On Aug 6, 7:46 pm, "M" wrote: > Where can I find a web page that describes resistor value designations? >
The easiest way was to get it out of the DigiKey catalog -- like here
I would just print this out and keep it on the desk as a reference. In the CAD package I was using (jurassic CAD - Tango) instead of inserting a 'resistor' and then having to add the part number and pc footprint, I had each resistor as a separate entity by value so when I would insert a "27K4", the company part number, footprint and such were already buried in there ready to go. Made a BOM much easier.
It's common in Europe to see the multiplier - in this case "k" - placed where the decimal point would be to avoid specks of flyshit on a page looking like a decimal point. So, a resistor designated 4k74 is the same as
4 point 74 k ohms. This may appear to be an 'odd' value of nearly, but not quite, the standard 4.7k (4k7) but may well be correct if the resistor is to be found in some precision circuit such as an opamp based filter with a specific cutoff frequency, or instrumentation amplifier requiring a precise gain figure. Such resistors are often calculated to this level of precision, and require to be made up from two or more values from one or several of the 'preferred' (standard) ranges of resitor values. 27k4 is 27.4k.
"681" is unlikely to be an 'actual' value of 681 ohms. If it is a surface mount resistor that is being specified, it is probably 680 ohms, as these resistors are physically marked like this. It's the equivalent of marking them with coloured bands, but using the actual figures instead. Thus, the third digit is the equivalent of the multiplier band on a coloured stripe resistor. In your example 681 would mean "6" "8" (number of noughts) "1" or
680 ohms (may also be shown as "680R" using the system in the first paragraph)
Just to confuse you, a resistor marked "680" in the surface mount scheme, will be 68 ohms, that's "6" "8" (number of noughts) "0" This could also be shown as 68R in the other scheme. A 0.47 ohm resistor would be shown as "0R47". A 100k resistor would be shown as "100k" (!!) and a 3.3 meg as "3M3" In surface mount they would be marked "104" and "335" respectively. Just for completeness, caps are often shown using the same schemes. So a cap shown on a schematic as "4n7" will be 4.7 nanofarads, or 4700pf. It will quite likely be physically marked using either resistor colour code stripes - yellow, purple, red - or the equivalent numbers in the same way as the surface mount resistors thus "472"
4K74 is not a standard E96 or E128 value. If the schematic value of
4K74 (4,740 ohms) is correct, it has to be a special or a select on test resistor, but if the latter, it should be so designated. The closest E96 or E128 value is 4K75 (4,750 ohms) and if the circuit is so critical that it can't tolerate a standard value only 10 ohms away then I would question the designer's intention or design ability.