When reading electrical circuit diagrams, beginners often encounter symbols whose meaning is not obvious without special knowledge. One of the most common elements that raise questions is the standard resistance symbol with an arrow through it. This graphic symbol is not random or erroneous - it carries specific technical information about the properties of the component.
Unlike a conventional resistor with a constant resistance, which is depicted simply as a rectangle (according to GOST) or a broken line (according to Western standards), the presence of an arrow indicates the possibility of changing the parameters. Variable resistor allows you to regulate the current or voltage in the circuit manually or automatically. Understanding this difference is critical to correctly reading documentation and assembling electronic devices.
In this article we will look in detail at how exactly such elements are marked, how they differ from each other and where they are used. You will learn to distinguish between potentiometers, rheostats and trimming resistors simply by looking at their symbols in the drawing.
Basic designation and operating principle
An arrow approaching or passing through the resistor body symbolizes a moving contact known as slider or wiper. By moving this pin along the resistive track, the user changes the resistance between the outer pins and the center tap. This allows you to smoothly adjust the electrical parameters of the circuit without breaking the connection.
Structurally, such elements can be made in the form of rotary knobs, slider mechanisms, or even multi-turn screws for precise adjustment. In the diagram, the arrow always indicates that a given node involves external intervention or automatic adjustment. Nominal resistance is indicated next to the symbol and means the maximum value that can be obtained between the extreme pins.
⚠️ Attention: Never connect a variable resistor motor to a circuit unless you are sure of its current position. A sudden surge in current with minimal resistance can damage sensitive circuit components.
There are two main types of such devices based on the nature of resistance changes: linear and nonlinear. In linear models, the resistance changes in proportion to the angle of rotation or the length of the slider's path. In nonlinear (logarithmic or exponential) the dependence is different, which is often used in audio technology to adjust the volume, since the human ear perceives sound logarithmically.
The arrow in the diagram is a universal sign of a variable parameter, be it manual adjustment by the user or automatic adjustment.
Types of adjustable resistors
Although they may look similar graphically, the functional purpose of each type of "arrow resistor" is different. The main difference lies in the control method and frequency of adjustments. Engineers divide them into three main categories, each of which has its own designation and application in electronic circuits.
The first type is potentiometers. They are designed for frequent changes of parameters by the user during operation of the device. Examples include volume controls in radios, monitor screen brightness, or temperature controls in household appliances. Such elements must have high mechanical strength and wear resistance.
Second type - trim resistors (trimmers). They are designed for infrequent adjustments, usually only during initial equipment setup or repair. They are often hidden inside the device body, and a slot for a screwdriver is put on the axle. On diagrams they can be designated in the same way as potentiometers, but sometimes they have additional differences in the form of a perpendicular line at the end of the arrow.
- 🎛️ Potentiometers: controlled by the user through a knob or slider on the device body.
- 🔧 Trimmers: require a tool (screwdriver) to adjust and are often hidden from view.
- 📉 Rheostats: are used to limit current and often have only two terminals involved.
Third type - rheostats. Although they may be identical in design to potentiometers, they are wired differently in the circuit. A rheostat is connected in series to the load circuit to regulate the current, while a potentiometer works as a voltage divider. This is reflected in the diagram by the way the pins are connected to the circuit.
Differences according to GOST and international standards
When working with documentation, it is important to remember that graphic symbols may vary depending on the standard adopted in the country where the circuit is produced. In Russia and the CIS countries the main document is GOST 2.728-74, which regulates the depiction of elements of electrical circuits. According to this standard, a constant resistor is a rectangle, and a variable resistor is a rectangle with an arrow going to the middle of one of the long sides.
In international practice, especially in schemes from the USA and Europe, the designation in the form of a broken line (zigzag) is more common. The arrow in this case also indicates a moving contact, but it is drawn differently - often passing diagonally through a zigzag or approaching its middle at an angle. This distinction can be confusing to the untrained professional.
| Parameter | GOST (Russia/CIS) | IEEE/ANSI (USA) | IEC (Europe) |
|---|---|---|---|
| Resistor shape | Rectangle | broken line | Rectangle |
| Engine designation | Arrow perpendicular | Arrow at an angle | Arrow perpendicular |
| Color | Black on white | Black on white | Black on white |
| Trimmer | Arrow with crossbar | Zigzag with T-arrow | Rectangle with T-arrow |
In addition, there are specific designations for pre-installed resistors. In GOST, for this purpose, a perpendicular segment is added to the end of the arrow, forming the letter “T”. In Western standards, the arrow can be completely filled in or have a T-shaped tip. Understanding these nuances allows you to accurately read any technical documentation.
What does the double arrow on a resistor mean?
A double arrow or an arrow slanting through the rectangle may represent a varistor (non-linear voltage-dependent resistor) or a thermistor if additional temperature symbols are added. However, the classic “arrow resistor” is precisely the adjustment mechanism.
Special types: thermistors and varistors
Do not confuse adjustable resistors with elements whose resistance changes automatically under the influence of external factors. In the diagrams you can also find resistors with an arrow, but they have their own characteristics. For example, thermistors (thermistors) change their properties depending on the ambient temperature or the flowing current.
In their designation, not just an arrow is added to the standard rectangle, but an arrow with a break or an additional symbol of degrees Celsius. There are two main types: NTC (negative temperature coefficient) and PTC (positive temperature coefficient). When heated, the resistance of the former decreases, and that of the latter increases. This is used in temperature sensors and protection systems.
Another important element is varistor. Its symbol is a rectangle with an arrow across it, often with a "U" or "V" next to it. Varistors change resistance sharply when the voltage changes. They are widely used in surge protectors and power supplies to protect against power surges in the network.
- 🌡️ Thermistors: react to temperature changes and are used in sensors.
- ⚡ Varistors: react to voltage surges and serve as circuit protection.
- 💡 Photoresistors: change resistance from light (indicated by arrows directed towards the resistor).
It is important not to confuse these elements with conventional adjustable resistors when replacing them. If you use a regular variable resistor instead of a thermistor in the thermostat circuit, the device will not respond to temperature changes, which can lead to overheating or equipment failure.
Practical application in circuits
Where exactly will you find the resistor with the arrow? There are many areas of application. In audio engineering, these are tone and balance controls. In power supplies - setting the output voltage. In measuring instruments - calibration of readings. Understanding the operating principle allows you not only to read the diagrams, but also to modify them.
For example, if you want to make a device in which the brightness of an LED can be adjusted, you will need to connect a variable resistor in series with the LED. By turning the knob, you will change the current passing through the crystal, changing the glow. However, for high-power LEDs this method is ineffective due to heat generation, and it is better to use PWM regulation.
⚠️ Attention: When replacing a burnt-out variable resistor, it is important to select not only the correct nominal resistance, but also the appropriate power dissipation. Installing an element of lower power will lead to its rapid overheating and failure.
It is also common to use potentiometers as position sensors. In the joysticks of game controllers or the accelerator pedals in cars with electronic throttle, it is the change in resistance that tells the controller how far the lever is deflected or the pedal is depressed.
When selecting an analogue of a variable resistor, pay attention to the type of characteristic (linear, logarithmic). Replacing linear with logarithmic will make the adjustment uneven and inconvenient.
Typical faults and diagnostics
Variable resistors are elements with mechanical movement, so they wear out more often than other circuit components. The main problem is poor contact between the track and the slider. This manifests itself in the form of a crackling sound in the speakers when the knob is turned, jumps in the readings on the display, or a complete loss of the signal.
Diagnosis begins with an external examination and “listening”. If you hear a characteristic noise when you rotate the knob, it means the contact is broken. For an accurate check, use a multimeter in resistance measurement mode. Having connected the probes to the extreme and middle terminals, slowly rotate the axis. The instrument needle (or numbers on the screen) should change smoothly, without jumps or breaks.
Cleaning the contact group with a special spray for electrical contacts often helps restore operation. However, if the graphite layer of the track is worn out, only replacing the element will help. When soldering a new resistor, be careful not to overheat it, as the plastic casing may warp, again causing poor contact.
☑️ Variable resistor diagnostics
Frequently asked questions (FAQ)
Is it possible to use a variable resistor instead of a fixed one?
Yes, it's possible. To do this, you need to connect the middle pin (engine) to one of the outer pins. This way you get a constant resistor with a fixed resistance equal to the track impedance. However, it is not economically feasible to use an expensive variable resistor as a cheap constant one.
What to do if there is no resistor of the required value?
You can use a variable resistor with a higher value. Set it to the middle position, turn on the circuit and, monitoring the parameters with a multimeter, select the desired value. After this, you can measure the resulting resistance and replace the variable element with a constant element of the nearest standard value.
What is the difference between a potentiometer and a rheostat in the diagram?
Graphically they can be indicated the same way. Wiring difference: The potentiometer uses all three pins (input, output, ground) to divide the voltage. A rheostat uses two terminals (one extreme and one middle) to change the resistance in the circuit. The third terminal of the rheostat is often left free or connected to the middle one for reliability.
Why does the sound change jerkily when I turn the volume knob?
This is a sign of wear on the resistive layer or contamination of the contact. Debris could have gotten inside or the track could have oxidized. Try quickly turning the handle from extreme position to extreme position several times. If this does not help, you will need to clean it with a spray or replace the element.