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 (compared in detail elsewhere on this blog). In this post, we only discuss thick film power resistors.
When choosing a thick film power resistor for any application it is important to consider more than the resistance value, power rating and tolerance. There any many factors that can affect the performance of the resistor, and the system as a whole.
In most applications, the objective will be to minimise the power resistor dimensions but thermal and mechanical issues can impact on the performance of the power resistor in the final application. Failure to consider these issues during the power resistor design phase may not only impact on the performance of the resistor it could also dramatically shorten the effective in- service life of the device.
Temperature must be managed appropriately to ensure the resistor performs to specification. A heatsink can be used to dissipate heat away from the resistor but it is also important to manage the system environment.
The resistor must be able to withstand any mechanical issues. Vibration or mechanical shock, thermal factors, moisture, altitude and quality specifications can all influence the resistor design process for a given application.
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. As the resistor is installed in series with the input line a large value resistor with high power handling capability is 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.
The 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, be low inductance and (ideally) low cost.
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 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 time period while minimising losses in the resistor.
The resistor must,therefore, be capable of dissipating significant amounts of energy in a relatively short 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.
A wide range of resistors are available as standard products from the major resistor manufacturers. However, when performance is critical or no standard device is fit for purpose it can be best to consult a specialist supplier expert in producing application specific thick film power resistor devices.