Basic Electronics – Battery Notes
If you have ever looked at a battery, you will find that there are usually two metal contacts. One is called the positive (+) side, and the other is called the negative side (-). If you connect a wire directly between these two poles, an electrical current will flow through the wire. An electrical current is made of electrons that flow through the wire much in the same way that water would flow through a hose. If I replace the wire with a bunch of electronic components, the current still flows, and it causes something useful to happen. For example, if I connect the two wires of a radio to the battery, I might hear the a baseball game. Most of electronics is the study of how we can connect things to a battery to make that current do something useful. As long as you keep the idea that all electronic circuits affect how the current gets from one side of the battery to the other, you will be able to figure out most electronic circuits.
Whats a circuit? A circuit is the path by which the electricity flows from one end of the battery to the other. A circuit could be as simple as a single length of wire, or as complicated as a mainframe computer. The principles, however, are always as simple as giving that current somewhere to go.
Circuits are usually described by pictures, called a schematic drawing, or schematic for short. It is usually the easiest way to describe the circuit, and to insure that someone else can interpret what the circuit is going to do. The following schematic shows some really common symbols that you might see in your work with robotics.
|Battery||There are many types of batteries, but they typically share the same schematic picture. The drawing has two ends to it. Notice that top has a long thin line. The bottom has a short fat line. The long thin line is the positive (+) terminal for the battery. The short fat line is the negative (-). The long bar is usually drawn at the top of the battery, but be careful because not everyone follows this practice.|
|Resistor||A resistor is a device that doesn’t allow current to flow as well as a normal length of wire. It resists the flow of current, hence the name. Resistence is measured in Ohms. The higher the Ohm value, the more it resists the flow of current. Resistors come in values from .1 ohm to 10 megaohms (ten million ohms). Common values used in robotics are 10k (10,000 ohms), 4.7k (4700 ohms), and 330 ohms. Note the use of K (kilo or 1000) as a short hand. If you see 10k on a schematic, it means 10 * 1000. The other major abbreviation is M (mega or 1,000,000).|
|Switch||A switch is a device that allows you to stop the flow of current entirely. These are usually mechanical devices that seperate two bits of metal. When the metal doesn’t touch, current doesn’t flow. When the metal touches, is called a closed circuit. When the metal doesn’t touch, is called an open circuit. (closed = ON, open = OFF)|
|Diode||A diode is an interesting device. It is a semi-conductor. The Diode is the most basic of semiconductors. Is function is to allow electricity to flow in one direction, but not the other. It is like a one way valve for electricity. By looking at the schematic symbol, you can see that has the shape of an arrow. The electrical current will flow in the same direction as the arrow. Thats how you know which way the diode is oriented in the schematic. There are special terms used for each side of the diode. The positive side of the diode is called the ANODE. The negative side is called the CATHODE. Current always flows from the ANODE to the CATHODE.|
|LED||LED stands for Light Emitting Diode. An LED is a diode with a unique property that when electricity flows through it, it emits light. These are extremely useful, require very little power to operate, and you will find them to be very useful. The ANODE and CATHODE are the same as on a regular diode. The way you know that it is an LED is by the little arrows pointing away from it. That is supposed to represent light energy being transmitted.|
There are hundreds of other parts you might see in electronic schematics, but these 5 are a good start for now. I will show some more parts later. Lets use these parts in the following schematic, and I will describe what we are looking at.
As you can see in the schematic drawing, there are schematic symbols for each part, and each is labeled with a name, and a value where appropriate. For example, the battery has a value of 6 volts, and the resistor has a value of 330 ohms. Even the LED labeled L1 has a value, which is RED.
The lines in the picture above are supposed to represent wires. Therefore, you will connect a wire between the batteries positive terminal and one terminal on the switch. Then connect the other terminal on the switch to one end of the resistor. The other end of the resistor to the anode of the diode, and the cathode of the diode to the negative terminal of the battery.
So, you might ask, what does the above circuit do? Lets do a quick analysis. If it is built and wired correctly, closing the switch SW1 will allow current to start to flow. The current will flow through resistor R1, which will prevent too much current from flowing. More on that later. The current will then flow across the Light Emitting Diode, L1, causing it to emit light. Opening in the switch will do the opposite by causing the current to stop.
Volts, Current, and Resistance
These are a few interesting concepts that are really easy to learn. Electricity flows a lot like water flows. Lets assume there is a tank of water sitting on a table. And, there is a hose connected to the bottom of the tank. This is just like a battery and a wire. The battery is full of electrons, and the wire is a place for the electrons to flow.
The water in the tank will generate some pressure in the hose. How much pressure depends on the size of the tank. Water pressure is measured in pounds per square inch (PSI). Electricity works in a very similar way. A battery generates ‘electrical pressure’, and is measured in a unit called a Volt.
If the end of the hose allows water to flow out of it, then there is flow through the hose. Lets assume it is flowing out onto the ground, which is where the water wants to go because of gravity. This is usually measured in gallons per minute. In electricity, if a wire is connected between the positive and negative terminals of the battery, then electrons start to flow. This is called Current, and is measured in Amperes, or Amps.
With the water, the amount of water that can flow depends on the size of the hose. A very small hose provides a very small path for the water. The smaller the hose, the smaller the water flow. Electricity has a similar thing, called resistance. Almost all electrical conductors have some amount of resistance to them, even plain wire. The lower the resistance, the more electrical current flows.
In moving water around, it turns out that water pressure and pipe diameter determine how much water can flow through the pipe. If you raise the water pressure in a fixed size pipe, then more water flows through it (at a faster rate!). You could also leave the water pressure alone and put in a bigger pipe. This would have the same affect.
In electricity, a similar relationship exists. The amount of current depends on the resistance and the voltage (i.e. pipe size and pressure). There is a little formula for calculating this. Its called Ohms law, and it says that Voltage = Current * Resistance. Using algebra, you can determine any one of these values by solving for it. So, Current = Voltage / Resistance, Resistance = Voltage / Current.
The important point is that these three values are related. Changing one of the values is going to affect the other two.
Now that I have told you this, let me explain why. It turns out that normal wire has very low resistance. Often, its in the neighborhood of .01 ohms, or even less. If you use a 6 volt battery, and connect a plain wire across its terminals, then the amount of theoretical current flow is Current = Voltage / Resistance, or Current = 6 / .01 which equals 600 amps. Now, it turns out that most batteries will only hold around 2 or 3 amps. This means the battery will drain almost immediately, which is not good. To use the water analogy, a very low resistance wire is like a really big pipe. In fact, in this case, the pipe is almost the same diameter as the tank. If you allow the water to flow, it will empty the tank all at once.
There is some additional danger involve here. It turns out that when electrons flow through a wire, they encounter some friction as they pass through the wire. Normally, this isn’t a problem. However, if too many electrons flow through a wire, then the wire will become very very hot, and could melt. You have seen this if you have ever looked at a normal light bulb. The way an light bulb works is by allowing a lot of current to flow through a very small wire. The wire turns white hot and would like to burn away. However, since light bulbs have no air in them, the wire cannot burn, so it is stuck being very very hot.
So, as a rule, you should never allow electricity to flow from one end of the battery to the other without using some sort of resistance. If this occurs, its called a short circuit, and will usually start to burn or at least smoke badly. Perhaps you have heard the term electrical short or shorted out when referring to a badly damaged electronic gadget. It isn’t a good situation.
Now, armed with that information, lets look at our first circuit again, this time looking at resistance.
When the switch is closed, current starts to flow. The switch itself has very little resistance, so we won’t count it. The current then flows through the resistor. The resistor will limit the amount of current that flows in this circuit. It turns out that the amount of current that flows through this circuit is determined by the total resistance of the circuit. In this example, only one device has resistance, therefore the total resistance is 330 ohms. The current then flows through the LED, which doesn’t have resistance, and then to the negative terminal of the battery. Using Ohms law, I can tell you that the amount of current flowing is
Current = 6 volts / 330 ohms
Current = .018 amps
This means the amount of current flowing through the circuit is .018 amps, or 18 MilliAmperes, which is abbreviated 18mA (milli, or lowercase ‘m’ means divided by 1000, and is commonly used to talk about amperage. 1 Amp, or 1000 mA, turns out to be a rather large amount of power.)
This resistance is extremely important in our circuit. First, it insures that we haven’t created an electrical short. Second, it turns out that the LED has some really small wires inside it, and cannot be allowed to carry very much current. In fact, most LED’s will burn up if you allow more than 30mA to pass through them. In this case, our resistor is filling the role of a ‘current limiting resistor’, which you will see alot in robotics and digital electronics. In most circuits that have a resistor connected in series with another part, the resistor is there to limit the amount of current that flows.
For series resistors (thats a resistor connected to another resistor, such as shown below), you add the two values to determine the total resistance.
For example, in circuit #2, I added an additional resistor. This means the resistance is added, so we now have 660 ohms resistance.
Current = 6 volts / 660 ohms
Current = .009 amps
Now the circuit only has 9 mA of current. This means that the LED will not shine as brightly. However, the circuit also doesn’t drain the battery as fast. In theory, the battery should now last twice as long.
- Voltage Current and Resistance are all inter-related in an electrical circuit. If you know two of them, you can solve for the third.
- Resistors are important components that limit the current flow
- Never short out a battery or a circuit. There MUST always be a resistance in a circuit
- LED’s require a current limiting resistor.
- Switches do not have resistance.
- Resistance in series can be added together.