Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

donate to eham
   Home   Help Search  
Pages: [1] 2 Next   Go Down
  Print  
Author Topic: ceramic disc caps  (Read 4760 times)
G4REK
Member

Posts: 1




Ignore
« on: March 22, 2013, 10:46:49 AM »

Just received a replacement  1000pf 500 volt working ceramic disc capacitor for my amplifier.......its only about a quarter of the size of my existing disc ceramic cap.......how can this be am puzzled ........am i missing something..are ultra modern caps smaller but still okay HuhHuh
      jim g4rek
Logged
W5CPT
Member

Posts: 652




Ignore
« Reply #1 on: March 22, 2013, 11:48:07 AM »

Yes - the new components are considerably smaller than those produced even 5 or 10 years ago.
Folks who rebuild Classic Tube Type equipment often will take apart the large cardboard covered caps and insert the new smaller replacements inside so, the unit retains its original look, while having the benefits of the modern parts.

Clint - W5CPT -
Logged
WB2WIK
Member

Posts: 21757




Ignore
« Reply #2 on: March 22, 2013, 03:16:09 PM »

But...ceramic mixes haven't changed all that much over the years.

The "size" of a ceramic cap depends on its design and construction, as well as the dielectric material (ceramic mix) used.  Very high density mixes like Y5P and such can create very "small" capacitors that are very unstable with temperature and applied voltage.  Class I dielectrics like NP0 have much lower dielectric constants and require the capacitors to be larger, but they're also very stable with temperature and voltage.

If the parts are much smaller than the original ones, it "might" be due to technology advances, or it might also be because they're not as good (stable)!

Whether that actually matters is purely application-dependent.
Logged
N4NYY
Member

Posts: 4944




Ignore
« Reply #3 on: March 22, 2013, 03:19:10 PM »

Most modern caps are superior than old caps. That have also shrunk in the process, likely from better and more accurate manufacturing techniques.
Logged
WB2WIK
Member

Posts: 21757




Ignore
« Reply #4 on: March 22, 2013, 05:32:32 PM »

Most modern caps are superior than old caps. That have also shrunk in the process, likely from better and more accurate manufacturing techniques.

The "superior" part applies to electrolytics...not so much for ceramics.

Ceramic caps in 2013 are made pretty much the way they were made in 1980, when it comes to discoidal designs.

They're still barium titanate, and the electrodes are made or printed about the same way. 

The big difference between "small" and "large" disc capacitors is which material mix is actually used.  Stable dielectrics occupy more space, and unstable ones don't.  But that was true 30-40-50 years ago, also.

Whether that matters is application dependent.
Logged
G3RZP
Member

Posts: 28




Ignore
« Reply #5 on: March 23, 2013, 07:06:15 AM »

Jim,

Where in the circuit is this capacitor?
Logged
G4REK
Member

Posts: 1




Ignore
« Reply #6 on: March 23, 2013, 07:30:21 AM »

Hi,
  Its the capacitor on the output from PA stage which goes to the aerial change over relay  and thence to the aerial socket
             jim g4rek
Logged
G3RZP
Member

Posts: 28




Ignore
« Reply #7 on: March 23, 2013, 10:48:08 AM »

What power is the TX - in fact what model? I'm not too sure about the Qs of the very high dielectric constant ceramics, and for the same Q, a larger capacitor has more area from which to get rid of heat.

If it's QRP, it probably doesn't matter. At the 100 watt level or above, you could get a problem.
Logged
G4REK
Member

Posts: 1




Ignore
« Reply #8 on: March 23, 2013, 01:29:50 PM »

Hi
   The output is 150 watts,  its a modified 10 meter amplifier......the original disc ceramic was large in comparison to the ones i bought on ebay recently only about a quarter of the size....i managed to break it sliding the bottom cover off it to have a look inside.....
     jim g4rek
Logged
KE3WD
Member

Posts: 5688




Ignore
« Reply #9 on: March 23, 2013, 05:00:06 PM »

Hi
   The output is 150 watts,  its a modified 10 meter amplifier......the original disc ceramic was large in comparison to the ones i bought on ebay recently only about a quarter of the size....i managed to break it sliding the bottom cover off it to have a look inside.....
     jim g4rek

If the new one has the same voltage rating or better, it should work there just fine. 

Size be damned.


73
Logged
VK2TIL
Member

Posts: 543




Ignore
« Reply #10 on: March 23, 2013, 05:55:03 PM »

Capacitors have a property called Equivalent Series Resistance (ESR); that is a "real" resistance that dissipates power in the form of heat.

ESR is made-up of lead resistance and dielectric losses.

Your 150-watt amplifier will drive roughly 2 amps into a 50-ohm system.

ESR of ceramics depends on the type of dielectric and is usually in the range of 0.1 ohms to 0.01 ohms for common types.

0.1 ohms at 2 amps will generate 0.4 watts whilst 0.10 ohms will generate 0.04 watts to be dissipated as heat.

Since you are unlikely to be able to measure ESR, next best thing is to try it and check the temperature of the capacitor while it's delivering power to an antenna or load; if it gets hot you should replace it with either a better one or two or three or more of appropriate value in parallel which will dissipate heat better.
Logged
G0VKT
Member

Posts: 71




Ignore
« Reply #11 on: March 25, 2013, 05:35:45 AM »

It could be that the old cap is a single layer type. So one dielectric layer with metallization on either side.
The new cap could be a multi-layer type which means more cap in a given volume. These tend to be in square or rectangular packages rather than round.

Or a different ceramic material has been used. The old cap could have a low K material which means a physically bigger cap is needed. Ceramic dielectics have come a long way in the last 20 years. But a higher K dilectric can also have worse performance in some respects.

I could write pages on this as I have spent nearly 20 years in this business.
Logged
NR4C
Member

Posts: 471




Ignore
« Reply #12 on: March 25, 2013, 06:56:17 AM »

....i managed to break it sliding the bottom cover off it to have a look inside.....
     jim g4rek

Excuse me, but what type of ceramic disc capacitor has a "Cover" that could be removed to look inside?

Are we all talking about the same component?

...bill nr4c
Logged
G4REK
Member

Posts: 1




Ignore
« Reply #13 on: March 25, 2013, 07:08:30 AM »

Of course it was the cover on the amplifier......although all the caps have removable lids some even have glass covers letting one look inside.....
     g4rek
Logged
WB2WIK
Member

Posts: 21757




Ignore
« Reply #14 on: March 25, 2013, 10:41:02 AM »

If it's a 1000pF capacitor used at 150W output to couple to a 50 Ohm antenna, at that power the capacitor would dissipate about 15W at 28 MHz (Xc = 5.7 Ohms).

That sounds like an accident waiting to happen if the amp really runs 150W and the normal load is 50 Ohms and that's the only output coupling cap to the antenna port.

Is it?
Logged
Pages: [1] 2 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!