LEDs and Resistors

LEDs and resistors in series are common in our daily life, but how does it work? Here we explain a problem you may face. 

Suppose you have stumbled upon this confusion about series circuit. That you have two 3V LEDs connected in series to a 12V battery. What will happen then? Because the first LED would have 9V across it and the second one 6V, so you may wonder is it possible to burn them? If you use a resistor, it will lower the available volt for the second LED, but how will you make it work?

Actually, an LED will always try to get its specified voltage across itself. Two 3V LEDs in series, connected to a 12V battery will force a lot of current through the LEDs and they will burn out. When you put a resistor into the circuit, the two LEDs will have 3V across each, and the remaining voltage will be across the resistor. Then you can use Ohm’s law to find the proper resistor for the current you want. For the most common LEDs, you can usually aim for 15-20 mA.

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Resistor Types

Resistors can be made of several different materials and methods. Here are a few types of resistor: lead arrangements, carbon composition, carbon film, carbon file, printed carbon resistor, metal film, thick and thin film, metal oxide film, foil resistor, wire wound, foil resistor, ammeter shunts and grid resistor. Different types have different features. Some are very accurate, some can withstand high temperatures, some can withstand high power, and some are cheap. Some are good for low noise applications, and some are good for high-power applications, for high-speed applications and for measurement circuits.

You don’t need to learn more all features of each resistor type if you need standard circuit board. What matters you is about the resistance value and how much power it can take. If your schematic does not tell you the necessary power rating for the resistor, and you don’t know how to calculate it, then you may try a standard 1/4W resistor. If it fails after a short while, you can exchange it for a higher wattage rating.
You can learn more about resistors from www.epcb.com

Soldering a Surface Mount Resistor

There are several ways to successfully solder SMD components on a circuit board. Some are easier to learn, and some require using special materials (like solder paste, which is a mixture of powdered solder and flux) or special equipment (like SMD solder stations). Actually, you don’t need fancy equipment to do SMD soldering at home. You can easily do it with a soldering iron.

Soldering a resistor is the simplest way for freshmen. At the beginning, you should apply flux to one pad on the circuit board in order to clean the pad and make it easier for the solder to fasten properly. And then touch the circuit board pad with the tip which covers some solder to let them pass on to the pad. Next, place the resistor on its ideal location, and fix it by soldering. The resistor should now be fastened on one side. Apply solder to the soldering tip again and touch the iron tip on the other side. Finally, you’d better to use a loupe to check the solder joints to make sure the connection is good. 

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