Learn the electrical symbols for basic electronic components, including passive components (resistors, capacitors, inductors, transformers), diodes, and thyristors.
Electrical symbols are a short way to indicate which components are involved in a circuit diagram. They provide a quick guide to a design for visual communication, an essential aspect of engineering. I can’t imagine a design review meeting that doesn’t involve a carefully drawn outline. Despite the proliferation of projectors and digital tablets and other things, I suspect that many engineers have not found a suitable substitute for physical schematic prints that can be closely examined and marked with a pencil.
Even if no one else is going to see your design, a good schematic can help you organize your thoughts, reflect on the functionality of a circuit, and find errors when they are very easy to correct (i.e. prior to The board has been sent to the fabulous house.
There is no doubt that some of this information is a bit elementary. However, if you read both articles to the end, I think you will find several details that will be new information for some readers and good reminders for many others.
Note: This guide will focus on North American symbols. If there is enough interest from the community, we will supplement it with symbols popular in other parts of the world.
Symbols for resistors
The general idea of a resistance symbol presents no difficulties, but putting this theory into practice is surprisingly difficult. How many peaks and valleys should there be? Does the first diagonal line go up or down? Should the number of peaks equal the number of valleys? What is the ideal slope of the diagonal lines? There is currently no agreed answer to all of these questions in the industry.
Of course, all these questions would be definitely solved if everyone just adopted me Symbol of resistance (which is undoubtedly the best in the world):
Electrical symbol for resistors.
Now we will move on to some components that are extensions of the basic resistor. First, there are resistors that have a non-fixed resistance, rheostats, and potentiometers.
If a device is simply a variable resistor, it is called rheostat. This is a two terminal device that allows the user to mechanically adjust the resistance between the terminals.
A three-terminal variable resistor is a potentiometer. The third terminal (called the wiper) allows the device to function as a variable voltage divider, although a potentiometer can be used as a rheostat by connecting the external circuit to the windshield wiper and one of the other two terminals.
Mechanical movement is not the only thing that can change the resistance of a component. A variable resistor that is controlled by light is called photoresistor or a LDR (light dependent resistor). As you might expect, these devices are useful when the behavior of a circuit must be influenced by the intensity of light; Take a look at this article to learn more.
Photoresist symbol, also known as LDR
If the resistance of a variable resistor is governed by temperature, we have a thermistor.
As the temperature increases, the resistance of an NTC thermistor (negative temperature coefficient) decreases, and the resistance of a PTC thermistor (positive temperature coefficient) increases.
NTC thermistor (left) and PTC thermistor (right)
Symbols for capacitors
The symbol for the capacitor, in contrast to the resistor, is very straightforward. The lines in the center of the symbol can be parallel or curved. When using a curved line, it indicates the negative terminal.
Ionized capacitors need a plus sign to indicate which side connects to the highest voltage. Even when using a curved line to show a negative terminal, I also recommend using the plus sign. This is much easier than trying to desolder and desolder a 0402 tantalum cap that the assembly center installed backwards because in a moment of abstraction the polarity convention for the curved line cap symbol was confused.
Electrical symbols for capacitors.
Symbol for inductors
Inductor symbols are even more complicated than resistance symbols. The symbol must somehow evoke a coil of wire. I don’t like the ones that are just a monotonous sequence of semicircles, but the extremely wacky versions seem a bit outlandish.
A happy medium can look like this:
Electrical symbol for inductors
I have the impression that some designers consider a ferrite bead to be more or less the same as an inductor. The two components are certainly similar but have different applications in my opinion and consequently the symbol for a ferrite bead must have something to distinguish it from an inductor. I don’t think there is any official guide here. My suggestion is adding a line or a narrow rectangle:
Electrical symbol for ferrite beads.
Symbols for transformers
A transformer is similar, in terms of physical structure and functionality, to two inductors that are placed in close proximity. This fact is effectively conveyed by the circuit symbol, which looks a lot like two inductors:
Electrical symbol for transformers.
The strong magnetic coupling between these two inductors (called windings when they are part of a transformer) allows efficient transfer of electrical energy from one winding to the other, despite the fact that there is no direct electrical connection. Thus, a transformer provides galvanic isolation for AC systems. It is also a convenient way to increase or decrease the amplitude of an AC voltage. (You can find more information on this concept on the textbook page for mutual inductance.) The vertical lines between the two inductors indicate the presence of a core material; Using a magnetic core results in a magnetic field that is stronger than what would be obtained if the core were simply air.
What are the dots on the transformer symbols?
You may have noticed transformer symbols that include dots. This is an important detail.
From a structural point of view, the dots indicate the relative orientation of the windings. From an electrical point of view, the dots indicate the phase relationship between the input and output signals.
If the windings are wound in the same direction, the input signal is in phase with the output signal. If they wind in opposite directions, there will be a 180 ° phase difference between the inlet and outlet; in other words, the transformer becomes an inverter. This reversal behavior is indicated by dots at opposite ends of the symbol.
Transformer point convention
Central thread transformers
A variation on the basic transformer theme is the center tapped transformer. A center tap is a terminal that originates from the center of a winding. This effectively splits the winding into two windings, each producing half the output voltage.
Electrical symbol for a central thread transformer.
Symbols for diodes
The basic diode symbol is an intuitive representation of the basic functionality of the diode: the triangle is like an arrow pointing in the direction of current flow and the line serves as a barrier to current flow in the opposite direction.
Electrical symbol for a diode.
Diodes come in a variety of flavors, and consequently there are quite a few different symbols.
A Zener diode, which functions as a gross voltage regulator when conducting a reverse current (that is, from cathode to anode), has the following symbol:
Electrical symbol for a Zener diode
Schottky diodes have a lower forward voltage drop and are useful in circuits, such as switching regulators, where a diode must rapidly switch between a conductive state and a non-conductive state. The symbol has a modified line that makes it look like an “S” for “Schottky” (I have no idea if this was the intention).
Electrical symbol for a Schottky diode
Light Emitting Diodes (LEDs) and Photodiodes
A pair of arrows is used to identify diodes that have functions related to light. The arrows point towards an LED, which indicates the generation of light, and point towards a photodiode, which indicates the reception of light.
Electrical symbol for an LED (left) and a photodiode (right)
Symbols for thyristors
Silicon Controlled Rectifiers (SCR)
A Silicon Controlled Rectifier (SCR) is like a diode in that it conducts current only from anode to cathode, but has an additional terminal, called a gate, that can be used to drive the device on. Here is the symbol:
Electrical symbol for a silicone controlled rectifier.
A TRIAC, short for “triode for alternating current”, is a type of A TRIAC that works as two SCRs connected in antiparallel, that is, cathode to anode and anode to cathode. This allows the device to conduct current in both directions (a characteristic that could easily be deduced from the circuit symbol). The gate provides a trigger action, as with an SCR.
Electrical symbol for a TRIAC
TRIACs are useful when you need to precisely control AC current, as in this dimmer project.
We have covered the schematic representations of some of the most common electronic components. In the next article, we will look at transistors and mechanical devices.