COLIN WONFOR: When/why did capacitor value standards change?

Colin Wonfor

Dear Colin, a question which has been nagging away for a few days now: why were capacitor value 'standards' changed around early 70s, and why such seemingly arbitrary numbers? In my Quad 33/303 there's several electrolytic caps featuring values such as 2uF, 12uF, 25uF, 50uF, 100uF..... When (and why) were values changed to 10uF, 22uF, 33uF, 47uF etc. ? Why for example, change the 50uF standard to 47uF, and yet 100uF remains a standard today, for example. Odd-squared! Cheers, John.

Blame the Yanks! All Cad packages I use are in Metric but all parts are in inches, Yanks again. Also the electrolytic capacitance tolerance is crap; some are +20% through to - 50% and so fixing a value is – as you say – arbitrary. Nowadays we can do much better, but the paperwork might take too long or something to fix.

Thank you for the question.