1
\$\begingroup\$

I noticed that the resistor kits I've seen appear to be based on E6 or E12, but they skip certain values in favor of near neighbors. For instance, the 1000 decade of my kit contains: 1k, 1.2k, 2k, 3.3k, 3.9k, 4.7k, and 5.1k. Is this because whoever put the kit together couldn't get a good price on 6.8k resistors (E6), but got a screaming deal on 2k and 5.1k (which aren't standard values until E24)? Or is it because these extra resistors are commonly needed in designs? (A nonstandard standard, if you will.)

Here are two decades from another kit:

  • 100 120 150 180 200 220 270 300 390 470 510 680 820
  • 1k 1.5k 2k 2.2k 3k 3.3k 3.9k 4.7k 5.1k 5.6k 6.8k 7.5k 8.2k

They're not even consistent with each other! Is this based on common existing designs, a factory's current surplus, or is there actually a better method than choosing an E series?

\$\endgroup\$
  • \$\begingroup\$ E series are designed to be "logarithmically complete"; the only better values are the exact ones you need. \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 24 '18 at 4:39
2
\$\begingroup\$

It depends why the kit has been put together.

If you buy a lab kit of E24 values, then you get a book with 50 of each value from 10 ohms to 1 Mohm, in the full E24.

If the kit has been put together to support a set of tutorial designs (my first kit was part of the 'Phillips EE20 Electronic Engineer Kit' which had germanium transistors in it), then there's no need for the full range.

Think about what resistors are used for. It's typically to set up a ratio, at some reasonable impedance level. The impedance level can be fairly coarse. As a designer, I rarely think tighter than 'does it have to be at the 1k, or the 10k, or the 100k level?' However, I often want to nail the ratios to a few percent, and be able to set ratios over a wide range.

Resistor ranges are constant ratio (or very nearly so). To take E24, I can can set up a 2:1 ratio with 1k/2k, 1.2k/2.4k, 1.5k/3k, 1.8k/3.6k, 7.5k/15k, and maybe a few others if I think for longer (like 1.1/2.2). I don't need all those pairs in my kit, just one pair in each decade will do for 2:1 ratios. And a few more for a few others.

Because of the economies of scale, it's far cheaper to provide a tutorial kit with 20 each of a dozen different values, than 5 each of 4 dozen values. It needs less organisation, and a smaller box, as well. Whether those values are from the E6 or E24 series makes very little difference to the price, E24 is used so widely in industry. What does make a difference is tolerance, but I'd probably want to use 1% throughout for a tutorial kit anyway.

As an aside, once the book has been in use in the design lab for a month, all the 1k and 10k resistors have gone! It's possible (just sayin') that designs end up with 1.2k resistors in them because all the 1k resistors had gone when the designer came to prototype it!

\$\endgroup\$
  • 3
    \$\begingroup\$ That last paragraph hits so true. "Hi, I'd like to order 20 count of each value, except 1k and 10k. We need TEN ZILLION of each of those." \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 24 '18 at 5:48
  • \$\begingroup\$ As far as I know, these kits are for general purpose tinkering, repairs, and experimentation. So I'm still not sure why someone would choose 12 resistors in a decade but not make them the E12 values. Perhaps because some variation will enable more ratios that would be possible with E12 alone, as long as the variation isn't the same in each decade. \$\endgroup\$ – piojo Jun 24 '18 at 15:23

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.