Resistors

Resistors are components that just about every electronic device uses. A resistor is a component that resists the flow of current. They do this by either extending the length of wire that the electricity must flow through or forcing the current to pass through a poor conductor, such as carbon.

It can protect components that have a specific current rating

It can change the function of the circuit

It can create a "dummy load" for a circuit in order for testing purposes

It can create by products such as heat that can be utilized for special purposes

The protection is necessary in order to prevent destruction of certain components that have a maximum current that can be passed through them. This is particularly true in components such as LEDs, which can suffer permanent damage and/or destruction if excessive current is passed through them.

The resistance can change the function of certain circuits, such as oscillators. Certain circuits use the level of current as a control for specific functions. For example, the 555 timer IC outputs pulsed electricity, the frequency of which is determined by the current level sent to one of its leads. This is very important.

The "dummy load" is useful when testing circuits in the lab since the actual load can be impractical for testing (such as a very large antenna). The resistor duplicates the resistance of the real load and makes the circuit act as though it is connected as it normally would.

Resistance creates heat losses in electrical circuits, but that is not always a bad thing. Most electrical heaters utilize this by running electricity through resistors with very low resistance, producing a lot of heat.

On a circuit diagram normally a resistor will have a letter after the value, for values less than 1,000, an ‘R’ is used. So 100 ohms will read 100R. From 1,000 ohms a ‘K’ is used and the number is divided by 1,000. So 1,000 ohms is read as 1K, 22,000 as 22K and 100,000 ohms as 100K. Lastly, from 1,000,000 ohms a ‘M’ is used and the number is divided by 1,000,000. So 1,000,000 ohms is 1M of course. Resistors are too small to have these numbers printed on them, instead they have coloured bands, which is explained further down.

Resistor values with a decimal point in circuit diagrams are expressed in 2 ways, say the circuit requires a 1.2k ohm resistor. The diagram might have it as 1.2K or it might appear as 1K2. The ‘K’ is put in place of the decimal point to prevent the value from being misread as 12K ohms. For resistors below 1k ohm an ‘R’ is used in place of the ‘K’. So 5.6 ohm resistor on the diagram would appear as 5.6R or 5R6.



The above is a picture of a 4-band 1/4w carbon film resistor. These are the most commonly used in electronic circuits due to their low cost and versatility. They come in 1/4w, 1/2w and 1w. You can tell the difference in power handling by the physical size of the package. A 1/4w resistor 7mm long by 2mm diameter, a 1/2w is about 9mm long by 3mm diameter and a 1w is 11mm long and 4mm in diameter. They usually have a tolerance of 5%.



These are 5-band metal film resistors, they have a much smaller tolerance than carbon film resistors, these have a tolerance of 1%. These are used where you need an exact value, such as a high quality audio preamplifier.



This is a ceramic wire wound resistor. They usually come with a power rating of 5w and 10w. These are used where a lot of power is going to be dissipated, such as that of a dummy load. They will be used in a high power audio amplifier.



This is wire wound nichrome wire. The purpose of nichrome wire is to produce heat and hence it’s used in electric heaters and stoves. Nichrome wire normally has a resistance of about 13.8 Ohms per metre. This coil came from a 2400w, 240v fan heater, it had 8 lengths just like the one pictured and they were used in series and parallel combinations to achieve low, medium and high power. The wire actually had a faint red glow on full power. Nichrome wire in heaters should be protected so stray hands don’t touch them, because that stray hand will get burnt.

Because carbon resistors are so small it’s impractical to print the resistance on it so instead they have 4 or 5 coloured bands. The number of bands relates to the tolerance of the resistor, the tolerance is how much variation there is likely to be. At 5% a 100 ohm resistor can be as low as 95 ohms or as high as 105 ohms. 4 bands are used when the tolerance is 5% or 10% and 5 bands are used when the tolerance is 1% or 2%. The 5th band is used to achieve more precision.



The 1st, 2nd (3rd) and multiplier bands are bunched together so you can see where to start from. This is helpful especially with 5-band resistors, which is harder to tell because the tolerance band is brown or red.

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Transistors

Transistors are electronic switching devices, which are the basis of nearly all electronic circuits. This page will give a brief outline of what they are, as well as different methods to interface analog transistors from digital circuitry.

Introduction

The simple explanation of a transistor is that it is a combination of three 'doped' pieces of semi-conductor material. The piece in the middle is called the Base (in Bipolar Junction Transistors), and the outside edges are the Collector, and the Emitter.

When current is put into the Base, it changes the voltage characteristics of the entire transistor, and so it is possible to control the current flowing from the Collector to the Emitter. So a small change of current on the base, results in a large change between the Collector and Emitter.

Bi-Polar Junction Transistors (BJT)
NPN

This is the simplest type of BJT to understand. As you can see in the diagram below, when you apply voltage to the base of the BJT, it turns on the transistor.

A more detailed explanation is that when current is applied onto the base, it changes the voltage difference between the collector and the base. This difference changes the bias within the transistor, causing current to flow from the collector to the emitter.

When there isn't a lot of charge on the base, there are areas within the semiconductor that aren't capable of carrying current from collector to emitter. This means that a lot of power is dissipated to drive the current through. When there is so much charge on the base that no more will fit, the transistor is said to be saturated. There are plenty of carriers for the current, and not much power is dissipated, making the transistor more efficient. This is only true when the transistors Emitter is connected directly to ground (Common Emitter).

This diagram shows how an NPN is turned on. When the base is turned off (connected to ground), there is no way to put current through the transistor, so the transistor is off. When the base voltage is raised, driving charge onto the base, it turns the transistor on.

PNP

The PNP isn't quite as simple. The base still controls the flow of current, but it is more or less opposite. In order to turn the transistor on the base is connected to ground (turned off). To turn the transistor off, voltage is applied to the base.

The reason for this is because of the type of semi-conductor used. When the base is connected to ground, loose electrons are taken away, creating 'holes'. These holes can be thought of as positive charges, and are capable of carrying current from the Emitter to the Collector.

A PNP transistor will saturate only when it is set up as a Common Emitter

The diagram below shows how this works.

This diagram shows how an NPN is turned on. When the base is turned off (connected to ground), there is no way to put current through the transistor, so the transistor is off. When the base voltage is raised, driving charge onto the base, it turns the transistor on.

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Inductor

An inductor is a coil of wire which may have a core of air, iron or other ferrous materials. Its electrical property is called inductance and the unit for this is the henry, symbol H. 1 Henry is very large so mH and µH are often used, 1000µH = 1mH and 1000mH = 1H. Iron and ferrite cores increase the inductance since they can become magnetized. Inductors are mainly used in tuned circuits and to block high frequency AC signals (they are sometimes called chokes).

They pass DC easily, but block AC signals, exactly the opposite of capacitors.

Inductance is a property that is possessed by all coils of wire containing electrical current. The current creates a magnetic field, which can in turn induce current flow if the original current decreases in magnitude or stops. Essentially, an inductor is like a capacitor, only stores energy in a magnetic field instead of an electric field. This makes it very useful for power supply filters that help maintain a fairly noiseless current. A transformer is essentially two inductors, where current flow through one inductor induces current flow in the second as a result of the magnetic field.

Inductors are most often found in audio electronics, power supplies, and radio tuning circuits. An inductor can easily be made by winding insulated wire around a ferrous rod. Thin gauge wire is easiest, since it can bend into smaller loops and is cheaper than large gauge wire.

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Transformator

Transformers are devices that utilize the property of inductance to step-up or step-down voltages. Transformers, however, only work with alternating current, since inductance only occurs when a magnetic field is changing, which is not the case with direct current. However, transformer usage is not limited to voltage modification, it is also capable of matching impedances between different electrical circuits, mating balanced and unbalanced circuits, and isolating dc between circuits, while allowing ac to pass.

The basic construction of a transformer consists of two seperate coils of wire wrapped around a core of iron, air, or any ferromagnetic material. While iron and ferromagnetic cores provide much higher coupling (efficiency of induction transfer), there are significant losses through heat generation in the core. Cores can also be shaped differently, such as the rod and E-core designs. The voltage modification is caused by the difference in number of coils on each wire. The wire you have the supply voltage on is referred to as the primary winding. The wire that is receiving the modified voltage is the secondary winding. The level of modification is determined by the ratio of turns between the primary and secondary windings. Current limiting is also provided by the gauge of the wire used in the windings. A thicker gauge wire allows higher amounts of current to go through, while a smaller gauge allows less. Transformers are the primary reason that power transmission and household outlets utilize alternating current. This is because transformers allow for efficient changes in voltage that allow power to be transferred all over the world at high voltages and low costs.

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Opamps: Operational Amplifiers

Operational amplifiers, often known as opamps, are nifty little integrated circuits that contain several transistors. These transistors are used to amplify or attenuate a signal, depending on the circuit that the opamp is placed in.

Why use an opamp when you can use a transistor? Because opamp packages are easier to control, generally more powerful, and also more versatile. An equivalent amplifier circuit might take numerous components, but the opamp places them all into a compact package, reducing the amplifier’s circuit board footprint and the number of components that could fail.

The classic opamp, and perhaps the most widely used, is the UA741. This device comes in several packages, including a compact 8 pin DIP as well as a 10, 14, and 20 pin version for other purposes. All of these are quite cheap, with the 8 pin version costing only $0.30 or so at most online retailers. You would pay considerably more to construct an amplifier circuit using pure transistors.

Opamps are used very frequently for signal conditioning circuits. Let’s say you have a sensor, perhaps a heartbeat sensor, and it is picking up a signal of several millivolts. You want the signal to be windowed to 0-5V so you can hook it up to an analog-to-digital converter. Sounds hard right? Not so!

The first step in designing your circuit is to determine the gain you will need. Let’s assume that the signal from your sensor has a peak of 5 millivolts. In that case, you can calculate the gain as:



To find the gain you need, you divide the desired output by the input, in this case 5 and 5E-3 (.005) volts, respectively. Here, we need a gain of one thousand. Can the opamp handle that magnitude? Yes!

Now we need to construct our circuit. The opamp is represented as a triangle:



The leads on the left are inputs. One of these has to be grounded and which one depends on whether your signal is inverted or not. To avoid inverting (flipping) your signal, you need to ground IN- and connect your signal to IN+.

The other goofy thing about the opamp is that it needs two VCC signals, unlike most other integrated circuits. They should be identical in magnitude, but opposite in polarity. For example, you might connect positive 12V to VCC+ and negative 12V to VCC-. Finally, we have Vout, which is the output signal.

Now, to perform the amplification, we need to tell the opamp what gain we want. To do this, we but two resistors in a circuit with it, forming the classic non-inverting amplifier circuit:



Here, we see the two resistors, R1 and R2. The approximate gain of the amplifier in this case would be the ratio of R2 to R1. The two resistors can be any values as long as they maintain that ratio. In our case, we could use a 1000 ohm resistor for R1 and a 1 megaohm resistor for R2.

Now if this circuit was actually constructed, Vin could be connected to the sensor and Vout could be connected to the ADC. By looking at the input and output signals on an oscilloscope, you would be able to see that the signal was clearly amplified. If no amplification is occurring or you only see noise, ensure that your circuit is connected properly, you have +12 and -12 (not ground) volts connected to the opamp’s VCC, and also make sure that everything has a common ground. These are the most common mistakes that I have committed when working with opamps.

Now you should be able to go out and use the opamp effectively. We have looked at the most common circuit configuration, but it is also possible to configure the opamp to do frequency-based filtering and other nifty things.

Source from www.freeinfosociety.com by Jonathan Dunder

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