The following explores wirewound vs metal film resistor. For those who are uninitiated, it may be surprising to discover that metal-based resistors tend to perform better than carbon-composite and carbon-film resistors.
This confusion typically occurs because many think of metal as a conductor. So how exactly do metal-based wirewound and metal film resistors work, and how do they compare?
The following guide will answer these questions by discussing what these resistor types are, how they fundamentally function and how they differ.
By the end of the guide, you should be able to decide which resistor type will suit your PCB assembly.
What is a Wirewound Resistor?
Wirewound resistors are the oldest types of resistors still in production and used today, and for good reason. They’re incredibly stable and deliver reliably low resistance values.
These resistor types get their name from the metallic wire wrapped around their non-conductive cores. The wire consists of a metal alloy with high resistivity, like manganin(copper-nickel-manganese) and nichrome (nickel-chromium). Other wire materials include:
- Copper
- Silver,
- Iron Chromium
- Iron Chromium Aluminum,
- Pure Metals (Tungsten).
The wire-wound resistor’s core can be a ceramic spool or metal cylinder. It may also comprise other non- or low-conductive materials such as glass, steatite (soapstone), plastics, and epoxy. When constructing the wire-wound resistor, the manufacturer first varnishes the core material with a non-conductive insulating coating such as enamel.
The manufacturer then winds the resistance wire tightly around the core. This allows it to dissolve heat from the wire.
You can find wire-wound resistors in four winding types:
- Normal coil winding: Uses a single wire coil typically looped around a spool.
- Bifilar winding: Uses a double-folder wire. While this results in low self-induction, the parasitic capacitance between the wires is usually high.
- Winding on flat former: In this construction, the winder uses a flat former (mica card) instead of a drum, barrel, or spool. The former’s shape brings the front wires closer to the back wires, canceling each other’s fields. The thinner the former, the more effective it is at minimizing self-induction and capacitance.
- Ayrton-Perry Winding: This is a variation of the flat former winding in which wires whose currents flow in the opposite direction are placed closely together. This process creates intersections between the wires, which ultimately minimizes self-induction and capacitance.
The type of winding impacts the amount of parasitic capacitance and inductance of the wirewound resistor.
Not only can you use wire-wound resistors as passive components in your PCB, but you can also modify them and use them as part of or fully as:
- Temperature sensors
- Current Sensors
- Potentiometers
Ultimately, potentiometers are essentially just wire-wound resistors with three terminals.
Old Tapped Wire Wound Resistor Isolated | Shutterstock
Types of Wirewound Resistors
There are two main types of Wirewound resistors:
Precision Wirewound Resistors
As the name implies, precision wire-wound resistors are intended for applications that require accuracy and reliability.
As such, electronic makers generally implement them in sensitive instrumentation, such as calibration equipment, audio frequency attenuators, and measurement bridges.
Precision wirewound resistors are still the most well-known forms. Their cores usually consist of thin mica, brass metals or ceramics. The properties and specifications of the average precision wire-wound resistor are as follows:
- Resistance Range: 1 milliohms to 100 megaohms
- Temperature Coefficient of Resistance (TCR (ppm/°C)): +/- 5
- Power Rating: 10 Milliwatts to 100 Watts
- Stability: Above average
- Tolerance: 0.1%
- Frequency Limit: 50 kHz
Power Wirewound Resistors
Fabricators and manufacturers use power wire-wound resistors for power-intensive applications. These applications include industrial power electronics, computers, medical equipment, etc.
There are different types of power wirewound resistors. You can identify each type by its shell or coating. For instance, power wirewound resistors with a vitreous enamel coating performs well in low temperatures but can break down in high temperatures.
On the other hand, power wire-wound resistors coated with silicone resins perform well in high temperatures (up to 300°C). They also have the lowest power dissipation levels.
Power wire-wound resistors with aluminum casing have the best heat dissipation values. This is largely due to their heat sink-like fins. The properties of wire-wound resistors are as follows:
- Resistance Range: 1 ohm to 22 kiloohms
- Temperature Coefficient of Resistance (TCR (ppm/°C)): +/- 25 to +/- 400
- Voltage Coefficient of Resistance (VCR (ppm/V)):
- Power Rating: 0.5 to 1000 Watts
- Stability: Above Average
- Tolerance: 0.1% to 1%
- Frequency Limit: 50 kHz
3d Render Wire Wound Resistor | Shutterstock
Wirewound Resistor Applications
Electronic makers and device manufacturers commonly implement wirewound resistors in the following applications:
- Power supplies (mainly for medical systems)
- Telephone switching systems
- Battery-testing equipment
- Engine monitoring systems
- Factory machinery and other tools
- Lighting controls
- Gas monitoring systems
- Transducer instrumentation
- X-ray equipment
- Flowmeters
- Personal computers
- Gaming consoles and accessories
- Metering systems
Advantages of Wirewound Resistors
Power wirewound resistors are incredibly stable and reliable. This is largely due to their low-temperature coefficient resistance and ability to function under tremendous heat.
They have been seen to be fully operational in environments with ambient temperatures as much as 170 °C.
In addition to these advantages, high-power and precision wirewound resistors produce low thermal noise and deliver high-blocking accuracy during operations.
Disadvantage of Wire Wound Resistors
Wirewound resistors (especially the precision type) struggle in high-frequency operations because of their construction.
As we mentioned, changing the coil or winding type can address this. However, the type of winding can impact the cost of the resistors.
Wirewound resistors with more elaborate windings tend to cost more. Another disadvantage of wirewound resistors—particularly the high-power versions—is that they can be relatively large and bulky. This makes it harder for electronic makers to incorporate them in compact designs.
Wire Resistance Wound On Porcelain White
What is a Metal Film Resistor?
Where metal foil resistors consist of micron thick bulk metal, metal film resistors consist of a ceramic core surrounded by a thin metal film.
The metal is the resistance material and an alloy such as nickel chromium (NiCr) comprises it. Other less-used materials include:
- Gold (with platinum)
- Tin
- Antimony
- Tantalum nitride
Its thickness determines the stability and resistance. The thicker the metal film, the more stable the resistor is, but the lower its resistance value.
The manufacturer may further calibrate the resistance value by cutting helical shapes into the metal film.
After the manufacturer applies and covers the leads on each side of the resistor, they apply a protective coat and mark it.
Metal film resistors are part of a class of film resistors. Other resistors in this classification include carbon-film and metal-oxide resistors. The properties of metal film resistors are as follows:
- Resistance Range: 1 to 10 MegaOhms
- Temperature Coefficient of Resistance (TCR (ppm/°C)): +/-50 to +/- 100
- Power Rating: 50 Milliwatts to 2 Watts
- Stability: Above average
- Tolerance: 0.05% to 1%
- Frequency Limit: 400 Mhz
Single Metal Film Precision Resistor Component
Metal Film Resistor Applications
The properties of metal film resistors make them most suitable for the following applications:
- Computer circuitry
- Power supplies
- Automotive electronics
- Industrial equipment
- Audio and video equipment
- Military equipment and hardware
- Consumer electronics
- Home appliances
Advantages of Metal Film Resistors
Metal film resistors are comparatively less expensive and more precise than wirewound resistors. Furthermore, metal film resistors are also more compact.
One of the biggest reasons for using metal film resistors is that they deliver the best frequency response among most (if not all) resistors.
Additionally, they have a low voltage coefficient, which makes them stable and low noise-producing.
These properties make them ideal for protecting sensitive electronic components in use cases with high-voltage spikes.
Metal film resistors also have a diverse tolerance range. You can find them in tolerances of 0.1, 0.25, 0.5, 0.1 and 2%.
Disadvantages of Metal Film Resistors
Power wirewound resistors are more suitable for high-power applications than metal film resistors. This is because metal film resistors generally have a lower power rating.
Power wirewound resistors also have better tolerance, stability and TCR than metal film resistors.
Regardless, metal film resistors are still superior to precision wirewound resistors because of their other advantages.
Metal Film Resistor On Cool Blur
Wirewound vs Metal Film Resistor: Closing Remarks
So, wirewound vs. metal film resistor—which is better? Wirewound resistors are more diverse than metal film resistors.
However, this doesn’t mean they’re better. Many believe that metal film resistors are superior replacements for wire wound resistors.
This is primarily because they’re less expensive and more compact.
However, power wirewound resistors deliver the highest accuracy and stability, whereas metal film resistors have the best frequency.
Ultimately, which one you pick should be based on your needs.
In addition, it’s important to pick a PCB manufacturer that can understand and fulfill your component and spacing needs.
