Electronic Circuit Design

[Bartie Spalitto]

CircuitLabprovides online, in-browser tools for schematic capture and circuit simulation. These tools allow students, hobbyists, and professional engineers to design and analyze analog and digital systems before ever building a prototype. Online schematic capture lets hobbyists easily share and discuss their designs, while online circuit simulation allows for quick design iteration and accelerated learning about electronics.

If you are an engineer looking for free PCB design software, then you have come to the right place. Here we bring to you 9 helpful circuit design simulator tools which are strong in features and soft on the pocket.

This website is the repository for the suite of open-source EDA (Electronic Design Automation) tools including Magic, IRSIM, Netgen, PCB, and XCircuit. These tools are all provided for free under the GNU Public License (GPL) or similar open-source license.

KiCad is an open-source software suite for Electronic Design Automation (EDA). The programs handle Schematic Capture and PCB Layout with Gerber output. The suite runs on Windows, Linux, and macOS and is licensed under GNU GPL v3.

SimulIDE is a simple real-time electronic circuit simulator, intended for hobbyists or students to learn and experiment with simple electronic circuits and microcontrollers, supporting PIC, AVR, and Arduino.

Gerbv is a viewer for Gerber RS-274X files, Excellon drill files, and CSV pick-and-place files. Gerbv is a native Linux application, and it runs on many common UNIX platforms. A Windows version is also available. Gerbv is free/open-source software.

Implementing decoupling schemes, efficient grounding, circuit safety measures, and capturing a schematic diagram are a few of the best electronic design practices. In this article, we will discuss the best circuit design practices that can build a well-planned board.

All this should be done keeping in mind the available budget. The next step is to build block diagrams that provide an overview of the circuit functioning and component groupings. Block diagrams are crucial for successful electronic circuit design and act as a reference material for future circuit analysis.

They also allow for a divide-and-conquer strategy where the diagram is broken down into different functional sections, each performing a specific task that can be implemented and tested in isolation. Knowledge and understanding of the operation of each of these blocks are necessary. Some blocks may have signal conditioning circuits so that the output can be read easily by the data acquisition system. For example, output from an LDR (light-dependent resistor) sensor needs to be converted to a voltage range that can be easily read by the microcontroller ADC. Circuits in these blocks could also be replicated in other designs requiring similar functions, promoting design reuse. To learn more about design reuse, see auto grouping of repetitive PCB design blocks.

Digital ICs work with certain logic levels to determine whether the input is interpreted as 0 or 1. For a VCC of 5V, voltage levels from 5V to 2.8V are considered logic 1, and 0 to 0.8 as logic 0. The voltage range between 0.9 to 2.7V will be an indeterminate region and the chip will interpret it either as a 0 or as a 1 causing the IC to behave incorrectly. This is called the floating state.

To prevent this, pull-up or pull-down resistors are used while designing electronic circuits that use digital ICs, microcontrollers, and switches. Pull-up resistors are used to fix the voltage close to VCC and pull-down resistors to pull the voltage close to GND point. Modern microcontrollers are now equipped with internal pull-up and pull-down resistors that can be activated using a code. Please check the datasheet and decide accordingly if these resistors need to be used or eliminated.

As a circuit board designer, you can make PCBs with multiple configurations and functionalities. Most often these configurations interfere with each other and cannot all coexist in a circuit. Hence, based on requirements, different sections of the PCB can be disabled or enabled easily by unsoldering or adding zero resistors accordingly.

You can divide the circuit into multiple sections by using zero resistors. This is helpful in isolating faulty segments that need troubleshooting. For example, it is easy to find noisy blocks and traces that cause crosstalk without having to cut up the circuit.

SMD zero resistors facilitate continuous connection and are used to replace jumpers. This will allow for an efficient single-layer board design instead of a double-layer. They are much cheaper than DIP switches or berg jumpers. Zero ohm resistors are also used as circuit breakers during supply overload preventing circuit damage.

In RF circuits, zero resistors can be inserted into an RF feed in series with the antenna trace. As the antenna is tuned, it can be replaced with other values. It is also used as a matching element in some RF applications.

Wired resistances i.e. traces connected by zero ohms work well for digital circuits. However, high-speed circuits will not work due to the high value of their parasitic inductance. The maximum permissible current, the enclosure type, the footprint on the PCB, the parasitic inductance, and the capacitance of the resistor are different parameters to be looked at while choosing a zero resistor.

Mixed-signal circuits are hybrid circuits that combine both analog and digital circuitry in a semiconductor die. This is also referred to as system-in-package. Depending on the design requirements, you should make use of modern low-cost microcontrollers as they have several built-in features. The most commonly used microcontroller peripherals are timers, pulse-width modulators (PWM), analog-to-digital converters (ADC), digital-to-analog converters (DAC), and serial/SPI/ I2C communication interfaces.

They can be easily programmed to function according to the requirements. This will avoid overkilling your circuit with too many discrete components to perform the same task. Integrating appropriate digital ICs and analog circuit sections with a microcontroller will increase the performance efficiency of the circuit while reducing its size and cost. A block diagram of a microcontroller is shown below.

Building a circuit that saves energy is important, especially for those which are operated with batteries. PWM is a type of modulation where the pulse width of a signal is modified based on the duty cycle.

A PWM signal with a 75% duty cycle will stay ON for 75% and OFF for 25% of the total time period, thus reducing the mean output power consumption. These signals can be generated using a reliable microcontroller or NE555 IC. It can be employed to save energy in simple LED and motor driving circuits. PWM waveforms are also used in power converter(s) topology to design highly efficient, lighter, and inexpensive switched-mode power supply designs.

Protecting circuit connections from ESD is extremely essential if the product is used in harsh conditions. Metal-oxide varistors, TVS (transient voltage suppressor) diode arrays, clamping diodes, and GDTs (gas discharge tubes) can be used to safeguard from ESD. Between the diodes and ICs, a resistance of a few tens of ohms can be placed to dissipate voltage surge. This should be especially implemented at communication interface connections, user button contacts, and other sensitive input/output connections.

For overvoltage protection, voltage limiters can be used in conjunction with electronic fuse chips and thermistors. Modern ICs now come with built-in protection circuits. Examples of voltage limiters are varistors, TVS diodes, and diode circuits (clipping and clamping). Fusechips can be either hotswap circuit ICs or power controllers.

Protection diodes should also be used when relays are being driven by semiconductors. If the power to the relay coil is shut off suddenly, the coil will generate a voltage spike that can damage the rest of the circuit. A flyback diode acts as a snubbing circuit, providing a discharge path for the coil in order to protect your circuit. Using microcontrollers for a protective shutdown circuitry should be avoided as they might freeze.

High-voltage capacitors used in the circuit must be provided with proper discharge paths. If devices in a circuit are deriving power from a network, a good understanding of class X/class Y safety capacitors and their proper placement is essential. These capacitors are used to filter AC signals and reduce EMI as shown in the figure below. They also mitigate the adverse effects of voltage transients and overvoltage surges.

Class X capacitors are typically placed between the line and neutral of the AC supply to reduce EMI/RFI due to differential mode noise. Class Y capacitors are ideally placed in between the AC supply and ground to minimize common mode noise. There are varying standards and classifications for safety capacitors that indicate their threshold capacities. You should choose the right capacitors based on the design needs.

Galvanic isolation is also used when two sections of a circuit have different ground potentials and a signal needs to be transferred between them. It prevents the flow of unwanted AC and DC currents between the two parts. Isolation breaks these ground loops that cause signal inaccuracies between the two systems, maintaining communication integrity. A popular option is to use transformers for stepping up/down voltages along with using electric fuses. The exchange of information between isolated sections is provided by using optoisolators, hall sensors, capacitors, or relays.

The next important part of the electronic circuit design process is to draw the schematic diagram using EDA tools like Altium, KiCAD, and Cadence Allegro. These tools provide several features like schematic capture, creating electronic components, simulating complex circuit functions, and BOM creation. They also include PCB layout packages, component footprints, 3D views, and output of manufacturing files. Choose an EDA tool that is comfortable and which provides adequate design support.

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