Learned something (the hard way) about mica capacitors

[pmillett]

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

 

[scott17]

Point taken.
It guess it pays to check those.
Thanks.

 

[12E1]

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.

 

[nigelwright7557]

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.

 

[TubeHead Johnny]

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.

 

[Robert Kesh]

Not just mica, i've seen this on metallized poly box caps too.

224 means 22 with four zeros pf, not 224n

 

[Blaireau]

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.

 

[Shoog]

Had the same issue with a 10R power resistor.
One was 10R and the other turned out to be 1R. Same banding I had soldered them in before I checked.

Shoog

 

[wicked1]

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.

I had the same idea, but all of my old mouser packages have faded, and I can't read them anymore!