Partlist Wednesday: Resistors

Every Wednesday we highlight a component from the updated partlist. This week: resistors.

If you don’t have a cheap resistor kit with common values, head to eBay and get one now. It’ll prevent last minute part orders and project delays. Sjaak likes this kit from MDFLY because it comes in plastic pages, only $15. Many of the kits on eBay come with a free capacitor kit too, search for “resistor kit”.

These are the most common values in our projects, we buy them by the reel:

  • 390R (LEDs @ 3.3volts)
  • 1K (LEDs @3v3 or 5v0, transistor base resistor)
  • 2K (LEDs @ 3v3 or 5v0, strong pull-up, transistor base resistor)
  • 10K (General purpose pull-up/down, trans. base resistor)
  • 100K (Weak pull-down)

Choosing a size deserves some consideration because you may be stuck with a stock of parts for a while. 0805 is a lot friendlier to new solderers, but 0603 isn’t much more difficult. We settled on 0603, and are migrating all active projects to that size. The parts are generally cheaper than 0805, and cost a lot less. 0603 is also easier to locate and route next to a SSOP or TQFP chip.

We like to use resistors rated for at least twice the current needed. 0603 resistors are typically 1/16 to 1/8 watt. If that’s not enough, reach for a larger package, use multiple resistors, or use a resistor array.

Common resistors are easy, but the exotic arrays and SMD potentiometers are harder to deal with. Frequent requests for the 10K resistor array on the Bus Pirate inspired the partlist update.

Resistor arrays or networks are several resistors stuck together. They are a lot easier to solder than it might appear. Double check the size. 0603 can mean several 0603 resistors stuck together, or several resistors the size of one 0603 resistor. Arrays also come in convex (solder on the points, as in the photo) and concave (solder in the indentations), but we’ve been able to use the parts interchangeably.

Standard through-hole potentiometers are 6mm. They come with the legs bent into several different configurations, but they can usually be contorted to fit for hand prototyping.

Surface mount potentiometers come in lots of sizes and shapes. We have experience with 4mm (Dangerous DSO) and 3mm (#twatch). 3mm is a bit difficult to solder and a real pain to adjust. They generally feel cheap and fragile. 4mm is a bit tougher and a lot easier to solder, though adjustment is still a bit temperamental.

This is meant as a summary of what we use, not an essay on resistors. Please share your experience in the comments.

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  1. Is there any specific reason why we use the 390 E12 resistor instead of the E6 series 330 ohm value? Does it happen to be cheaper, or did it just happen to be laying around first / was entirely arbitrary?

  2. Just about to post the same question as Kenneth. This is my means of subscribing to the answer. :)

  3. Honestly it was just there, and it makes the LEDs a little dimmer. I actually wrote that up wrong (not fixed), the 390 is for 3v3 LEDs, not 5v0 LEDs.

  4. I got some of the MDFLY SMT resistor kits, including the one you linked to:

    These are in 6-ring binder pages, and it would be nice to get a binder to keep the pages tidy and in order. The page is 4-5/8″ x 6-7/8″ in size, the holes are on 3/4″ centers except holes 3 and 4 are 2″ apart, and the 1st and 6th holes are 5″ apart. Does this match the “day-timer” ™ personal organizer format? Is there a cheap source for an empty binder that would fit this page size?

    Amazon has a few such as the $8 “Loose Leaf Binder 3 3/4 by 6 3/4” from “All pro software” but they don’t specify the exact hole spacing. Customer reviews indicate there is more than one standard hole spacing, unfortunately.

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