Suppose you want to operate an LED with a direct current of 12 V. However, the LED indicates that it is only designed for a voltage of 2.1 V. To prevent the LED from flying around your ears immediately after connection, you need to pre-connect a component that achieves an ohmic resistance: A technical resistor. But what exactly is a resistance, and how does it work? The most important information and relevant articles are available in the following:
What is resistance?
What resistor types are available?
What fastening technologies are available?
Example for calculating an electrical resistance
FAQ: The most frequently asked questions about resistance
What is resistance?
A resistor is a two-pole passive component which is very frequently used in electrical engineering. It is used to regulate the connection between the applied voltage and the current in a circuit. The electrical resistance is measured in the unit Ω (Ohm) and is usually abbreviated to the upper case R (resistance) in circuit diagrams.
Switching signal of a resistor
A circuit can also contain several resistors. Then you must distinguish whether the resistors are connected in parallel or in series. Depending on the arrangement, the current and voltage are distributed differently between the individual resistors!
Click on this link to get more information about the switching of resistors
What resistor types are available?
Linear resistors follow Ohm's law, i.e. the applied voltage (U) is directly proportional to the flowing current (I). In contrast, with non-linear resistors (also called parameter-dependant resistors), the resistance value (R) depends on other parameters such as temperature or light incidence.
Example resistance characteristic for linear resistors: U and I are directly proportional to each other
Linear resistors include:
- Film resistors
- Surge Protection
- High Power Wirewound Resistors
- Resistor Networks
- Resistor wire
Example of resistance characteristic for non-linear resistors: U and I are not directly proportional to each other
Non-linear resistors include:
- Thermistor (NTC thermistors)
- PTC thermistors, PTC)
A layer resistor consists of a protective layer , a resistance layer and two supply lines . Depending on the material of the resistance layer, there are carbon film resistors and metal film resistors . Carbon film resistors are particularly suitable for the high frequency range. Metal film resistors can be manufactured with great accuracy and have a low tolerance.
|Metal Film Resistor||Carbon film resistor|
|Technical difference|| |
They tend to have lower tolerance, which makes them more precise.
Not very temperature-dependant, but the resistance increases
Higher tolerance, so they are less precise.
As the heat increases, the resistance decreases.
Is used mainly in the high frequency range.
|Optical difference|| |
Blue or green cover
5 or 6 color rings
4 color rings
Metal film resistor with blue coating and 5 color rings
Carbon film resistor with ochre coating and 4 color rings
This is the meaning of the color code on the resistors
When you decode the color code, you can calculate and determine the resistance value (in ohms) . With the help of up to six rings with different colors, you can read all the relevant resistance data comfortably from a color code table.
The first two or three rings give the resistance value . The fourth color ring tells you by which you still have to multiply the result. For example, if the colors of the first four rings are orange, red, blue and brown, the value is 326×10 Ω = 3.26 kΩ (kilo-ohm). It is also important to note the tolerance indicated in the fifth ring. Depending on the type of resistor, the tolerances range from 0.1 to 10 percent. If you are looking for a component with extremely precise resistance value, you should stick to the colors green, blue and violet. The sixth ring indicates the temperature coefficient.
Metal film resistors with 4 or 5 color rings.
The exact values can be found in the corresponding color code table.
In addition, each resistor is provided with an indication of its maximum load capacity. The power that a technical resistor converts into heat is calculated according to the formula P = U∙I . (For alternating current, the effective values are used.) The current and voltage values applied to the resistor must always be such that the product does not exceed its maximum load capacity. Otherwise the resistance will break.
Resistor color code table for resistors with 5 rings
|Ring 1||Ring 2||Ring 3||Ring 4||Ring 5|
|Brown||1||1||1||x 10||1 %.|
|violet||7||7||7||x 10,000,000||0.1 %.|
|silver||-||-||-||x 0.01||10 %|
Resistor color code table for 4-ring resistors
|Ring 1||Ring 2||Ring 3||Ring 4|
|Brown||1||1||x 10||1 %.|
|violet||7||7||x 10,000,000||0.1 %.|
|silver||-||-||x 0.01||10 %|
Simple resistance determination thanks to a Vitrometer
Determine the resistance with a Vitrometer.
After comparing the color code with the resistance clock, you can easily read out the value of your resistance.
With the Vitrometer from Conrad Components, 5 and 6 color codes can be read out.
The other resistance clock helps to decipher 4-way color codes or 5-way color codes.
You can use both Vitrometers to read the E 6, E 12, E 24, E 48 and E 96 series.
With these components, which essentially consist of a resistor, you can protect electrical devices and cables against dangerous overvoltages . Gas filled surge arresters are called gas arresters.
If the permissible voltage flowing through the surge conductor is exceeded, the gas ionizes and forms a conductive arc. The overvoltage is thus dissipated.
- Response time: Duration from event time to signal output (may be some nanoseconds)
- Nominal leakage current: Currents that flow to the protective conductor
- For protecting electronic components and systems
- For the protection of telecommunication components and systems
- LV HRC fuses for low voltage in power distribution and telecommunications
High Power Wirewound Resistors
Power resistors are resistors that can deliver higher power . In electronics, resistors with more than 0.5 W are referred to as power resistance. The term high-load resistor is also often used. Characteristic for power resistors is that they are often provided with heat sinks or heat dissipators. Aluminum housings or heat-dissipating surfaces also contribute to better heat dissipation. These different forms result in different power resistances.
- Heating resistors
- Discharge resistors
- Protective resistors
- Current or voltage limitation
Axial wired metal film resistor.
Resistor networks and resistance wire
To save components, you can use resistor networks or wires.
Networks contain several resistors at once. This allows you to reduce the number of components used in your electrical device.
You can also buy resistance from the roll: Resistance wires indicate how high their resistance value is per meter of wire. The wires reach a high age without loss of resistance and are very resilient. However, they are only suitable for applications that require a precise resistance value.
Thermistors (temperature-dependant resistors)
Thermistors are defined by the fact that the resistance value is temperature-dependant. Two different temperature characteristics must be distinguished, which behave exactly mirror-image to each other. On the one hand these are the thermistors and on the other hand the PTC thermistors . The temperature dependence of the thermistors is indicated in the data sheets on the basis of formulas, characteristic curves or tables. In addition, you should be aware of the medium (e.g. vacuum, flowing air, liquid, etc.) in which you are buying the thermistor. You want to use the component. The choice of the medium changes the thermal conductivity of the thermistor.
Advantages of the thermistors:
- Very small designs possible
- Large range of nominal values
- Strong temperature dependence of the resistance value
- Temperature Sensors
Thermistor (NTC thermistor)
Thermistors have a negative temperature coefficient (NTC). This means that the resistance decreases as the temperature increases. The temperature can either depend on the ambient temperature or on the self-heating of the appliance.
Thermistors are made of ceramic materials based on metal oxides.
thermistors are excellent for recording the ambient temperature. They indicate the temperature and give an evaluable resistance value.
here, the self-heating of the resistors is used. When current flows through the thermistor, the component heats up after a certain time. As the temperature increases, the resistance value decreases, at a certain value an output pulse is generated, as the current can flow freely.
thermistors are also used as sensors, for example to detect the height of a liquid level. This is possible because they have different resistance values in different media (water, air, etc.).
PTC thermistors have a positive temperature coefficient. This means that the resistance increases as the temperature increases. The temperature can either depend on the ambient temperature or on the self-heating of the component. PTC thermistors are made from ceramic materials based on barium titanat.
Temperature sensors for coarse states
PTC thermistors can be used for temperature measurement. However, they are less accurate than thermistors. The output values are then rather "too warm", "normal" or "too cold".
in order to be used as a temperature monitoring circuit or overtemperature protection circuit, PTC thermistors are installed directly in the current path to be protected. This is often the case for transformers or motors, for example. If the temperature is too high, the PTC thermistors reduce the current flow or even shut down the device completely.
Utilization of self-heating
The self-heating of the PTC thermistors is used, for example, for the overcurrent limitation, the time delay or when switching through current pulses. The PTC thermistor is heated by the current flow and the resistance increases. Current flow is limited.
With a varistor, the resistance value can be set variably with the help of the applied voltage. As the voltage increases, the resistance value decreases . This is why they are also called Voltage Dependant Resistor (VDR). Varistors are usually made of metal oxide (MOV). High-resistance blocking layers form a network of series and parallel connections between the individual zinc oxide grains (crystalline microstructure). If the voltage is exceeded, the blocking layers are removed and the varistor becomes low-resistance . This threshold voltage depends on the sintering time and temperature.
What you must pay attention to when selecting the varistor:
- Maximum permissible operating voltage : highest voltage that may be applied continuously
- Varistor voltage : Electrical voltage applied when 1 mA flows through
- Protection level (response voltage): Voltage drop at currents > 1mA; maximum protection level = highest voltage that may occur
- Maximum leakage current: Current that may flow at most when the maximum permissible operating voltage is applied
What fastening technologies are available?
SMD Metal Film Resistors
All types of resistors are also available as SMD resistors. The difference is in the type of attachment: The resistors shown above are attached to the PCB using THT (through hole technology), while SMD resistors are mounted using surface-mounted technology (SMT). So you don't have any connecting wires. Instead, SMD resistors are miniature resistors that are placed directly on the PCB. There are round (MELF) and rectangular (chip) SMD resistors.
- Due to its smaller dimensions, it is suitable for use in small electrical devices
- Inductances generated in the connecting wires are eliminated
A round SMD resistor is also called MELF
A square-shaped SMD resistor, on the other hand, is called A CHIP.
Resistors that are processed using THT are offered with different connections:
- Screw connection (left image)
- Radial wired (middle image)
- Axial wired (right image)
Depending on the specific circuit, one of the connections may prove to be better or worse suited.
Pipe resistance with screw connection
Radially wired high-load resistance
Axial wired resistor
Example for calculating a resistance
The circuit diagram shows the starting position of the resistance to be calculated.
Using the example of the LED lamp (2.1 V) mentioned at the beginning, you can now practice how a series resistor is calculated in concrete terms. Suppose the LED is to be operated at an operating voltage of 12 V with a current of 20 mA. The circuit diagram opposite shows the starting position.
According to the addition rule for series-connected resistors, the series resistor R must intercept a voltage of UR = 9.9 V at a current of 0.02 A. According to Ohm's law UR = R∙I, this corresponds to a resistance value of R = 495 Ω. The series resistor must have at least this value so that your LED does not smort through.
Resistors are not available to buy with any value, but only in a particular grid called resistor series or E-series. For example, the E3 series contains 24 allowed values between 1 Ω and 47 MΩ. The number behind the E is closely linked to the tolerance range of the resistance. For example, the E96 series is common for resistors with a 1% tolerance.
This link provides detailed information about e-series
Depending on the tolerance you specify for your series resistor, you can now select the next highest resistance in the appropriate E-series. In the E12 series, this would be a resistance value of 560 Ω. The E24 series would have a value of 510 Ω. And in the E96 series, you would get pretty close to the calculated value at Ω 499.
If you are already expecting: What power does the series resistor need to provide to protect your LED effectively? From the formula P = U∙I you get directly the value 9.9 V ∙ 0.02 A = 0.198 W. So if you select a resistor with a maximum load capacity of 0.25 W, you are on the safe side.
Power = Voltage * Current
FAQ: The most frequently asked questions about resistance
What data should you pay attention to when selecting the resistance?
- Rated Current - A current consumed when the resistor is supplied with rated voltage and the rated power is output
- Saturation Current (saturation current): Increasing the electrical voltage does not increase the current
- Power rating (rated power): Maximum continuous power at which the resistor can be operated without damage
- Rated Resistance: Is the resistance value of the component
- Tolerance: Maximum permissible deviation from the nominal resistance
- Leakage: Maximum current that the component must withstand, otherwise it will be destroyed
What are ground resistances and how do they differ from layer resistances?
Ground resistors are among the oldest types of resistance . In principle, they consist of a piece of poorly conductive material (coal, for example), which is provided with two connections. Due to their simple design, ground resistances have a strong noise. In addition, its effect is highly nonlinear . In contrast, only a thin layer of the poorly conductive material is used in layer resistors. This allows the properties of the resistor to be controlled much more precisely.
CAR mechanics often use the term "mass resistance" as a synonym for the contact resistance of car lamp holders . For example, if a light glows darker, they say that the socket is grounded. This is caused by the fact that the car body is exposed to corrosion, which creates a mass between the lamp holder and the car body.
This has nothing to do with the structure of the resistance, but simply means that the socket has a resistance to the body (which should not actually be the case).
This phenomenon occurs not only in the lights, but also horns become quieter and windscreen wipers slower.
What is a reactive resistor?
In AC technology, resistance is a complex number, which means it can affect both the magnitude and phase of the current or voltage. The complex resistor is also known as impedance. The imaginary part of the impedance is called reactive resistance . For example, a capacitor in an AC circuit has a zero-different reactive resistance, although no current can flow through it in a DC circuit at all. The term reactive resistance is due to the fact that no (thermal) power loss occurs at such resistors.