Blower resistor

 
Blower resistor

What is a blower resistor? Blower resistors are resistors which are used to control the fan speed of automotive blowers. The fan speed can be changed either by switching the blower resistor resistance mechanically, using a rotating lever, or electronically by the air conditioning system. The change in resistance then limits the current through the motor, which dictates the speed at which the blower fan works. Blower resistors, being a mechanical component, are prone to wear and are the most common point of failure in a car’s heating system. This article will focus on mechanical blower resistors, their construction and troubleshooting. Construction A blower fan is connected to the negative battery terminal (also called ground) on one end and to the positive battery terminal through a blower resistor on the other end. The blower resistor is connected in series with the blower fan. This means that the current running through the blower motor, and thus is speed is controlled by the resistor value. The user chooses a suitable fan speed by using a selector to connect one of the resistors in the blower resistor pack. Blower resistors are made of several resistors with different resistances. There are also two additional circuits used for the off state and the highest fan speed state. In the off state, the blower motor is disconnected from the power supply. In the highest speed state, the blower resistor is bypassed completely and the fan is connected directly to the car’s battery, which allows maximum current through the motor. The lower the resistance of the selected resistor in a pack, the higher the current that flows through the blower fan, and the faster the fan will turn. Troubleshooting The individual resistors inside the pack are usually wire wound and they may fail by burning out from [... read more]

Shunt resistor

 
Shunt resistor

Definition shunt resistor A shunt resistor is used to measure electric current, alternating or direct. This is done by measuring the voltage drop across the resistor. Shunt resistor for current measuring A device to measure electric current is called an ammeter. Most modern ammeters measure the voltage drop over a precision resistor with a known resistance. The current flow is calculated by using Ohm’s law: Most ammeters have an inbuilt resistor to measure the current. However, when the current is too high for the ammeter, a different setup is required. The solution is to place the ammeter in parallel with an accurate shunt resistor.  Another term that is sometimes used for this type of resistor is ammeter shunt. Usually this is a high precision manganin resistor with a low resistance value. The current is divided over the shunt and the ammeter, such that only a small (known) percentage flows through the ammeter.  In this way, large currents can still be measured. By correctly scaling the ammeter, the actual amperage can be directly measured. Using this configuration, in theory the maximum amperage that can be measured is endless. However, the voltage rating of the measurement device must not be exceeded. This means that the maximum current multiplied by the resistance value, cannot be higher than the voltage rating. Also, the resistance value should be as low as possible to limit the interference with the circuit. On the contrary, the resolution gets smaller the smaller the resistance and thus the voltage drop is. Example of calculation As an example a shunt resistor is used with a resistance of 1 mOhm. The resistor is placed in a circuit, and a voltage drop of 30 millivolts is measured across the resistor. This means that the current is equal to the voltage divided over the [... read more]

Heater resistor

 
Heater resistor

What are heater resistors? Heater resistors are used whenever an electronic device needs to generate heat for some reason. They are designed as a special type of power resistor to provide a reliable and controllable source of heat. A heating resistor can produce convective heat, meaning it heats up the surrounding air, or radiant heat, meaning it heats other objects directly through a phenomenon called infrared radiation. Radiant heating requires the heater resistor to be placed within line of sight of the object that is to be heated, while convective heating sometimes utilizes fans to blow air over heater resistors in order to increase the heating effectiveness. Heater resistor definition Heater resistors are a special type of power resistors whose main purpose is to convert electrical energy into heat. Radiant heating Wirewound radiant heater Wire-wound radiant heaters are essentially wire-wound power resistors. The heated object receives heat by absorbing infrared rays emitted by the glowing-hot wire. Some of the energy is given off as light in the visible spectrum as well. A reflector is often added behind the resistance heating element in order to direct as much heat as possible in the desired direction. The wire can be exposed or enclosed in a tube to protect it from damage. This is especially useful if there is a risk of water drops falling on the element, which could cause thermal stress damage. Wire-wound radiant heaters are often used in bathrooms or outdoor uses where the intent is to heat a person without having to heat up the surrounding air first. Halogen radiant heater Halogen radiant heaters, often called quartz heaters, are similar in design to halogen light bulbs. They are most often made of a quartz tube with a tungsten resistive filament inside of it. The air from the tube is [... read more]

Pull up resistor / Pull down resistor

 
Pull up resistor / Pull down resistor

What are pull-up resistors? Pull-up resistors are resistors used in logic circuits to ensure a well-defined logical level at a pin under all conditions. As a reminder, digital logic circuits have three logic states: high, low and floating (or high impedance). The high-impedance state occurs when the pin is not pulled to a high or low logic level, but is left “floating” instead. A good illustration of this is an unconnected input pin of a microcontroller. It is neither in a high or low logic state, and a microcontroller might unpredictably interpret the input value as either a logical high or logical low. Pull-up resistors are used to solve the dilemma for the microcontroller by pulling the value to a logical high state, as seen in the figure. If there weren’t for the pull-up resistor, the MCU’s input would be floating when the switch is open and brought down only when the switch is closed. Pull-up resistors are not a special kind of resistors; they are simple fixed-value resistors connected between the voltage supply (usually +5V) and the appropriate pin, which results in defining the input or output voltage in the absence of a driving signal. A typical pull-up resistor value is 4.7kΩ, but can vary depending on the application, as will be discussed later in this article. Pull-up resistor definition Pull-up resistors are resistors which are used to ensure that a wire is pulled to a high logical level in the absence of an input signal. What are pull-down resistors? Pull-down resistors work in the same manner as pull-up resistors, except that they pull the pin to a logical low value. They are connected between ground and the appropriate pin on a device. An example of a pull-down resistor in a digital circuit can be seen in the figure. [... read more]

Resistor for LED

 
Resistor for LED

Resistors in Light Emitting Diode (LED) Circuits An LED (Light Emitting Diode) emits light when an electric current passes through it. The simplest circuit to power an LED is a voltage source with a resistor and an LED in series. Such a resistor is often called a ballast resistor. The ballast resistor is used to limit the current through the LED and to prevent that it burns. If the voltage source is equal to the voltage drop of the LED, no resistor is required.  The resistance of the ballast resistor is easy to calculate with Ohm’s law and Kirchhoff’s circuit laws. The rated LED voltage is subtracted from the voltage source, and then divided by the desired LED operating current: Where V is the voltage source, VLED is the LED voltage and I the LED current. This way you can find the right resistor for LED. LEDs are also available in an integrated package with the correct resistor for LED operation. This simple circuit might be used as a power-on indicator for a DVD player or a computer monitor.  Although this simple circuit is widely used in consumer electronics, it is not very efficient since the surplus of energy of the voltage source is dissipated by the ballast resistor. Therefore, sometimes more complex circuits are applied with better energy efficiency. Example of simple LED circuit In the following example an LED with a voltage of 2 volts and an amperage of 20 mili-amperes must be connected to a 12 volts supply. The ballast resistor can be calculated using the formula: The resistor must have a resistance of 333 ohm. If the precise value is not available, choose the next value that is higher. LED in a series circuit Often multiple LEDs are connected to a single voltage source with a [... read more]

Resistors in series

 
Resistors in series

How to calculate the equivalent resistance value for resistors in series?   In many electrical circuits resistors are connected in series or parallel. A designer might for example combine several resistors with standard values (E-series) to reach a specific resistance value. For series connection, the current through each resistor is equal. There is only one path for the current to follow. The voltage drop however, is proportional to the resistance of each individual resistor. The equivalent resistance of several resistors in series is given by: The voltage across each resistor is calculated with Ohm’s law: Example Consider a circuit as shown in the picture below. Two resistors R1 and R2 connected in series are subject to a constant current I. How can we calculate the voltage drop for each resistor and how can we determine the equivalent resistance value for the two resistors? The current through each resistor is equal. Knowing this, and using Ohm’s law we get the voltage drop for R1 and R2: The equivalent resistance is equal to the sum of R1 and R2: This corresponds with the voltage drops that we calculated: Networks with resistors in parallel and series Take a look at the article resistors in parallel to find practical examples of how to solve a resistor network with resistors that are connected in series and parallel. The following video might help to get a quick understanding in solving resistor networks.

Resistors in parallel

 
Resistors in parallel

How to calculate the equivalent resistance value for resistors in parallel? Resistors are often connected in series or parallel to create more complex networks. An example of 3 resistors in parallel is shown in the picture above. The voltage across resistors in parallel is the same for each resistor. The current however, is in proportion to the resistance of each individual resistor. The equivalent resistance of several resistors in parallel is given by: The current through each resistor is given by: To quickly calculate the equivalent resistance value of two resistors in parallel, you can use the parallel resistor calculator. example A circuit designer needs to install a resistor with 9 ohms and can choose from the E-12 series of preferred values(.., 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, ..).  The value of 9 ohms is unfortunately not available in this series. He decides to connect to standard values in parallel with an equivalent resistance of 9 ohms. The equivalent resistance value for 2 resistors in parallel is calculated with these steps:  The above equation shows that if R1 is equal to R2  Req is half of the value of one of the two resistors. For a Req of 9 ohms, R1 and R2 should therefore have a value of 2×9=18 ohms. This happens to be a standard value from the E-series. As a solution finally, the designer connects two resistors of 18 ohms in parallel as shown in the figure right. How to solve a network with resistors in parallel and series? A more complex resistor network can be solved by systematic grouping of resistors. In the picture below three resistors are connected. Resistors R2 and R3 are connected in series. They are in parallel with resistor R1. To solve the network, the resistors [... read more]

Power resistor

 
Power resistor

What are power resistors? Power resistors are designed to withstand and dissipate large amounts of power. In general they have a power rating of at least 5 Watt. They are made from materials with a high thermal conductivity, allowing efficient cooling. They are often designed to be coupled with heat sinks to be able to dissipate the high amount of power. Some might even need forced air or liquid cooling while under maximum load. Some are wire wound, some are made from wire grids for ease of cooling, but the common thing for all power resistors is that they are built to dissipate the most power while keeping their size as small as possible. An example use for power resistors are load banks used to dissipate power generated during engine braking in vehicles using electrical motors, such as locomotives or trams. Definition A power resistor is a resistor designed and manufactured to dissipate large amounts of power in a compact physical package. Types and construction Wirewound resistors Wire wound resistors are made by winding a metal wire around a solid form, often made of ceramic, fiberglass or plastic. Metal caps are attached to the end of the winding and metallic leads are attached to the ends. The end product is often coated with a non-conductive paint or enamel to offer some protection from the environment. Wire wound resistors can be built to withstand high temperatures, sometimes up to 450 °C. These resistors are often built to tight tolerances thanks to the material used, an alloy of nickel and chrome called Nichrome. The body of the device is then coated with a non-conductive paint, enamel or plastic. Winding types There are several winding methods. Some of them are: helical winding, edge-winding and bifilar winding. The helical type is the ordinary winding [... read more]

Resistor applications

 

Resistor are used in a wide variety of applications. This category contains various pages explaining different resistor applications. Resistors are used in electronic circuit, where they can be used in series or parallel. For information about resistors in series, their behavior, calculations and circuit applications, please visit: Resistors in series For information about resistors connected in parallel, their behavior, calculations and circuit applications, please visit: Resistors in parallel