1. How to Make (Almost) Anything Electrical Unit 3
DC Circuits

2. Direct Current
Direct current (DC) is the unidirectional flow of electric charge, electrons flowing from where they are concentrated (negative) towards areas missing electrons (positive). Direct current is produced by sources such as batteries, power supplies, thermocouples, solar cells, or dynamos. DC may flow in a conductor, such as a wire, but can also flow through semiconductors, insulators, or even through a vacuum as in electron or ion beams. The electric current flows in a constant direction, distinguishing it from alternating current (AC). The simplest of all to understand, kinda like water going thru a hose. First used by Thomas Edison.

3. Circuits – pathways for electricity – systems
A direct current circuit is an electrical circuit that consists of any combination of voltage sources, constant current sources or pathways and resistors or loads. Circuit voltages and currents are independent of time, i.e. a particular voltage or current dies not depend on the past value of any circuit voltage or current. This implies that the system of equations that represent a DC circuit do not involve integrals or derivatives with respect to time.

4. Resistor Color Code

5. The mathematics of DC circuits – Ohm’s Law
Ohm’s law - The law was named after the German physicist Georg Ohm, who, in a treatise published in 1827 described measurements of applied voltage and current thru simple electrical circuits containing various lengths of wire.
The equation = I (current in amps) = V (voltage in volts) / R (resistance in Ohms).
Examples simple circuits in circuits.io

6. The mathematics of DC circuits – Watt’s Law
Watt’s law - named after the Scottish engineer James Watt ( ) for his contribution to the development of the steam engine.
The equation = W (power in watts) = V (voltage in volts) * I (current in amps).
We use this as a measure of power transferred from one entity to another, like from the power plant to your house.
Examples of wattage in circuits.io

7. The mathematics of DC circuits – Kirchoff’s Law
Kirchhoff's circuit laws are two equalities that deal with the current and potential difference (commonly known as voltage) in the limped element model of complex (series and parallel) electrical circuits. They were first described in 1845 by German physicist Gustav Kirchoff.
The first rule deals with current; at any node (junction) in an electrical circuit, the sum of currents flowing into that node is equal to the sum of currents flowing out of that node.
The second rule deals with voltage; the algebraic sum of the products of the resistances of the conductors and the currents in them in a closed loop is equal to the total voltage available in that loop.
Simply, resistances in a series circuit add up, and the voltage drops across the resistances add up to the total voltage.
In a parallel circuit, the voltage drop across resistors is the same, but the current flow thru them is different based on their resistance, current flows add up to the total current flow in the circuit.
Examples of series and parallel circuits in circuits.io

8. What if it doesn’t work when I build it?
Troubleshooting basics – cause, frequently, it won’t work the first time….but don’t panic!!
The steps in troubleshooting are:
Detect
Isolate
Remove / Replace
Verify the fix

9. Troubleshooting Step 1 – Detect
In order to know if something is not working correctly, you first have to know how it is supposed to work.
In electronics we use mathematical modeling and simulation to build circuits and analyze their behavior before building the real circuit.
Detection, therefore, is the identification of a not expected behavior based on what correct behavior is.

10. Troubleshooting Step 2 – Isolation
Now that a problem is detected, we isolate it to a replaceable component or an interface between components.
Look for failure symptoms, and based on the functional block diagram of the system, divide the circuit into increasingly smaller chunks until you isolate the fault.
Like when they plug your car into a computer to diagnose a problem at the dealership.

11. Troubleshooting Step 3 – Remove / Replace Step 4 – Verify the Fix
In this step you remove and replace the faulty component, or fix the interface, or fix the computer code.
Determining what will be a replaceable component is an important economic consideration during design.
Often this decision also considers whether special tools / test equipment will be required, whether the removed component is discarded or repaired.
Step 4 – Verify the Fix
This step tests the system to ensure the fault has been correctly identified, isolated, and repaired.