[HomeBrew] Questions about passive components

[Glenn Little WB4UIV]

The EIA has a set of standard values.
These are E12 for 10% values.
E24 for 5% values E48 for 2% and E96 for 1% values.
For 10% values there are 12 logarithmic steps.
These are 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 and 82.
10+10%=11, 12-10%=10.8 (about 11), 12+10%=13.2, 15-10%=13.5, 15+10%=16.5 
and so on.
For a circuit that can accept a 10% part, all values can be covered by 
this sequence.
The same is true for 5%, 2%, 1% and all other tolerances.
Your 49.9 Ohm part is 1%.

The standard values are selected so that any value can be covered within 
the desired tolerance range.
The same is true for capacitors and zener diodes.

  Hope this helps
Search on resistor standard values for more information.

73
Glenn
WB4UIV

On 1/22/2016 10:01 AM, R. Blackman wrote:
> ** Please do NOT cross-post messages when posting to HOMEBREW **
>
> Hello to group.
> Not much activity these days so I thought a discussion would be fun.
> Question:
>
> why do components have the 'standard' values that the have?
> for example:  resistors have 47 , 470, 4.7K 47K etc,  instead of perhaps
> 48 which is easier math  for 12 volts
> Why does a 1/4 watt resistor have the value 49.9 instead of 50 ohms? (
> just checked digikey)
> Capacitors:  seem to like the numbers 22, 33  or 68 for example.
>
> does this refer back to the old days of high voltage systems?
>
> Happy to hear any ideas
>
> Have fun
>
> Richard VA3NDO
>
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-- 
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Glenn Little                ARRL Technical Specialist   QCWA  LM 28417
Amateur Callsign:  WB4UIV            wb4uiv at arrl.net    AMSAT LM 2178
QTH:  Goose Creek, SC USA (EM92xx)  USSVI LM   NRA LM   SBE ARRL TAPR
"It is not the class of license that the Amateur holds but the class
of the Amateur that holds the license"
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