How do you start when designing circuits?

Answer 1

The answer to this question must start with another question, "what kind of circuit?" There are residential house wiring circuits, 3-phase power circuits, electronic circuits, analog and digital circuits, etc...

There are many parameters involved in creating a well designed circuit, and would take an entire electrical engineering degree to understand them all. However, let's just go over the basics to give you an idea.

You should begin with the load parameters. The load is whatever is being powered and becomes the focal point of the circuit. Sometimes the load may vary, or a circuit can be designed for a wide range of loads. Nevertheless, you always want to design a circuit so the the load doesn't burn out or get damaged in any other way. Therefore, I like to begin with the maximum power that the load can consume. Power can be calculated as voltage*current, (voltage)^2/resistance, or (current)^2*resistance.

Say for instance, I am building an amplifier circuit to drive a speaker. The maximum power rating on the speaker may be 50 W. Most speakers are 4 or 8 ohms. Therefore, the 50 W maximum power rating sets the limit on the current and voltage used to drive the speaker according to the above relationships for power. Now, you have reduced one parameter, power, into two parameters, voltage and current.

Next, look at the maximum current and voltage rating of the load. If the maximum voltage is, say 10 V, then determine the amount of current needed at 10 V to produce 50 W. If this is above the maximum current rating, then something is wrong. It should be well below the maximum current allowable. So the next step is reduce the voltage to, say 5 V, which is only half the maximum. Now recalculate the current needed to reach 50 W and decide if this is above the maximum current rating.

Much of electrical engineering is this trial and error process. As you work your way from the load to the source, make sure that you never exceed any of the maximum ratings, whether it be voltage, current, or power. When you are finished with the design, go back and recalculate everything to ensure that it meets specifications.

For more advanced circuits that may involve frequency responses, parameters such as capacitance and inductance become extremely important.

Finally, designing a residential house wiring circuit has parameters that cannot change, such as the voltage. If you go across one hot and the neutral, the voltage is about 120 V. If you go across both hots, the voltage is about 240 V. This makes things much easier and all that you have to worry about is current and power.

Designing a circuit that functions well is an art, takes lots of practice, and requires many checks. I hope this article can get you started in the right direction!