Characteristics of Wirewound Resistors?
A wirewound resistor is an electrical passive device that limits or restricts current within a circuit. The resistive element is a wire made of varying alloys that is insulated by a non-conductive material like a vitreous enamel that can block or deflect exterior heat. The wire is typically made of nickel-chromium or a copper-nickel-manganese alloy called Manganin, both of which have a high resistivity. Other common materials used include ceramic, plastic, and glass.
Wirewound resistors are one of the oldest types of power resistors that are still manufactured today for a number of their advantageous characteristics. Some of these advantages include their low resistance values, high power ratings, superior surge handling capability, incredible precision, and strictly regulated temperature coefficients.
In fact, this kind of power resistor is used in a variety of applications for these reasons, including motor controls, assembly equipment, and robotics. Other markets that wirewound resistors serve include telecommunications, gas monitoring systems, industrial control systems, and telephone switching systems. Moreover, with the creation of new solid-state power devices with high voltage ratings, the demand for components with high power, high voltage, and high current ratings has led the need for wirewound resistors to resurface.
Wirewound construction processes vary widely. The choice of materials and manufacturing method depends on the way the resistor will be implemented in a circuit. While all are made with a wire that wraps around a core, the resistance value relies on the resistivity of the wire, the cross section, and the length. These characteristics are what enable the wirewound resistor to obtain a high precision resistance.
Moreover, to achieve a high resistance, the diameter of the wire must be very small, but very long in length. If the length of the wire is long, then the resistance will be high due to the electrons traveling the lengthy distance of the wire. As a result, as the current travels through the wire, electrons and atoms may collide, resulting in a loss of energy in the form of heat. To that end, only a small amount of energy will flow through the resistor. To achieve lower power ratings, the wire must be very thin, which can make handling even more tedious. In contrast, for applications that require resistors to have a high power rating of 50 W or more, their construction parameters are different. As such, the wire diameter is larger and, as a result, more robust.
Additionally, wirewound resistors are made of alloys due to their high temperature coefficient of resistance (TCR). In the case that your resistor is exposed to high temperatures, a pure metal such as tungsten is often used. The TCR is measured in units of ppm/°C and is used to measure the extent to which resistance will change as the temperature changes. Keep in mind that for applications requiring a high precision measurement, the difference between the material of the wire and the connection can result in a negative effect. At the intersection of these materials, a change in temperature can generate a small unwanted electric current across the resistor, known as a thermal-electric effect.
Another factor to consider when looking at wirewound resistors as your electrical component of choice is the parasitic capacitance and inductance they carry. Capacitance is defined as the ability of a resistor to store electrical energy in the form of an electrical charge. In some cases, the capacitance can have an undesirable effect, for example, in high-frequency alternating current applications found in audio and radio signals. However, parasitic capacitance does not have a significant effect on direct currents such as cellphones, as well as hybrid and electric vehicles.
Since wirewound resistors serve as inductors, they have the worst high frequency properties among other resistor types. The parasitic effects can be mitigated by the way the winding is applied to the resistor. Some special winding types include: Bifilar winding, winding on a flat former, and Ayrtn-Perry winding. Bifilar winding is a winding type where the wire is folded double, which results in a very low self induction, but a large parasitic capacitance. When the wire is wound on a flat former, this can reduce its capacitance. Lastly, Ayrton-Perry windings are more commonly used in demanding circuits wherein two opposite windings are applied closely together so that the winding is free from self induction. Additionally, the two winding being so close can also minimize the capacitance.
There are two types of wirewound resistors, precision and power, which can be used in a variety of applications.Precision wirewound resistors are oftentimes used in precision audio frequency (AF) attenuators, measurement bridges, and calibration equipment.In the case of power wirewound resistors, this type exists for very high power applications and can serve as a fuse and circuit breaker.
If you find yourself in need of power or precision wirewound resistors, parts, or components for your applications, Accelerating RFQs offers a simple way for buyers, mechanics, technicians, engineers, and logisticians in the aerospace and aviation industry to procure the parts they need when they need them. As an AS9120B, ISO 9001:2015, and FAA AC 00-56B accredited member of the Aviation Suppliers Association, our dedication to quality is clear. With a wide network of supply chain locations across the US, Canada, and the UK, our team can supply and deliver aircraft parts in a schedule that suits you! For more information, or a quote, feel free to contact us!
