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How to Choose the Right PCB Resistor

Simon / 2021-03-15

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Are you looking forward to designing your first PCB board? Well, there are a plethora of components used when designing a PCB board. Of all the components employed, none can beat the workability and performance of a popular component known as a resistor. Supposedly, you might be having a clue of what the resistor looks like. If you are clueless, look at your circuit board and you will find several of them all over the board's surface. They are primarily designed to monitor and control the flow of current and ensure that LED lights work smoothly. With that in mind, how do we define resistor? What are the types available? Are all types of resistors ideal for your tasks? Well, read on to find out!


What Is A Resistor?


A resistor is one of the small, unique, and vital passive electrical components employed virtually in all electronic circuit boards. There are many reasons why electronic manufacturers consider using resistors. Why do you think electric devices don't burn out? Alternatively, why do you think resistors are placed slightly behind the LED lights in circuits? They are not situated there for fun and definitely, they work to ensure that your device doesn't burn out when there is excess or irregular electric current.


The resistor's value is considered resistance, which is mathematically calculated and measured in Ohms. In case you did physics or electronic course, then you can attest that your tutor mentioned Ohm's law and how it is applied. When working with resistance, therefore, you must be ready to apply this Ohm's law over and over again. If you are new to Ohm's law, then it is a resistance that happens when 1 Ampere of current passes via a resistor with a voltage drop of one across the resistor's terminal. Usually, Ohm's law is calculated as: 


  • R= V/I


This means that resistance is inversely proportional to voltage and current.


Resistors are arguably employed for different purposes. These can be delimited electric current, heat generation, control gain, voltage division, matching/loading circuits, and solve time constants. Resistors are available locally and commercially with resistance values exceeding 9 magnitude orders. Additionally, they are also best suited for electric brakes so that they can dissipate kinetic energy (K.E) from trains or electronics.


What Is SMD Resistor?


SMD resistor is a deliberate type of device that embraces surface mount technology to offer significant benefits concerning automated manufacture and space-saving of PCBs. They are normally employed in broad quantities. Most industrial and consumer electronic devices use SMT because it enhances manufacturing, allowing vast automation levels and boost reliability. This aspect offers high functionality levels to be realized within ideal size and minimizes the costs. Regarding the quantity of components used, SMD resistors are the types of unique components in almost all electronic devices. These resistors offer similar functionality and workability as traditional leaded resistors but with much minimal power dissipation, capacitance, and stray inductance, among others.


These resistors are readily available in the most popular standard values, including E192 and E3. They are yet available in different sizes and many of them being minute and challenging to handle. They are commonly referred to as chip resistors due to their rectangular shape. They are made of ceramic or alumina substrate and that is why they have metalized sections at the end of their body.


The electrode end connection bases are positioned onto this area, which is fired to make sure they are strongly held. There is a resistive thin-film material deposited, which can be a metal film or metal oxide. The thickness, length, and type of material employed dictate its resistance. In most cases, the YIG laser is a resistive material trimmed to achieve the ideal resistance. Once the right resistance is achieved, the resistor is enclosed using a successive protective coat, which helps hinder the ingress of contaminants and moisture while preventing mechanical damage.


The reason why SMD resistors are made using metal film or metal oxide and safeguarded using strong coating is to make them stable and reliable while ensuring they have great time tolerance and temperature. The SMD resistor terminal ends are an integral part of the overall workability and performance of the resistor. The connection between terminal ends and internal connection mainly employs a nickel layer. The tin layer is employed on the outer layer to offer outstanding solderability.


Different Types of PCB Resistor


There are many types of resistors categorized into two; fixed and variable resistors.


  • Fixed resistors


Just as its name signifies, it is a type of resistor with fixed tolerance and resistance despite the changes in external aspects such as light, temperature, among others. They are broadly used resistors in many electronic circuit devices. Examples of fixed resistors can be;


1. Carbon composition


This type of resistor was previously very common. However, it is very rare to see it being used today since new technology has introduced unique types of resistors that are cheaper, small, and offer better performance. It was large and influenced by the high-temperature coefficient. It was also influenced by an erratic and large irreversible transformation in resistance due to age or heat.


2. Carbon film


This was developed during the time of transistor technology. They used low power and were highly inductive. They were secured by ceramic tube or conformal epoxy and tolerated a temperature coefficient of 100-900 degrees Celsius.


3. Metal-oxide film


This is the current widely employed type of resistor. Instead of carbon film, it utilizes metal oxide film. Furthermore, the film uses conformal epoxy coating to safeguard it. It also has a high-temperature coefficient, providing it a superb performance.


4. Metal film


This type is almost the same as the metal oxide type in terms of visual and performance. The difference is that rather than utilizing metal oxide, it employs metal film, which can be nickel alloy.


5. Wire wound resistor


This is a kind of resistor that is normally used in high-power device applications. They have a winding wire designed with a relatively higher resistance than the normal ones.


6. Surface mount resistors


These are the primary format employed for electronic elements. They are simple to apply in automated manufacturing and are designed to offer high-performance levels. They normally employ the same technology as other types but in a format of surface mount.


  • Variable


These types of resistors encompass a slider and fixed resistor compounds. This helps tap into the primary resistor compound, providing up to three-element connections. One is connected to the slider and two are linked to the fixed compound. This makes the element to be perceived as a variable divider in case all the three are employed. Examples of variable resistors can be rheostats, potentiometers, and digital resistors.


They are kinds of resistors with fixed resistance values and are applied for voltage division and sensor sensitivity.


How to Choose the Right Resistor?


Due to many types of resistors, many people can find it challenging to choose the right option. This can be tricky because you must understand the type of resistor you will use for your PCB design. Here are the three steps that can guide you: 


1. Know the resistance you require


This is where Ohm's law is applied, which also means that you must know how this is calculated. You can employ any of the following formulas;


  • V= IR

  • R= V/I

  • I= V/R

  • Where V = voltage

  • R = resistance

  • I = Current


2. Know the power required


You also need to know how much power your preferred resistor will require to dissipate. This means that you must have the right formula in mind. You can use the following formula;


  • P = V2/R

  • Where V2 = voltage squared

  • P= power in Watts

  • R = resistance


3. Picking your resistor


Having power rating and resistance values, the next thing is identifying the ideal resistor from an element distributor. Always consider the standard type of resistor if you want to make your PCB board long-lasting.


How to Solder Resistor on PCB?


There are three steps involved when soldering resistor on PCB, which include: 


1. Picking soldering materials

2. Soldering preparation

3. Soldering process


  • Picking soldering materials


These can be a wand, solder wick, and solder sucker. The wand is the iron section that grasps the tip. It is also the part you will handle. It is mostly made of rubber or other nonconductor materials. Solder wick is the eraser or desoldering braid. A solder vacuum is a right tool for getting rid of solder left when desoldering elements.


  • Soldering preparation


Before soldering the board must be thoroughly clean. The iron must be warmed up to keep the board at the right temperature when soldering. You also need to create space to work on. This can be a moistened sponge to place your soldering iron stand. The next thing is coating the tip and cleaning it.


  • Soldering process


Soldering on resistor on PCB starts with surface preparation. This means ensuring a clean board surface to have low resistance and a strong solder joint. The next thing is component placement. This will entails soldering each resistor at a time. The best way is to start with small components and place other large components. This helps ensure there is enough space and the board is stable during soldering. Next is applying heat on a little solder amount to the iron's tip. This ensures the conduction of heat to the board and other elements. Next is solder placement on the joints. Confirm if the solder in the joints is perfectly placed.


Conclusion


Resistors are perceived as simple electronic elements; however, you need to know that there are different types of them available. You must know the different options available and know how to choose the right one for your specific needs.