Power resistors are used in a wide range of applications. They are primarily used as protection devices but are also found in power dissipation applications. The most common types of power resistor technology are wirewound and thick film. They are compared in detail elsewhere on this blog.
Thick film power resistors are used in a wide range of applications to prevent damage to sensitive components. They are typically used to prevent damage under transient conditions. These include start up conditions, switching applications, and electromagnetic interference.
Inrush Current Limiting
Electric devices such as motors can draw several times their normal operating current at startup. These short duration high currents can damage sensitive components and switches and/or blow fuses.
In many circuits, thermistors are used to limit inrush currents but resistors may be used in lower power, relatively simple or low cost applications. The resistor is installed in series with the input line. A large value resistor with high power handling capability is therefore required.
Snubber resistors are used to protect sensitive electronic components from voltage spikes generated when a switch is opened or the supply is interrupted. A simple snubber circuit consists of a capacitor and resistor connected in series across the switch.
Matching of the resistor and capacitor is crucial. The resistor dissipates the energy stored in the capacitor. It must be capable of dissipating that energy and be low inductance and ideally low cost. The low inductance requirement tends to rule out wirewound resistors.
High current or voltage pulses can be generated by a number of factors. The pulse can be repetitive or a single transient event.
Pulse resistors are used to absorb and dissipate the energy of the pulse to prevent damage to sensitive components. A thick film power resistor is an ideal choice for pulse applications as they tend to be small in size for a given power rating. The structure of thick film resistors means they are relatively easy to use with heatsink devices. They are also low inductance and relatively low cost.
Bleeder resistors are generally used as safety devices to dissipate the energy stored in capacitors at switch off. They are often connected across the load. The challenge is to dissipate the energy stored in the capacitors within a reasonable period of time while minimising losses in the resistor.
The resistor must therefore be capable of dissipating significant amounts of energy in a relatively short period of time. Choosing an appropriate substrate material and heatsink combination can make thick film technology an ideal choice.
To decelerate an electric motor, kinetic energy is transformed back into electrical energy. The electrical energy is then dissipated as heat. Where large motors are involved wirewound resistors tend to be used. Thick film power resistors tend to be used in more specialist applications.
Thick film power resistors are used in a wide range of applications where component failure cannot be tolerated. The resistor design and manufacturing process is the key to success.