There are so many applications for power resistors that they cannot be easily tabulated. Discussed here are a few practical, useful applications that you will find interesting. First an image with some typical tubular power resistors.
A few common power resistor applications
The resistance heater is a very common appliance in temperate climates – they range from 1 to 2kW and operate on 115 or 230VAC. Some are radiant (glow red) while others are forced air. While some manufactures boast the highest efficiency, they are actually all 100% efficient in converting electricity to heat. The inefficiency actually occurs during power generation due to the Rankine cycle steam system efficiency.
A current sensing resistor generally has a very low resistance value, is in the order of 0.01 to 0.1Ω that is inserted in series with a circuit in such a way that it has minimal effect upon the circuit. Accuracy of meter shunts is generally ±1%. Sometimes they are simply a length or strip of copper-nickel resistance wire or flat stock – this is done only where accuracy is not an issue. Typical voltage developed across sensing resistors and shunts is in the order of 50 to 200mV or so. Sometimes they have four leads to provide a Kelvin connection.
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Snubber resistors absorb current transients caused by rapid changes in voltage. These are often connected across semiconductors to help protect against destructive voltage transients. To reduce average power dissipation, a capacitor is connected in series with the resistor. A non-inductive resistor element is essential. Typical snubber resistances are in the order of 50 to 200Ω. Power ratings are generally under 10W. In semiconductor applications, a diode is sometimes connected across the resistor – this is called a polarized snubber.
A Quencharc device is an R-C snubber device that is applied across relay contacts to reduce arcing.
A bleeder resistor is required to discharge power supply capacitors when the power is removed so that the equipment may be safely serviced. Minimally, it is sized to discharge the capacitors to below 45V within 45sec. Since they tend to waste power, the object is to make the bleeder resistance as high as possible while at the same time meeting discharge time specifications. In laboratory power supplies, the bleeder resistors generally discharge the capacitors within about 5sec.
In low voltage circuits, a safety bleeder is often used to ultimately discharge the capacitors and/or keep them discharged. The safety bleeder is in the order of 100K to 1M for minimal power dissipation.
Experimenters should have a bleeder resistor in their tool box. While capacitors may be shorted via a screw driver, such discharges can be destructive and sometimes dangerous. Shorting a capacitor with the proper value resistor is a very safe, simple and effective procedure.
Perhaps you have noticed a “crack” or “zap” when you connected power to a capacitor limited power supply. This is caused by the high peak current that initially charges the limiting resistor. A low value of resistance connected in series with the circuit greatly reduces this problem. This resistor must absorb a high peak power or energy safely. A film type resistor is not recommended as they tend to be unreliable for this application. A wirewound, carbon or ceramic composition resistor is recommended.
Ballast resistors provide a means of balancing load currents in multiple branches of semiconductor circuits. This is required because semiconductors have range of forward voltage drops as well as a negative temperature coefficient of voltage. Because of this, parallel connected devices share current poorly and one device tends to “hog” the current. This is also true when paralleling LEDs. The design objective is to keep the voltage drop across such resistors to a minimum in order to maintain greatest efficiency, but high enough in voltage drop to effect reasonable current sharing. A typical ballast resistor voltage drop ranges from about 0.1 to 0.5V or so.
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We have all experimented with circuits where the load is simulated rather than actual. One such application is the 50Ω dummy load for a transmitter. By using a dummy load you are preventing spurious signals from being radiated from an antenna. Such resistors are generally arrays of carbon composition resistors connected in parallel between two plates. This provides a non-inductive load at radio frequencies.
A dynamic braking resistor is connected across a shunt DC motor in order to decelerate it quickly to a stop. The motor, when coasting, acts as a generator so its stored rotational energy may be transferred to a resistor. In this application, the resistor is initially overloaded by a factor of 5 or so due to the temporary nature of its use.
The electronic dissipater is used in AC motor drive speed controls. It is similar to the dynamic braking feature used in DC motor controls. When an AC motor speed control inverter reduces motor speed, the rotational energy is transferred into the DC bus capacitor thus causing its voltage to increase substantially. To prevent excessive bus capacitor voltage, the electronic dissipater temporarily connects a resistor across the capacitor until the voltage drops to a specified level. This application requires both the resistor and an ED controller – it is generally purchased as an AC drive accessory.
For the future
Dynamic Braking in a DC motor