How to Select a Resistor

When I was working on the next version of Common Parts Library, I realized that it would be helpful to explain why you might pick one type of passive component over another. We started off by writing &#;How to Select a Capacitor&#; last month, and are following it up with a close look at resistors this month.

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In this blog, we will explain all the different types of resistors, their merits and demerits, and popular applications. We have included some recommendations for commonly used resistor series with high supply chain availability from the Common Parts Library and Seeed Studio&#;s Open Parts Library, and have linked to pre-created search on Octopart.

Let&#;s dive into the world of resistors:

Resistors 

 

 

Resistors are two-terminal components used for limiting current, voltage division, and timing applications. Resistors complement &#;active&#; components like op-amps, micro-controllers, or integrated circuits for variety of operations like biasing, filtering, and pulling up I/O lines. Variable resistors can be used to change the properties of a circuit. Current sensing resistors are  used to measure the current in a circuit. The unit of resistance is Ohms (&#;).

TYPES There are several different types of resistors which differ by power rating, size, performance, and cost. Below are some of the common resistor types: chip SMD resistors, through-hole resistors, wirewound resistors, current sense resistors, thermistors, and potentiometers. We've also included their characteristics, applications, package information as well as info on part selection.

I. Chip SMD Resistors 

Chip surface-mount resistors provide a size advantage over through-hole resistors, so they are great for printed circuit boards (PCBs). Some of their common applications are pull-up/pull-down operations to ensure a signal is at valid logic level when external devices are removed, voltage divisions, current limiting, and filtering signals at certain frequencies in high-pass/low-pass/band-pass filters. You can use 0&#; resistors as jumpers to disable a sub-section of a circuit.

There are two kinds of Chip SMD Resistors:

1. Thin Film resistors are used in high precision applications like audio, medical, or test equipment. They have lower variation (0.1%-2%), a lower temperature coefficient (5 ppm/K), and are less noisy compared to thick film resistors. However, they are more expensive.
2. Thick Film resistors are the most common type of resistor, and are used for most applications. They have higher variation (1%-5%), a higher temperature coefficient (50 ppm/K), and are noisier than thin film resistors. If there are no specific performance requirements, thick film resistors are usually the preferred choice.

Package: , , , , and packages are the most common. The numbers represent the dimensions in the imperial system, with being 0.04 X 0.02 inches and being 0.06 X 0.03 inches and so on.

Part Selection:

Yageo&#;s RCFR series for 0&#; to 10M&#; rated at 0.063 W (1/16 W) Yageo&#;s RCFR series for 0&#; to 10M&#; rated at 0.1 W (1/10 W) [CPL] Yageo&#;s RCFR series for 0&#; to 10M&#; rated at 0.125W (1/8 W) Yageo&#;s RCFR series for 0&#; to 10M&#; rated at 0.25W (1/4 W) or

Panasonic&#;s ERJ-2RK series for 10&#; to 1M&#; rated at 0.063 W (1/16 W) Panasonic&#;s ERJ-3EK series for 10&#; to 2.2M&#; rated at 0.1 W (1/10 W) [CPL] Panasonic&#;s ERJ-6EN series for 10&#; to 2.2M&#; rated at 0.125W (1/8 W) Panasonic&#;s ERJ-8EN series for 10&#; to 2.2M&#; rated at 0.25W (1/4 W) or

Vishay&#;s CRCW series for 0&#; to 10M&#; rated at 0.063 W (1/16 W) Vishay&#;s CRCW series for 0&#; to 470M&#; rated at 0.1 W (1/10 W) [CPL] Vishay&#;s CRCW series for 0&#; to 470M&#; rated at 0.125W (1/8 W) Vishay&#;s CRCW series for 0&#; to 470M&#;rated at 0.25W (1/4 W)

II. Through-Hole Resistors 

Through-Hole resistors in axial leaded packages are popular and widely used esp. while prototyping as they are easily replaceable and can be used with breadboards. They are used for pull-up/pull-down, voltage division, current limiting, and filtering like chip SMD resistors. There are various types of through-hole resistors. Carbon film and metal film are the most popular.

Carbon film resistors have more variation in resistance value (2%-10%). They are most commonly available in E12 (± 10%), E24 (± 5%), and E48 (±2%) packages. Most applications have replaced carbon film resistors with metal film resistors. Carbon film resistors&#; temperature coefficient (TC) is usually negative &#; around -500ppm/K &#; but the exact value depends on the value of resistance and dimension.

Metal film resistors have less variation in resistance value (0.1%-2%) and have higher stability. They are most commonly available in E48 (±2%), E96 ( ±1 %) and E192 (±0.5%, ±0.25% and ±0.1%) packages. Because they have better performance than carbon film resistors and are inexpensive, they are used in most applications. Their temperature coefficient (TC) is around ±100 ppm/K- some parts have positive TC while others have negative TC.

Carbon composition resistors were widely used in electronic devices fifty years ago, but because of their large variations and poor stability, they have been replaced by carbon or metal film resistors. However, they have good high frequency characteristics and are good at withstanding high energy pulses, and are still used today in welding equipment and high voltage power supplies.

Metal oxide resistors were the first alternatives to carbon composition resistors, but are being replaced by metal film resistors for most applications. They are still used for high endurance applications as they can handle higher temperatures and have higher power ratings (>1W).

The EIA Decade Resistor Values table can be used to match tolerances with values. For example, standard E12 (± 10%) decade values are 100&#;, 120&#;, 150&#;, 180&#;, 220&#;, 270&#;, 330&#;, 390&#;, 470&#;, 560&#;, 680&#;, and 820&#;.

Color codes are used to represent the values in through-hole resistors. You can use this Color Code Calculator to decode the color bands.

Part Selection:
Carbon Film Resistors: Yageo&#;s CFR-25JB series for 1&#; to 10M&#; rated at 0.25W
Metal Film Resistors: Yageo&#;s MFR-25FBF series for 10&#; to 1M&#; rated at 0.25W [CPL]

III. Wirewound Resistors 

Wirewound resistors have high-resistance wire wrapped around an insulating core. They can provide very high power ratings (up to W) and can operate at very high temperatures (up to 300 degrees C). They also have good long term stability &#;  a change of 15-50 ppm/year compared to metal film resistors, which have a change of 200-600 ppm/year. They are the best choice for noise performance followed by metal film resistors.

Disadvantages: They are only available in low ohmic range (0.1&#; to 100k&#;). Because the wire is wound to create resistance, they have their own inductance, so they have the worst high frequency properties among all resistor types. They are also more expensive than other common types of resistors.

Applications: They are commonly used in circuit breakers and as fuses because of their high power capabilities.

Part Selection:
Ohmite&#;s HS-25 series for 0.1&#; to 30k&#; rated at 25W Vishay&#;s RH025 series for 0.05&#; to 95.2k&#; rated at 25W

IV. Current Sense Resistors

 

Current sense resistors, also referred to as &#;shunt&#; resistors, are used to monitor currents in a circuit by translating them into voltage drops that can be easily measured. They are low-resistance resistors with higher power ratings to handle large currents. One of their applications is current limiting with stepper motor driver chips.

Most current sense resistors have either two or four terminals. In the four terminal version, which is also called a Kelvin configuration, current is applied at two opposite terminals and voltage is sensed across the other two terminals. This configuration reduces the effect of temperature dependence on resistance and greatly improves stability of the sense circuit. It is used for applications that require high accuracy and temperature stability.

Two Terminal:
SMD:
 Ohmite&#;s MCS series for 0.005&#; to 0.05&#; rated at 1W [CPL]
 Vishay&#;s WSLP series for 0.005&#; to 0.05&#; rated at 1W [CPL]
Through-Hole:
Ohmite&#;s 12F series for 0.005&#; to 0.05&#; rated at 2W [CPL]
Vishay&#;s LVR03R series for 0.01&#; to 0.2&#; rated at 3W

Four Terminal (Kelvin Configuration):
SMD: Ohmite&#;s FC4L series for 0.001&#; to 0.05&#; rated at 2W
Through-Hole: Ohmite&#;s 13FP series for 0.005&#; to 0.1&#; rated at 3W

V. Thermistors 

Thermistors are resistors whose resistance changes significantly with a change in temperature.

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Use NTC thermistors when you need a change in resistance over a wide range of temperatures. Their resistance decreases with temperature, and they are good choices for temperature sensors between -55C and 200C.

Use PTC thermistors when you require a sudden change in resistance at a certain temperature. They are popular for over-current protection applications. The hold current refers to the current when the PTC thermistor is definitely &#;short,&#; and the trip current refers to the current when an PTC thermistor is definitely &#;open.&#;

PTC Resistors:
Bourns&#; MF-MSMF series for hold currents from 300mA to 2.6A.
Littelfuse&#;s L series for hold currents from 100mA to 3.5A
NTC Resistors:
Murata&#;s NCP18WF series for 100&#; to 470k&#;
Panasonic&#;s ERT-J1 series for 22&#; to 150k&#;

VI. Potentiometers 

 

Potentiometers provide variable resistance that can be be used in variety of applications such as amplifier gain control, circuit tuning and so on. Trimmer potentiometers (or trimpots) are small potentiometers that can be mounted on a PCB and adjusted using a screwdriver. Trimpots can either be SMD or through-hole and can have either have top or side adjusting orientation. They can also be either single-turn or multi-turn. Single-turn potentiometers are used in applications like amplifiers, which only require a single control. Multi-turn potentiometers are used for more precise control, and they can have up to 25 turns.

SMD:
Bourns&#; TC33X-2 series for 100&#; to 1M&#;  [CPL]
Panasonic&#;s EVM-3YS series for 100&#; to 1M&#;
Single-Turn Potentiometers:
Bourns&#; P series for 10&#; to 1M&#; [CPL]
BI Technologies&#; 25PR series for 100&#; to 100k&#;
Multi-Turn Potentiometers:
Bourns&#; W series for 10&#; to 5M&#; [CPL]
Vishay&#;s T93YA series for 10&#; to 1M&#;

This guide covers some of the most popular types of resistors. In addition to these, there are MELF resistors, metal foil resistors, ceramic resistors, varistors, photoresistors and resistor arrays, which all have unique advantages in either precision, performance, or compactness. However, in most electronic projects you are likely to see one of the six types that we have discussed in this guide. If you have any comments or suggestions on the part selection, drop us a note in our Slack chat room or in comments below. A guide on how to select inductors is coming next. Stay tuned!

When designing printed circuit boards (PCBs), resistors are one of the most commonly used components. Choosing the right resistor for your PCB design is crucial, as it impacts performance, reliability and cost. With many types, sizes and specifications of resistors available, selecting the optimal one can get confusing. This comprehensive guide provides circuit designers a systematic approach to pick the right resistors for their PCBs.

Types of PCB Resistors

There are several types of resistors used on PCBs, each better suited for certain applications:

Carbon Film Resistors

• Made of carbon film deposited on ceramic rod or cylindrical body
• Low cost and widely used general purpose resistors
• Resistance range: 1 ohm to 22 megohms
• Tolerance: +/-5% to +/- 0.5%
• Temperature coefficient: 250- ppm/°C
• Power rating: 1/8 watt to 2 watts
• Pros: Inexpensive, readily available
• Cons: Lower precision, more noise

Metal Film Resistors

• Made of thin metal film over ceramic body
• Improved properties over carbon film
• Resistance range: 1 ohm to 1 megohm
• Tolerance: +/-1% to +/-0.01%
• Temperature coefficient: +/-15 to +/-25 ppm/°C
• Power rating: 1/8 watt to 1 watt
• Pros: Better stability and tolerance
• Cons: More expensive than carbon film

Metal Oxide Film Resistors

• Made of metal oxide film on ceramic substrate
• Superior performance over metal film resistors
• Resistance range: 10 ohms to 22 megohms
• Tolerance: +/-0.5% to +/-0.01%
• Temperature coefficient: +/-1 ppm/°C to +/-25 ppm/°C
• Power rating: 1/8 watt to 1 watt
• Pros: High stability, precision
• Cons: More expensive

Wirewound Resistors

• Made of metal wire wound around ceramic core
• Very low resistance values possible
• Resistance range: 0.1 ohm to 10 kohms
• Tolerance: +/-5% to +/-0.02%
• Temperature coefficient: 20-60 ppm/°C
• Power rating: 1 to 10 watts
• Pros: Low resistance values, high power rating
• Cons: Inductance effects at high frequencies

Thick and Thin Film Resistors

• Screen printed resistive paste over substrate
• Often used in hybrid circuits and thermal management
• Resistance range: 1 ohm to 1 Mohms
• Tolerance: +/-1% to +/-25%
• Temperature coefficient: +/-50 to +/- ppm/°C
• Power rating: 0.1 to 1 watt
• Pros: Wide resistance range, low cost
• Cons: Lower precision and stability

PCB Resistor Sizes

Resistors come in a variety of physical sizes. The major size standards are:

• Very small surface mount resistors
• Length: 1 mm
• Width: 0.5 mm
• Used in high density boards

• Smaller SMD resistors
• Length: 1.6 mm
• Width: 0.8 mm
• Good balance of size and power rating

• Most common SMD resistor size
• Length: 2 mm
• Width: 1.25 mm
• Ease of handling and soldering

• Larger SMD resistors
• Length: 3.2 mm
• Width: 1.6 mm
• Higher power handling capacity

Axial Lead Resistors

• Through-hole resistors with leads
• Diameter: 3 to 10 mm
• Used for ease of prototyping and servicing

Chassis Mount Resistors

• Through-hole power resistors
• Used for high power applications
• Mounted on heat sinks to dissipate heat

Key Parameters and Ratings

Beyond physical size, resistors have electrical parameters and power ratings that must be considered for PCB design:

Resistance Value

• Measured in ohms (Ω)
• Wide range available from milliohms to gigaohms

Tolerance

• Deviation from nominal resistance value
• Usually +/-1%, +/-5% or +/-10% tolerance
• Tighter tolerance increases cost

Temperature Coefficient

• Indicates resistance change with temperature
• Expressed in ppm/°C (parts per million per °C change)
• Lower coefficient maintains stability over temperature

Power Rating

• Maximum power a resistor can handle without overheating
• Range from 1/8 watt for SMD up to hundreds of watts
• Higher wattage resistors often need heat sinks

Voltage Rating

• Maximum voltage that can be applied without arc over
• 500V or greater voltage ratings common

How to Select the Right PCB Resistor

R=1KΩ Resistor

Here are some tips for choosing the optimal resistor for your PCB design needs:

1. Determine Required Resistance Value

• Select resistor resistance to achieve desired voltage drops and current limits in your circuit.

2. Identify Size Constraints

• Consider board space &#; smaller SMD sizes needed for high density layouts.
• Larger resistors can handle more power dissipation.

3. Choose Suitable Tolerance

• Tighter tolerance increases accuracy and performance.
• Looser tolerance reduces costs.

4. Check Temperature Coefficient

• Lower temperature coefficient enhances stability in temperature changes.
• Important for precision and reliability over a wide temperature range.

5. Verify Adequate Power Rating

• Select resistor power rating sufficient for highest voltage/current expected.
• Use larger resistors or add heat sinking if high power needed.

6. Check Voltage Rating

• Ensure voltage rating exceeds maximum voltage including transients.
• Higher voltage rating provides design headroom.

7. Select Appropriate Type

• Carbon film &#; inexpensive general purpose use
• Metal film/oxide &#; precision and stability
• Wirewound &#; very low resistance values
• Thick/thin film &#; wide resistance range

Real-World Example

Let&#;s go through a practical example of selecting the right resistor for a particular PCB design requirement:

Design Requirements

1. Input voltage = 12V
2. Load current = 500mA
3. Need 5V output to load
4. +/-10% voltage regulation acceptable
5. Board space limited &#; size needed
6. Operating temperature range -20°C to +85°C

Resistor Selection

1. Target resistance value = (12V &#; 5V) / 0.5A = 14 ohms
2. 14 ohm +/-10% tolerance needed &#; use 15 ohm +/-10% resistor
3. chip size chosen for compact size
4. Metal film resistor provides stability over temperature range
5. 1/4 watt power rating suffices given calculated power dissipation
6. Resistor voltage rating > 12V input

Selected Component:

• 15 ohm +/-10% tolerance
• size SMD chip
• 1/4 watt metal film resistor
• 50V voltage rating

This meets all the criteria &#; resistance value, tolerance, power rating, size, and voltage rating. Metal film provides precision and stability over the operating temperature range.

Resistor Marking Codes

Resistors

Resistors use a compact coding system to label resistance and tolerance. Here are some common marking schemes:

3 or 4 Digit Code

• First two digits &#; significant figures of resistance in ohms
• Third digit &#; decimal multiplier (number of zeros to add)
• Fourth digit &#; tolerance (1 = +/-1%, 5= +/-5%)

Examples:

• 472 = 47 x 10^2 ohms = 4.7k ohms +/-5% tolerance
• = 824 x 10^1 ohms = 82k ohms +/-1% tolerance

Letter Code

• Letters indicate significant figure of resistance value
• Following number indicates decimal multiplier

Examples:

• R20 = R x 10^0 ohms = 0.22 ohms
• C220 = C x 10^2 ohms = 22 pF

Color Bands

• Colored stripes indicate resistance similarly to resistor color code
• Additional stripe shows tolerance

To determine resistance from color bands:

1. Read bands from left to right
2. First two bands = first two digits
3. Third band = decimal multiplier
4. Fourth band = tolerance

Small Case Letters

For very low resistance values below 10 ohms, small case letters represent significant figures.

Examples:

• 1r0 = 1 x 10^-1 ohms = 0.1 ohms
• 75m0 = 75 x 10^-3 ohms = 75 milliohms

This covers the most common labeling schemes found on PCB resistors.

Frequently Asked Questions

How to Read Resistor by Color Code

Here are some common FAQs about choosing resistors for printed circuit boards:

What is the difference between SMD and through-hole resistors?

SMD (surface mount device) resistors are small, flat chips that are soldered directly onto the surface of PCBs. Through-hole resistors have axial leads that are inserted into holes on the board. SMDs save space while through-hole makes prototyping and servicing easier.

When should wirewound resistors be used?

Wirewound resistors are best for very low resistance values below 10 ohms where other types are not readily available. The wire winding can create inductance though, so avoid them at high frequencies.

How are resistor power ratings affected by operating temperature?

Power ratings are often derated or reduced at higher ambient temperatures. The hotter the environment, the lower the usable power rating. Resistor datasheets include power derating curves.

Can multiple resistors be used in parallel or series on a PCB?

Yes, parallel and series resistor combinations on a PCB can provide resistance values difficult to obtain with a single component. Paralleling provides lower resistance while series connections increase resistance.

How are precision thin film resistors different from other types?

Thin film resistors typically achieve much tighter tolerances down to +/-0.01%. They use special materials and manufacturing processes to create very uniform and stable resistance values for precision applications.

Conclusion

Selecting the optimal resistor requires considering multiple parameters &#; resistance value, tolerance, size, power rating, and temperature coefficient. Matching these specifications to your PCB&#;s requirements results in a design with the right precision, stability and reliability. Modern resistor materials, tight manufacturing tolerances, and anti-surge designs provide circuit designers an extensive palette for their board. By following the guidelines in this article, you can confidently choose the perfect resistor type for your next PCB design.

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