on the topic. I'd fell comfortable using two 450 volt electrolytic caps in series for voltages in the 750 volt range. I wouldn't be that confident if I was using two 450 caps at 900 volts. That's how high voltage electrolytic cap were made, using two caps in series in one housing. You won't find equalizing resistors hidden inside of them.

I still see folks arguing that equalizing resistors and caps are needed across avalanche mode diodes used in series. The ARRL had it wrong in the handbooks for years.

By the way, that schematic shows a 700 volt RMS winding with a bridge and two 450 volt caps in series! 1.414 x 700 volts is close to 1000 volts. Equalizing resistors ain't gonna save them caps when the variac is cranked up.

Pete


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 Post subject: Re: Schematics for a Capacitor direct leakage tester
PostPosted: Jul Thu 21, 2011 4:35 am 
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Joined: Jul Sun 17, 2011 5:11 am
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Alan Douglas wrote:
I got around that problem by using a 1000V capacitor. To answer an earlier question, yes the printed schematic was missing the "k" on the bleeder resistor across the capacitor. I don't remember what I used, but ordinary carbon comps aren't rated for 1000V; if you're going to use two in series you might as well use them for equalizers also, with a pair of 450V electrolytics. My unit was built in the 1970s from available parts. Incidentally I wrote it up for the old Antique Radio Club of America Gazette and it was rejected; the editor felt it was too dangerous and might make them liable for accidents.

edit: "published reference"? A lot of authors kept parrotting obsolete information without ever thinking it through. Two electrolytics in series are self-equalizing.
http://antiqueradios.com/forums/viewtopic.php?t=153805


I'm so literal minded I watched that resistor go up in smoke. It's a good piece of tooling and it allowed me to get rid of a lot of stuff I had, mostly old Mallorys I got from somewhere. Touching the leads will give you a thrill though. I measured it with my variac and the most I got was about 425v so I dragged my old Superior Adjust-a-volt that looks like it was dredged up from the bottom of a river and it'll give me 150v which oughtta get me where I need to be. And there are a few other power transformers here.

A good project that has a number of uses.

Thanks for the idea, Alan.


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 Post subject: Re: Schematics for a Capacitor direct leakage tester
PostPosted: Jul Thu 21, 2011 6:27 pm 
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Joined: Oct Sat 20, 2007 3:36 am
Posts: 14393
Location: Southern NH, 03076
Quote:
Equalizing resistors went out of style years ago, that was 1950's technology.

Dave


Thats about the worse advice Ive read on here in a long time.

Show me one commercial HV PS that doesnt use them.

Carl


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 Post subject: Re: Schematics for a Capacitor direct leakage tester
PostPosted: Jul Sun 24, 2011 10:17 pm 
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Joined: Nov Fri 26, 2010 3:44 am
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Location: Oak Hill, Virginia
I did some checking and found:

ARRL Handbook, 2010. Endorses the use of equalizing resistors for high voltage power supplies. See the Power Supply chapter, page 7.29.

It's a possible test question for the current Amateur Radio Extra Class license. "When several electrolytic capacitors are connected in series to increase the operating voltage of a power supply filter circuit, why should resistors be connected across each capacitor?" Note; the answer is NOT, "the resistors are not needed". Since the FCC is using this as a question, it sounds like good advise to me.

A nice article "Strategies to Repair or Replace Old Electrolytic Capacitors" by Tim Reese, reese@nmr.mgh.harvard.edu, goes into considerable detail on it. See the section on SERIES-CONNECTED CAPACITORS:. http://www.nmr.mgh.harvard.edu/~reese/electrolytics/. An important take-away on this article and others, is that it is important that wattage of the resistor not be too low to handle the leakage current and voltage across it. I.E. a 1/2 watt resistor may not be a good idea.

There's an interesting article by power supply manufacturer:
http://www.ameritron.com/Product.php?pr ... =AL-1200JQ

I think there's sufficient published information in favor of using equalizing or balancing resistors, so whenever I'm dealing with high voltages, I'm going to use them to play it safe.

Tony


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 Post subject: Re: Schematics for a Capacitor direct leakage tester
PostPosted: Jul Sun 24, 2011 10:34 pm 
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Location: Pocasset, Cape Cod, MA
The license question could refer to more than two in series ("several"), the way they are typically used in linear amplifier power supplies, where the capacitance values tend to be large. Balancing resistors are useful here, and also serve as bleeders which you need anyway. My point is that they are not needed when using a pair of small 350V or 450V electrolytics to replace one 500V capacitor in a radio.


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 Post subject: Re: Schematics for a Capacitor direct leakage tester
PostPosted: Jul Mon 25, 2011 1:32 am 
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Location: British Columbia
Why would you even question using the resistor network on series caps??? I work on very large commercial power supplies ( back engineer ) for a living. I have never seen one without this network. This silliness about whether to use or not is just plain stupid.......And CHEAP! Spend the extra 1$ and do the job right! Do your research on how high voltage caps are built. The two pieces that are in the same can are from the same batch and in the same amount of chemical, thats why there are no stabilizing resistors internally on a single can. Want even more proof?????? Next time your 20uF/700 Volt can becomes half it's capacitance ( happens quite often) Open it up. Wow...The reason it's half capacitance is because the other 10uF section is now a "jumper." How do you think that happened? No rocket science here people.


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 Post subject: Re: Schematics for a Capacitor direct leakage tester
PostPosted: Jul Mon 25, 2011 1:49 am 
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The FCC question pool is administered by the VECs.

The ARRL has had enough mistakes in their handbooks to
make them a questionable resource. A lot of material was
never peer reviewed.

For the heck of it I sent an email to Vishay technical support
to see what they have to say. If I get an answer, I'll post it
here. It has been an interesting discussion with good points
from both sides.

Pete


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 Post subject: Re: Schematics for a Capacitor direct leakage tester
PostPosted: Jul Mon 25, 2011 4:17 am 
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Pete,

Surely you don't believe it makes any difference whether the test is administered by a VEC, FCC or the most experienced engineering professor? Once test questions and answers are carefully produced, it makes little difference who presents it. By the way, to administer a test for an Extra Class license, the testers (3) must also be an Extra Class license holder - not someone who knows nothing of electronics or test taking/giving.

Of course few books are perfect, just like free advise, but since the 2010 ARRL Handbook is in it's eighty-seventh edition, goes through considerable review, and is widely accepted in radio communications, I think I'll trust it.

Since you did not comment on the article from Tim Reese at harvard.edu or ameritron.com, possibly you are OK with them?

Consider that an email to Vishay technical support is not likely to go through a careful review procedure by several experts as done with a major publication such as the ARRL Handbook, or in the production of a commercial product that can have a real impact to the company's future. But please do post what they say. Maybe a check with other manufacturers would also be a good idea.

Tony


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 Post subject: Re: Schematics for a Capacitor direct leakage tester
PostPosted: Jul Mon 25, 2011 4:52 am 
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Tony

Who generates the question pools used by the VECs, and what are their
credentials?

Many of the chapters in the ARRL Handbook are written by contract writers.
Unfortunately, all too often "peer review" takes place after publication and not
before. There were some major shakeups when certain amplifier design
articles were scrutinized and debunked by another well known ham and
RF amplifier designer.

In the early 1960s the Handbook had the color codes wrong for molded
chokes, even after being wrong for many editions. It took a bit to get
them to correct the mistake.

The ARRL Handbooks were still showing RC networks across series diodes in
HV power supplies well into the late 80s (page 27-37 1986 edition for
example). Old habits die hard.

I'm willing to keep an open mind for the time being.

Pete k1zjh


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 Post subject: Re: Schematics for a Capacitor direct leakage tester
PostPosted: Jul Mon 25, 2011 3:10 pm 
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Received an answer from Vishay. Basically, their position is that the leakage current
can vary enough for one of the caps to go as high as 2X the voltage of the other.
Thus, even the if the leakage current is within spec, it would appear the energy charge
held by the caps wouldn't be equal.

Pete


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 Post subject: Re: Schematics for a Capacitor direct leakage tester
PostPosted: Jul Tue 26, 2011 12:19 am 
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Unequal energy storage is definitely a concern for capacitors running near their rated voltage, or carrying large ripple currents, or with more than two in series. 2x voltage difference is 4x energy storage difference. For light-duty applications however, such as this leakage tester, or a radio with just a bit more than 450V B+, equalizing resistors will cause more failures than they prevent.


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 Post subject: Re: Schematics for a Capacitor direct leakage tester
PostPosted: Jul Tue 26, 2011 2:24 am 
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Joined: Oct Sun 11, 2009 10:06 am
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Alan Douglas wrote:
For light-duty applications however, such as this leakage tester, or a radio with just a bit more than 450V B+, equalizing resistors will cause more failures than they prevent.


Hows that? Please explain. Oh ya...and were not talking about a 15 watt divider either.


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 Post subject: Re: Schematics for a Capacitor direct leakage tester
PostPosted: Jul Tue 26, 2011 2:55 am 
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More parts = more chances for failure.


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 Post subject: Re: Schematics for a Capacitor direct leakage tester
PostPosted: Jul Tue 26, 2011 3:14 am 
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Location: Oak Hill, Virginia
Pete,

You claim to have an open mind but you question the validity of the FCC examination questions, the VECs that administer the test, the ARRL handbook, the "contract" writers that write it, and have ignored the other references that I have provided.

What will it take to convince you that equalizing resistors are a good idea for series capacitors in a high voltage circuit?

Unless you can provide documentation from several respected publishers and manufacturers, I'll consider the issue closed.

Tony


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 Post subject: Re: Schematics for a Capacitor direct leakage tester
PostPosted: Jul Tue 26, 2011 4:31 am 
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Joined: Oct Sun 11, 2009 10:06 am
Posts: 1441
Location: British Columbia
Alan Douglas wrote:
More parts = more chances for failure.


Actually, those parts prevent failure. If the divider is designed properly, the resistors will outlast the caps. A capacitors life is rated in hours. Resistors have no "life span" rating.


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 Post subject: Re: Schematics for a Capacitor direct leakage tester
PostPosted: Jul Tue 26, 2011 3:30 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 25986
Location: Pocasset, Cape Cod, MA
You've never replaced a resistor in a radio?

You don't know how long modern electrolytics will last; you only know how long old ones lasted. The purity of materials has improved considerably in recent years. You only need to measure the leakage current of modern parts to see that.


Last edited by Alan Douglas on Jul Tue 26, 2011 3:37 pm, edited 1 time in total.

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