For God’s sake, SMD parts are the norm since the fall of the Berlin wall, yet many people still boast about their excellent resistor-reading skills, like it’s 1920. Yes, it was invented over 100 years ago! Yes, they are used for breadboard tinkering for obvious reasons, but let me show You to this magical new technology, developed in the 1960s: SMD components. Continue the introduction with an exercise: what are the values of these resistors?
Oh, the bottom one is 3R3, so it must be 3.3Ω. Easy-peasy. R means Radix, a.k.a The Dot. 3 dot 3, 3.3Ω. That’s it. But what about the top one?
Color coding
Okay, now the top one. Our first usual problem is: read from left to right, or right to left? The gold ring gives a hint: 5% tolerance and read from left to right in this case. But if it were brown, red, green, blue, or grey, it would complicate things.
The first digit and second digit is at least clear. The multiplier follows the same scale for most part. So: red – purple – orange. 2-7-1000. 27kΩ, 5%.
Last but not least: think about people with colour blindness. It’s a common joke, that men can only distinguish 16 colours, but some can even do less. Why mess with them?
How to calculate SMD codes?
The example 3R3 was a super easy one, but we have more! 3 common methods exist: 3-digit, 4-digit and EIA-96.
3-digit codes
Let’s have this 27kΩ example and convert it to 3-digit code! The system is similar to the colour rings: 2 significant digits, one multiplier. 27 is straightforward, and multiply it with 1000. The third digit is n in 10n, so 103 means the 3rd digit will be 3. So the code for 27kΩ is 273. Oh, I just found some resistors with 103 written on them. Can you calculate the value? Hint: it’s 10kΩ.
4-digit codes
It’s also similar: 3 significant digits and a multiplier. 27kΩ is just slightly trickier this time. 27kΩ = 27000Ω. The first 3 digits will be 270, and we left with “100”. Also known as 102. “2702” means 27kΩ. And what does 1301 mean? Hint: 1.3kΩ.
EIA-96
Call me a hater, but this is what makes the system overcomplicated. I know, the E12, E24, E49 and E96 scales for 10%, 5%, 2% and 1% resistors exist. But I don’t expect anyone to remember the scale values. The first 2 digits (01-96) are the numbers in the E96 scale, multiplied with a letter. Another thing to remember…
| Z | 0.001 |
| Y / R | 0.01 |
| X / S | 0.1 |
| A | 1 |
| B / H | 10 |
| C | 100 |
| D | 1000 |
| E | 10,000 |
| F | 100,000 |
Repairability
Love is in the air! And smoke. Ah, the distinct Amper-smell! Some electronics just died from current-overdose, but You decided to repair it anyway. Let’s start with the fuses first, maybe it was just an accident.
A quick lesson: my colleagues made a painful mistake at a housewarming party. (know your god damn limits and don’t act as a know-it-all) Long story short: they tried to install the thermostat, killed the boiler. Oh, it was chilly outside. And a Friday evening, so the chance of a cold weekend was too damn high. The repairman came on Monday, and tried to install a new mainboard for $150. After a quick check, just replaced the $1 fuse and it was good as new! So yes, sometimes it’s that easy, but assume you did that and it’s still not working. Let’s continue and find some spectacularly dead components. And we just found some deep-fried resistors!
What happens, when components get overheated? Bingo! They get discoloured! It can happen in various ways, but in most cases burned components get a bit blacker. You can watch my video here, I burned some resistors for your pleasure. Since detailed schematics are rare, we have to find the value of the dead component. Desoldering and measuring is absolutely pointless, so we have to look up the codes. It was already hard to tell the difference between brown, red and orange (many manufacturers use awful paints), and it’s impossible if the colour rings are charred!
SMD components also win here. They have numbers on them! Even when they burn, numbers are much more readable. However, they can still fail in an unreadable way, nothing is guaranteed.
Not all SMD components
As Mats Engstrom pointed our: we can still get a huge middle finger. Next time you pop something open, be prepared for unmarked resistors. Manufacturers try to spare every cent, so they sell plain components too. This is bad because we are evolving backwards. Chip capacitors and inductors usually miss any marking, and it makes manufacturing harder. I would have been happy to see capacitors with at least a hint for its capacity.
