4 Advice to Choose a all industial resistor manufacturer

What are the key characteristics and specifications that affect the choice of resistor? Factors that should be taken into consideration include initial tolerance and value selection. However, the tolerance or variation of the value of a resistor is affected by multiple parameters, as explained below.

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Temperature Coefficient

This is a measure of the variation of the nominal value as a result of temperature changes. Generally quoted as a single value in parts per million per degree centigrade (or Kelvin), it can be positive or negative. The equation for calculating the resistance at a given temperature is:

Rt=Ro[1+α(T-To)]

Where Ro is nominal value for room temperature resistance, To is the temperature at which the nominal resistance is given, T is operating temperature and α is the TCR.

Put simply, a 1 M&#; resistor with a TCR of 50ppm/K will change by 50&#; per 1 degree of temperature rise or fall. This may not sound like much but consider if you were using this resistor as the gain resistor in a x10 non-inverting amplifier circuit with 0.3v on the + input. The worst-case change in output could be as much as 7.5mv which is equivalent to about 5LSBs in a 5v 12-bit ADC circuit. This kind of change can be quite noticeable in precision design. Remember also that the TCR is quoted as ±x ppm/C so it is feasible, although unlikely, that the second resistor in the circuit could change in the opposite direction hence double the possible error. Finally, it&#;s worth noting that some precision resistors quote variable TCRs over the temperature range the circuit is operating in, and this can complicate the design process significantly.

Resistor Ageing or Stability

Ageing and stability are a complex amalgam of multiple changes to the value of a resistance value over time and are the result of temperature cycling, high-temperature operation, humidity ingress and so on. Typically, the value will lead to an increase in resistance over time as conduction atoms migrate within the device.

Thermal Resistance

The thermal resistance is a measure of how well the resistor can dissipate power into the environment. In practice, engineers use thermal resistance to model the heat dissipation for a system &#; it is thought of as a set of series &#;thermal resistors&#;, each representing one element of the heat dissipation of the system.

This is mainly important if the design means the resistor is running at or near its maximum value and can significantly affect the long-term reliability of the system. An example of where this parameter could be used is to calculate the size of a PCB pad or ground plane requirement that would be used to keep the resistor&#;s value and operating temperature within acceptable limits.

Thermal and Power Rating

All resistors come with a maximum power rating, specified in watts. This can be anything from 1/8th watt right up to 10s of watts for power resistors. In a first pass analysis, the engineer would check that the resistor is operating within its rated value. The equation for calculating this is P=I² R, where p is the power dissipated in the resistor, i is the current flowing and R is the resistance. Sadly, things can be more complicated than this; for exact work, the engineer needs to take account of the thermal derating curve for the resistor. This specifies the amount by which the designer needs to de-rate the maximum power dissipation above a given temperature.

This might seem theoretical as often the de-rating kicks in at quite high temperatures, but a power circuit in an enclosed housing in a hot region can often exceed the cut in point and the maximum power dissipation will need to be reduced appropriately. It&#;s also worth noting that the maximum operating voltage of a resistor is de-rated with power dissipation.

Resistor Noise

Any electronic component that has flowing electrons is going to be a source of noise, and resistors are no different in this respect. In high gain amplifier systems or when dealing with very low voltage signals, it needs to be considered.

The major contributor to noise in a resistor is thermal noise caused by the random fluctuation of electrons in the resistive material. It is generally modelled as white noise (i.e. a constant RMS voltage over the frequency range) and is given by the equation E=&#;4RkT&#;F where E is the RMS noise voltage, R is the resistance value, k is Boltzmann&#;s constant, T is the temperature and Δf is the bandwidth of the system.

It is possible to lessen system noise by reducing the resistance, the operating temperature or the system&#;s bandwidth. Additionally, there is another type of resistor noise called current noise which is a result of the electron flow in devices. It is rarely specified but can be compared if the standard numbers using IEC are available from the manufacturer.

High-Frequency Behaviour

The final challenge to consider is the high-frequency performance of the particular resistor. In simple terms, you can model a resistor as a series inductor, feeding the resistor which has a parasitic capacitor in parallel with it.

At frequencies as low as 100Mhz (even for surface mount resistors which have lower parasitic values than through-hole parts) the parallel capacitance can start to dominate, and the impedance will drop below nominal. At a higher frequency still, the inductance may predominate, and the impedance will start to increase from its minima and may well end up above the nominal value.

Learn what exactly is a resistor, how does it work, and how do you choose the right one for your first PCB design?

Are you planning to start on your first PCB design? There are so many types of components that you&#;ll end up using, but none can beat the infamous of them all &#; the simple resistor. If you have ever looked at a circuit board, you&#;ll find resistors are all over the place, controlling the current flow and making those LEDs light up. But what exactly is a resistor, how does it work, and how in the world do you choose the right one for your first PCB design?

Fear not, we have you covered with everything you might need to know.

So&#;What is a Resistor?

Resistors are one of several passive electrical components, and what they do is relatively simple yet vital &#; creating resistance in the flow of an electric current. Have you ever seen a LED light up? That was made possible thanks to the trusty resistor. By placing a resistor behind a LED in a circuit, you get all of the brilliant lights without anything burning out!

The value of a resistor is its resistance, measured in Ohms (&#;).  If you have ever taken a basic electronics course, then your instructor likely drilled Ohm&#;s Law into your head. You will use Ohm&#;s Law time and time again when dealing with resistors. More to come on this:

Finding a resistor symbol on a schematic is easy. The international symbol is a standard rectangular shape, but the US standard has the zigzag line that makes it easy to identify. Regardless of the form, both styles have a set of terminals connecting the ends.

What Are the Different Types of Resistors?

There&#;s a ton of resistors floating about that are divided into two categories &#; construction type and resistance material. Let&#;s cover both:

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Construction Type

• Fixed Resistors &#; As the name implies, these resistors have a fixed-resistance and tolerance regardless of any changes in external factors like temperature, light, etc.
• Variable Resistors &#; These parts have a modifiable resistance. The potentiometer is a great example, which has a dial that can be turned to ramp up or down the resistance. Other variable resistors include the trimpot and rheostat.
• Physical Quality Resistors &#; These resistors are like chameleons and can change their resistance based on a variety of physical properties, including temperature, light levels, and even magnetic fields. Physical quality resistors include the thermistor, photoresistor, varistor, and magneto-resistor.

Resistance Material

Resistors can also be broken down into the actual material they are made from, which has a massive effect on how they resist current. These materials include:

• Carbon composition
• Carbon film
• Metal film
• Thick and thin film
• Foil
• Wire Wound

Carbon composition is an older technique that has been around for a while and produces a resistor with a low degree of precision. You&#;ll still find these for use in applications where high energy pulses occur.

Of all the resistor material types, wire wounds are the oldest of them all, and you&#;ll still find these in use when you need precise resistance for high power applications. These ancient resistors are widely known for being reliable, even at low resistance values.

Today, metal and metal oxide resistors are the most widely used, and are better for providing a stable tolerance and resistance, while also being less influenced by changes in temperature.

How Do You Use Resistors?

You&#;ll find resistors being utilized in many applications beyond just resisting current. Other applications include dividing voltage, generating heat, matching and loading circuits, controlling gain, and fixing time constraints. In more practical applications, you&#;ll find large resistors being used to power electric brakes in trains, which helps to release all of the stored kinetic energy.

Here are some other cool applications that the versatile resistor is used for:

• Measuring electrical current &#; You can measure the voltage drop across a precision resistor that has a known resistance when it&#;s connected to a circuit. This is calculated using Ohm&#;s Law.
• Powering LEDs &#; Giving a LED too much of a current will burn out that beautiful light. By connecting a resistor behind a LED, you can control how much current the LED receives to keep the light shining.
• Powering blower motors &#; That ventilation system in your car is being driven by a blower motor, and a special resistor is used to control the speed of the fan. This resistor type is called, not surprisingly, the blower motor resistor!

How do You Measure a Resistor?

The value that you will see time and time again is resistance (R). This value is displayed in different ways, and there are currently two standards for measuring how resistance appears with either color-coded markers or SMD codes.

Color-Coding

You might be familiar with the color-coding system if you have ever tinkered with a breadboard. This technique was invented in the &#;s, and the resistance and tolerance values are displayed by several colored bands painted on the body of the resistor.

Most of the resistors that you look at will have four colored bands. Here&#;s how they breakdown:

• The first two bands determine the primary digits of the resistance value.
• The third band determines the multiplying factor, which will give a resistance value.
• And lastly, the fourth band provides you with a tolerance value.

All of the different colors on a resistor correspond to different numbers. You can use a handy resistor color code calculator to quickly determine these values in the future.

SMD Resistors

Not every resistor is large enough to be identified by color-coding, especially when using Surface Mount Devices, or SMDs. To compensate for the smaller space, SMD resistors are given a numerical-based code. If you take a look at a modern circuit board, you&#;ll notice that SMD resistors are also all about the same size. This helps to standardize the manufacturing process with those rapid-fire pick-and-place machines.

How do I Choose the Right Resistor?

Ok, time for the most important part, learning how to figure out precisely what kind of resistor you need for your first PCB design. We have broken this down into three easy steps, which includes:

1. Calculating your required resistance
2. Calculating your power rating
3. And lastly, selecting a resistor based on these two values.

Step 1 &#; Calculating Your Resistance

This is where you will use Ohm&#;s Law to calculate your resistance. You can use one of the standard formulas below when your voltage (V) and current (I) are known.

Step 2 &#; Calculating Your Power Rating

Next, you need to figure out how much power your resistor is going to need to dissipate. This can be calculated with the following formula:

In this formula, P is your power in Watts, V is the voltage drop across the resistor, and R is the resistance of the resistor in Ohms. Here&#;s a quick example of how this formula would work in action:

In the circuit above, we have a LED that has a 2V voltage, a resistor with a 350 Ohm (Ω) value, and a power supply giving us 9V. So how much power will dissipate in this resistor? Let&#;s add it up. We first need to find the voltage drop of the resistor, which is 9V from the battery and 2V from the LED, so:

9V &#; 2V = 7V

You can then plug in all of this information into your formula:

P = 7V*7V / 350 Ohm = 0.14 Watts

Step 3 &#; Choosing a Resistor

Now that you have your resistance and power rating values, it&#;s time to pick an actual resistor from a component distributor. We always recommend sticking with standard resistors that will be carried in stock by every distributor. Staying with standard resistor types will make your life a whole lot easier once it&#;s time to manufacture. Three dependable component suppliers that you can find some quality parts from include Digikey, Mouser, and Farnell/Newark.

The Resistance is Strong in This One

So there you go, everything you might possibly ever need to know about resistors for your first PCB design project. Resistors have so much versatility, and you&#;ll find yourself using them time and time again in every electronics project that you complete. The next time you need to choose a resistor, remember the simple three-step process &#; 1. calculate your resistance, 2. then your power rating, 3. and then find a supplier!

Now, before you run off making your own resistor symbols and footprints in your PCB design software, wouldn&#;t it just be easier if they were already done for you? They already are! Check out the vast amount of free parts libraries available only in Fusion 360. Try Fusion 360 electronics for free today.

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