"Explain Like I'm 5": Common Parts Markings and Polarity August 18, 2016 11:22
In the five years we've been selling kits, we've received a couple thousand support tickets. And I've found that ~90% of issues come down to two common errors:
- Parts in the wrong place
- Parts in the wrong orientation
While these errors are easily avoidable, they're not "dumb" errors. Identifying parts can be really confusing, since every kind of part follows a different convention.
In this edition of "Explain Like I'm 5" we'll cover how to identify the value and orientation of the most common parts.
Resistors feature colored bands which indicate their value (resistance) and tolerance (resistance range). To identify a resistor, check the colors of the bands against a color code chart or look them up on a color code calculator.
However, if you have a multi-meter there's an even better option! Set your meter to read resistance (Ω symbol) and probe either side of the resistor. Just keep in mind that the actual resistance will vary based on the tolerance of the resistor. For example, the resistor in the photo below is rated at 910Ω with a 1% tolerance and measures 904Ω.
Resistors are not polarized, so there's no correct orientation for them.
Electrolytic Cap Value
Electrolytics are one of the least forgiving components: they are polarized and they tend to explode spectacularly when they're inserted backwards. On the bright side, they're perhaps the most completely and clearly marked parts there are.
Electrolytics' value (capacitance) and voltage rating are marked right on the body, with units specified and everything!
Electrolytic Cap Orientation
And manufacturers are so serious about making sure you don't reverse electrolytics that they marked their polarity twice. How nice of them! An electrolytic's positive lead is longer and the negative lead is marked on the body with a stripe and minus signs.
If only all caps were as clearly marked as electrolytics. Most caps are marked with a three-digit code for value and one letter for tolerance.
The three-digit code indicates the cap's value in picofarads. The first two digits are the first digits of the value, and the third digit is the number of zeroes. So, in the photo below, the cap on the left is 100pF (10 + one zero) and the cap on the right is 100,000pF (10 + four zeroes).
If, like me, you can never remember the metric units, you can use an online converter to convert that 100,000pF to something more readable like 100nF.
Unlike electrolytics, most other caps are not polarized. The very few exceptions, such as tantalums, have their polarity marked on their bodies.
Diodes' names are marked right on the body (though you may need a magnifying glass to read them).
A gray or black stripe marks the cathode (negative) lead. Just align the stripe on the diode with the stripe on the PCB and you're set.
It doesn't get easier than identifying an LED. Searching for the "red LED" from the BOM? It'll be the red one.
And just like electrolytic caps, an LED's positive lead is longer than the negative lead.
Transistors are very straightforward to identify because, instead of a value, they have a model number which is marked on the body.
Since different transistors have different names for their leads, the most reliable way to identify orientation is by shape. Simply match the shape of the transistor's body to the shape marked on the PCB.
Integrated Circuits (ICs)
Like transistors, ICs have a model number which is marked on the body. There's often a batch number, too, which can be ignored.
IC manufacturers indicate orientation in a couple different ways. First is with a notch on one side the body (between pins 1 and 8). This notch is usually shown on the PCB as well. Second is with a dot next to pin 1.