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: 250-1000 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
- 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 +/-2500 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:
- 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:
- Measured in ohms (Ω)
- Wide range available from milliohms to gigaohms
- Deviation from nominal resistance value
- Usually +/-1%, +/-5% or +/-10% tolerance
- Tighter tolerance increases cost
- Indicates resistance change with temperature
- Expressed in ppm/°C (parts per million per °C change)
- Lower coefficient maintains stability over temperature
- 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
- Maximum voltage that can be applied without arc over
- 500V or greater voltage ratings common
How to Select the Right PCB 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
Let’s go through a practical example of selecting the right resistor for a particular PCB design requirement:
- Input voltage = 12V
- Load current = 500mA
- Need 5V output to load
- +/-10% voltage regulation acceptable
- Board space limited – 0603 size needed
- Operating temperature range -20°C to +85°C
- Target resistance value = (12V – 5V) / 0.5A = 14 ohms
- 14 ohm +/-10% tolerance needed – use 15 ohm +/-10% resistor
- 0603 chip size chosen for compact size
- Metal film resistor provides stability over temperature range
- 1/4 watt power rating suffices given calculated power dissipation
- Resistor voltage rating > 12V input
- 15 ohm +/-10% tolerance
- 0603 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 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%)
- 472 = 47 x 10^2 ohms = 4.7k ohms +/-5% tolerance
- 8241 = 824 x 10^1 ohms = 82k ohms +/-1% tolerance
- Letters indicate significant figure of resistance value
- Following number indicates decimal multiplier
- R20 = R x 10^0 ohms = 0.22 ohms
- C220 = C x 10^2 ohms = 22 pF
- Colored stripes indicate resistance similarly to resistor color code
- Additional stripe shows tolerance
To determine resistance from color bands:
- Read bands from left to right
- First two bands = first two digits
- Third band = decimal multiplier
- Fourth band = tolerance
Small Case Letters
For very low resistance values below 10 ohms, small case letters represent significant figures.
- 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
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.
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.