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2nd February 2013, 10:25 PM
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#1 |
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diyAudio Member
Join Date: May 2005
Location: Dallas (but I am not a Texan!)
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Learned something (the hard way) about mica capacitors
It turns out if you pull a mica capacitor out of a drawer and it says "470", there is a 50% chance that it is 470pF, and a 50% chance it is 47pF!
I was puzzling over why the second channel of the 5B/254M amp I just put together was oscillating when I biased it over 30mA. Looked all over, could not see any differences. I looked over the mica caps... they were the same (two said "100" and two said "470". Hmmm, but they were slightly different... Turns out one said "470 +/-5%"and one said "470J" ("J" means 5% too). But the one that was labelled 470J turned out to be 47pF! DOH! Pete |
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3rd February 2013, 12:01 AM
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#3 |
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diyAudio Member
Join Date: May 2012
Location: Near Huntingdon, Cambridgeshire, UK
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In case anyone not familiar with this numbering is wondering, here's a little explanation...
470 can mean 470, but it can also mean 47 and no zeros, so 47pF as above. 471 then means 47 and one zero, so really 470pF 472 then means 47 and two zeros, so really 4,700pF 473 then means 47 and three zeros, so really 47,000pF etc. |
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3rd February 2013, 12:10 AM
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#4 |
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diyAudio Member
Join Date: Apr 2008
Location: Carlisle, England
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I always keep components in their original packets so I know what they are without having to measure them.
Also the packets have the order code so I can check on the website if there is any uncertainty.
__________________
http://www.murtonpikesystems.co.uk PCBCAD50 pcb design software. |
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3rd February 2013, 01:18 AM
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#5 |
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diyAudio Member
Join Date: Jan 2005
Location: The emerald city, Seattle, WA.
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It's nice to have a DMM that measures capacitance as well. I always check my caps before they get soldered in a circuit just to be on the safe side. I think I paid $30 for mine at PE.
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"No man really becomes a fool until he stops asking questions" - Charles Proteus Steinmetz |
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3rd February 2013, 01:25 AM
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#6 |
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diyAudio Member
Join Date: Oct 2012
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Not just mica, i've seen this on metallized poly box caps too.
224 means 22 with four zeros pf, not 224n |
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3rd February 2013, 08:05 AM
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#7 |
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diyAudio Member
Join Date: May 2011
Location: Dijon
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This type of ambiguous marking is used on many capacitors, especially ceramic ones. A cheap RLC bridge can avoid disapointments. Measurement is the only way to be sure about the value when you begin to doubt.
__________________
http://constructions.f6fkn.com |
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3rd February 2013, 01:03 PM
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#9 | |
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diyAudio Member
Join Date: Jan 2008
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Quote:
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