Blog #20 - Simple Circuits

This shows a simple circuit that everyone has used at one time or another. Basically, it’s a flashlight with the external case and the on/off switch removed. The device consists of a single electrochemical cell and an electric light bulb. This pictorial representation also shows the conductors, which attach to the light bulb and the battery. The conductors provide a current path between the battery and the light bulb.

In this circuit, electrons travel from the negative terminal of the cell through the bulb element and back to the positive terminal of the cell, “leapfrogging” from atom to atom in the metal wire and the bulb filament. That’s how most electricians and engineers look at this situation. But theoretical current or conventional current, tells us that the current actually goes from the positive cell pole to the negative cell pole.

In order to draw a schematic diagram of this flashlight, you need to know three schematic symbols. The electrochemical cell (the battery), the conductors, and the bulb, as shown above.

Once you know the symbols, you can assemble them in a logical manner based on the appearance of the circuit in the pictorial drawing. Start by drawing the cell symbol. You can think of the cell as the heart of the circuit because it supplies all of the power for the device; it “pumps the electrons” through everything! Next comes the symbol for the light bulb, which you can draw at any point near the cell. Using this example, you should try to make the schematic symbols fall in line with the way the pictorial diagram appears. This layout places the light bulb above the cell.

Now that you’ve drawn the two major symbols, you can use the conductor symbols (plain straight and solid black lines) to hook them together. Notice that the pictorial drawing shows two conductors. Therefore, the schematic diagram also has two conductors. This is by no means the only way that you can represent this simple circuit in schematic form.

Any schematic representation will require the use of the same three basic symbols: cell, bulb, and conductors. The only changes that can occur involve the positioning of the component symbols on the page. This shows two different alternatives for portraying the same circuit. All three of these diagrams are electrically equivalent, but they look somewhat different as a result of the relative positions of the components on the page.

Let’s change this circuit a little bit, in order to gain proficiency in reading and writing schematics. This shows the same basic flash-light circuit, but an additional cell and a switch have been added. This configuration is quite common for flashlights sold in North America. By examining this pictorial drawing, you can see that any schematic representation will need symbols for the cells, the conductors, the light bulb, and the switch.

 This shows the symbols that you’ll need to produce an accurate and complete schematic drawing of this circuit. A Battery, a wire, a light and this time a switch. Again, you should draw the symbols in the same basic order as the components are wired in the circuit.

This is the resulting schematic…Note that the two cell symbols are drawn separately, connected in series, with polarity markings provided for each one. In the series connection, the positive terminal of one cell goes to the negative terminal of the other. The same two conductors are used from the cell terminals, but you need a third one to connect the switch to the light bulb, and you might also need a fourth one to connect the two cells together to form a battery (unless the cells rest directly against each other, a common state of affairs inside commercially manufactured flashlights).

Now you know what a common two-cell flashlight looks like when represented with schematic symbology. The next time that you switch one of those things on, you can imagine the switch symbol moving from the off (or open) position to the on (or closed) position.

The above is a pictorial representation of a device called a field-strength meter. Wireless communications engineers sometimes use this type of meter to see whether or not an RF electromagnetic field exists at a given location. You’ll find this little circuit quite handy if you enjoy amateur radio, or if you need to locate the source of something that’s causing RF interference. The circuit consists of an antenna, an RF diode, a micro-ammeter, which is (a sensitive current meter graduated in millionths of an ampere), and a coil.

In order to draw this circuit schematically, you need to know the symbols for an antenna, a coil, a micro-ammeter, and a diode,

Using the same method as before, you can draw the schematic by connecting the symbols in the same geometric sequence as the components they represent appear in the circuit.

A schematic of the field-strength meter shown before pictorially is drawn here involving nothing more than the substitution of the schematic symbols for the pictorial symbols. As before, the parts need not be physically placed in the same positions as the schematic diagram suggests, but they must be interconnected precisely as indicated in the schematic. When you build a circuit from a schematic diagram that you trust, you should double-check and triple-check your actual component interconnections to make sure that they agree with the schematic. If you try to build the circuit shown and make a mistake in the wiring connections, you cannot expect it to work. In more sophisticated devices and systems, wiring mistakes can cause component damage, and once in a while, give rise to dangerous situations!

In this circuit, an electromagnetic field induces a Radiofrequency current in the antenna and coil. That current is high-frequency AC. The diode rectifies the AC wave by “chopping off” either the positive half or the negative half of every cycle (depending on the diode’s polarity) to produce pulsating DC like the output of a simple rectifier. The micro-ammeter registers this current. As the strength of the electromagnetic field increases, the current increases, and the meter reading goes up.

Now let’s look at something that’s a little more complicated. This is a schematic diagram of a power supply that produces pure, battery-like DC from utility AC. As you read this diagram from left to right, you’ll see that a power plug goes to the transformer's primary winding through a fuse. At the top of the transformer's secondary winding, a rectifier diode is connected in series.  Following the diode, an electrolytic capacitor, (note the polarity sign) is connected between the output of the rectifier and the bottom of the transformer secondary. A fixed resistor is connected in parallel with the capacitor. 

The DC output appears at the extreme right. The physical size and weight of a real-world power supply, which you can build on the basis of this schematic, will depend on the voltage and current that you need to get from it. 

Because DC power supplies have polarized outputs, positive and negative signs indicate the output voltage polarity. Any power supply that uses a single diode, capacitor, and resistor will have this same basic configuration. Whether the output is 5 Volts at 1 Amp or 5000 Volts at 50 Amps, the schematic drawing will look the same. The schematic says nothing about how many volts or amperes the transformer, diode, capacitor, and resistor are meant to handle. You could add special features, such as a voltage regulator, overcurrent protector, voltmeter, or ammeter to your circuit and insert the symbols at the proper points in the schematic; but all half-wave DC power supplies are built around “cores” whose diagrams look like this.

In this circuit, utility AC appears at the plug on the left. The AC travels through the fuse and flows in the transformer primary. In the secondary, AC also flows, but the voltage across the transformer secondary might be higher or lower than the voltage across the primary (depending on the transformer specifications). The diode allows current to flow only one way; in this case, the current can go only from left to right, (the same direction of the arrow). As a result, pulsating DC comes out of the diode. 

The capacitor gets rid of the pulsations, called ripple, on the DC output from the diode. The resistor discharges or bleeds, the capacitor when you unplug the whole device from the utility outlet.

In this schematic, (which is the same circuit as the previous one shown), however, in this case, each component has an alphabetic numeric designation. Now you can see that this power supply uses a transformer with a primary winding rated at 125 Volts and a secondary winding that yields 12 Volts. The circuit has a diode rated at 50 peak inverse volts, (PIV) and a forward current of 1 Amp; a 100-microfarad, 50 Volts capacitor; and a 10,000 ohm, 1 Watt carbon resistor. The fuse is rated at 0.5 Amps and 125 Volts.

The letters that identify each component are more or less standard. Notice that each letter is followed by the number 1. The designation T1, for instance, indicates that the component is a transformer and that it’s the first such component referenced. If this circuit had two transformers, then one of them would bear the label T1 and the other one would bear the label T2. The numbers reference the position or order on the components list. They serve no other purpose.

The diode carries the reference designator D1, with D serving as the standard abbreviation for most diodes. Standardization is not universal, though! In some instances, the diode might bear the label SR1, where the letters SR stand for silicon rectifier. Some Zener diodes are labeled as ZD1, ZD2, and so on. This labeling makes little difference as long as you write the component designations next to the corresponding symbols. If you replaced the designation D1 with SR1, your readers would still know that the abbreviation went with the symbol for the diode, as long as you made sure to put the abbreviation close enough to the symbol.

In this situation, you don’t have to include a number next to each component designation because only one of each component is used to make up the entire schematic! You could simply write P for the plug, F for the fuse, T for the transformer, D for the diode, C for the capacitor, and R for the resistor; or, if you had confidence that your readers knew all the symbols, you could leave out designators altogether! Nevertheless, the standard diagramming practice requires that you always include a letter and a number, even if only one of a certain component type exists in the whole circuit.

In complicated electronic systems, several hundred components of the same type (resistors, for example) might exist, many of which come from the same family. For instance, if you see the designation R101, then you know that the system contains at least 101 resistors. If you want to know the type and value of resistor R101, you will have to look up R101 in the components list to find its specifications.

You can use this schematic to build a power supply with a peak output of about 18 Volts DC. But before each component was referenced, the schematic had no practical use.

Table 1 shows the standard letter designations for most types of electronic components that you’ll encounter in schematic diagrams. Some of these designations can vary in real-world documentation, depending upon the idiosyncrasies of the person making the drawing or designing the circuit. You should find it easy to memorize the information in Table 1 because most of the designations merely comprise the first letters of the component names. If the component has a complex name, such as a silicon-controlled rectifier, the first letters from each of the three words are used, so you get SCR1. A resistor is designated by R, a capacitor by C, a fuse by F, and so on. Conflicts do arise, of course. If you want to designate a relay, you need to use some letter other than R because R indicates a resistor! The same thing happens if you want to label a crystal; you can’t use C because that letter refers to a capacitor. Look through Table 1 from time to time as you read and draw schematic diagrams, and eventually, you’ll absorb all the information in there.

The circuit above has a full-wave bridge rectifier along with a better ripple filter than the simple capacitor used in the previous power supply. The inductor, L1, is a filter choke, which, along with capacitor C1, does an excellent job of “smoothing” out the DC so it resembles what comes from a 12-V battery, (pure DC with no ripple).

The above schematic shows a voltage-doubler power supply. The two capacitors, C1 and C2, charge up from the full transformer secondary output after the current goes through diodes D1 and D2. Because the two capacitors are connected in series, they act like two batteries in series, giving you twice the voltage. But there’s a catch! A voltage doubler power supply works well only at low current levels. If you try to draw too much current from one of these power supplies, you’ll “draw down” the capacitors and the voltage will decrease.

Previously, and above, the letter designations are the same for each component type, but the numbers advance, one by one, up to the total number of units. So, for example, in the top circuit, you see diodes D1 through D5 because the circuit contains five diodes. (The Zener diode to the right of R1 has the letter D just like the rectifier diodes have, but you can tell it’s a Zener diode because of the bent line in the symbol.) All the other components have only one of each type. In the lower circuit, you see two diodes, two capacitors, and two resistors, so the numbers for D, C, and R, go up to 2. The transformer is all alone, so you see only the number 1 following the letter T.

Even though multiple components might all have the same value (820 ohms, for example, or 50 microfarads), they must nevertheless get separate numerical designations when two or more of them exist in a single circuit.

Schematics don’t reveal every physical detail of a device, the way a photograph or detailed pictorial would do. Schematics depict schemes, that’s all! The schematic diagram for a device allows engineers and technicians to make the correct electrical connections when putting it together, and to locate the various components when testing, adjusting, debugging, or troubleshooting it. If you find all this talk overly philosophical, maybe a real-world example will clear things up. Remember that solid lines in schematic drawings represent conductors. However, a conductor doesn’t have to be a length of wire. It might be part of a component lead, or perhaps a foil run on a printed circuit board (the latter-day equivalent of a connecting wire). Whether or not a separate length of wire is needed to interconnect two components will depend on how close together those components are in the physical layout.

Examine this simple schematic. The circuit contains three resistors, all of which go together in a parallel arrangement. Taking the schematic literally, a conductor connects the left-hand side of R1 to the left-hand side of R2. Another conductor goes between the left-hand side of R2 and the left-hand side of R3. Two other conductors connect the right-hand sides of the components. In practice, the connections might be made with wires attached to the resistor leads, but if the components are close enough together the leads themselves can form the interconnections.

Naturally, if you want to follow good engineering principles, you’ll want to make all of your electronic circuits as compact (and dependable) as possible by using a minimum amount of point-to-point wiring and trying to make the component leads serve for interconnection purposes whenever you can. Of course, in the above example, if the three resistors had to go in different parts of the circuit separated by some physical distance, then you would need to use interconnecting conductors between them. However, as you design the physical layout of a circuit, you should try to minimize the overall length (that is, the total length) of all the interconnecting wires or foil runs combined.

Engineers and technicians use schematic diagrams to create electronic devices, but these diagrams can also prove invaluable for troubleshooting equipment when problems develop. Knowing how to read schematic diagrams, however, is not enough. You also need to know what tasks the various components actually perform, as well as how the diverse circuits work together in a complete system. No matter how proficient you might get at electronics troubleshooting, seemingly simple repair jobs can explode into major headaches without complete, accurate, and clear schematic representations of the hardware.

Remember! Schematic diagrams clarify circuits. They present the circuit elements in a logical and easy-to-understand manner. They tell you very little, if anything, about the component layouts in actual devices.

When you build a circuit from a schematic drawing, the physical object rarely bears much physical resemblance to the schematic. It’s impractical to build a complex electronic circuit by placing the components in the exact same geometrical relationship as they appear in the schematic. The diagrams purposely spread out the components on the page for easy reading. Schematic diagrams are two-dimensional, whereas real-world electronic components are three-dimensional. You need only to look inside of a major electronic device, such as a television set or computer, to realize the complexities that you’d face in troubleshooting a complex system without the help of a schematic diagram.

If you know a fair amount about electronic components and how they operate in various circuits, then you can use a schematic diagram to get a good idea (without any equipment testing) of where a particular problem might occur. Then, by testing various circuit parameters at these critical points and comparing your findings with what the schematic diagram indicates should be present, you can make a quick assessment of the trouble. For example, if a schematic diagram shows a direct connection between two components in a circuit, and a check with an ohmmeter reveals a high resistance between the two, then you can assume that a conductor is broken or a contact has been shaken loose. If a schematic diagram shows only a capacitor between two components (with no other circuit routes around it) and a reading with your ohmmeter shows zero ohms or only a couple of ohms, you can assume that the capacitor has shorted out and you’ll have to replace it.

Beginners to electronics troubleshooting and diagram reading sometimes assume that a professional can instantly isolate a problem to the component level by looking at the schematic. This idealized state of affairs might prevail for a few simple circuits, but in complex designs, the situation grows a lot more involved. Often, the schematic diagram allows a technician to make educated guesses as to where or what the trouble might be, but an exhaustive diagnosis will nearly always require testing. A particular malfunction in an electronic device will not necessarily have a single, easy-to-identify cause. Often there are many possible causes, and the technician must whittle the situ- ation down to a single cause by following a process of elimination.

Suppose that a circuit will not activate, and no voltage can be detected through testing at any contact point indicated by the schematic. Chances are good that no current is passing through the circuit at all. However, you don’t know from this observation exactly what has caused the failure. Has one of the components in the power supply become defective? Has the line cord been accidentally pulled from the wall outlet? Has a conductor broken between the output of the power supply and the input to the electronic device? Has the fuse blown?

In a scenario of this sort, you will almost certainly want to consult the schematic diagram as you go through all of the standard test procedures. You might wish to find the contact point that serves as the power supply output, indicated on the schematic. If you test the volt- age at this point and it appears normal, then you can assume that the problem lies somewhere further along in the circuit. The schematic diagram and the test instrument readings allow you to methodically search out and isolate the problem by starting at a point in the circuit where operation is normal and proceeding forward until you get to the point where the circuit shows some abnormality.

Continuing with the same example, if no output comes from the power supply, you know that you must search backward toward the trouble point. You will continue testing until you reach a point of normal operation and then proceed from there. Using a schematic diagram, you’ll follow your progress and thereby narrow the problem area down to something between two points, (the point farthest back from the output at which the problem exists, and the point furthest forward from the input where things test normal). Chances are good that this narrowing process will isolate the trouble to a single component or circuit connection.

You could follow all of the foregoing steps without a schematic diagram, although it would take you a lot longer to do it, and it would increase the risk of your making a mistake. As you become more experienced in the art of electronics troubleshooting, the information contained in schematic drawings becomes increasingly valuable.

Looking back at the flashlight circuit. Although the schematic diagram does not say so, the two batteries in series should yield a DC potential of 3 Volts because a typical flashlight cell provides 1.5 Volts, and DC voltages add up in series connections. Some schematic diagrams provide voltage test points and maximum or minimum readings that you should expect, but this simple example doesn’t.

Suppose that the flashlight has stopped working, and you decide to test the circuit with a volt-ohm-milliammeter, also called a multi-meter, with the help of the flashlight schematic. First of all, you can measure the individual voltages across the cells. With the meter’s positive probe placed at the positive cell terminal and the negative probe at the negative terminal, you should get a reading of 1.5 volts across each cell. If both read zero, then you know that both cells have lost all their electrical charge. 

If one cell reads normal and the other one reads zero, then in theory you should only have to replace the one that reads zero. (In practice, it’s a good idea to replace entire sets of cells all at once, even if some of them still test okay). If both cells read normal, then you can test the voltage across the bulb. 

Here, you should expect a reading of 3 Volts under normal operation with the switch closed. If you do indeed observe 3 Volts here, then you can diagnose the problem by looking at the schematic. The bulb must have burned out! The schematic shows you that current must go through the light bulb if the bulb can conduct, so it must light up. If voltage is available at the base of the light bulb, then current will flow through the element unless it has opened up. But of course, if the bulb filament has broken apart, no current can flow through the bulb, so it won’t light up. In fact, with a burned-out bulb, no current will flow anywhere at all in the circuit.

On the other hand, let’s say that you get a normal reading at the batteries, but no reading whatsoever at the light bulb. Obviously, a break must exist in the circuit between these two circuit points. Three conductors are involved here:  one between the negative terminal of the battery and one side of the bulb,  another between the positive battery terminal and the switch, and another between the switch and the other side of the bulb. Obviously, one of the conductors has broken (or a contact has been lost where the conductor attaches to the battery), or maybe the switch is defective. While you keep an eye on the schematic, you can test for a defective switch by placing the negative meter probe on the negative battery terminal and the positive probe on the input to the switch. If you see a normal voltage reading, then the switch must be defective. 

If you still get no voltage reading, then one of the conductors has come loose or broken.

Admittedly, the scenario just described presents only a basic example of troubleshooting using a schematic diagram—almost as simple as things can get! But imagine that the flashlight circuit is highly complex, one you know nothing about. Then the schematic diagram becomes an invaluable aid and a necessary adjunct to the standard test procedures with the VOM. This same basic test procedure will be used over and over again when testing highly complex electronic circuits of a similar nature. In most instances, no matter how complicated the circuit design looks, it’s actually a combination of many simple circuits. But if you have to do a comprehensive troubleshooting operation, you might have to test each and every one of those circuits individually.

Here is a diagram for a somewhat more complicated, real-world electronic circuit presented in a form intended to assist a troubleshooting technician. The circuit has a single, NPN, bipolar transistor along with some resistors and capacitors. Note, that test points, (abbreviated TP), exist at three different locations. TP1 at the emitter of the transistor, TP2 at the base of the transistor, and TP3 at the collector of the transistor. If you need to troubleshoot this circuit, (which happens to be a low-power amplifier of the sort you might find in a vintage radio receiver or hi-fi set), you’ll connect your Volt-Ohm-Meter between chassis ground and each one of these three points in turn. You’ll carefully note the meter readings and compare them with known normal values.

This circuit receives a weak input AC signal, (such as the output of an ultrasonic pickup) and boosts it so that it can drive a device that consumes significant power, (such as a switching circuit). The general signal flow goes from left to right. The original AC signal enters at the input terminals, passes through capacitor C2, and reaches the base, (the left-hand electrode) of transistor Q1. The base acts as an adjustable “current valve”, that causes large current fluctuations through Q1 as the electrons flow from ground through R1 to the emitter, (the bottom electrode), then onward to the collector, (the top electrode), and out through R4 to the positive power supply terminal. Capacitors C2 and C3 allow the AC signals to pass while blocking DC from the power supply so that the DC won’t upset the operation of external circuits. Capacitor C1 keeps the transistor’s emitter at a constant DC voltage while allowing the input signal to enter unimpeded. The resistors R2 and R3 have values carefully chosen to place precisely the right DC voltage, called bias, on the base of Q1, ensuring that the transistor will work as well as it possibly can in this application.

In many electronic circuits, actual voltages can deviate from design values by up to 20 percent. If this information is important, you’ll usually find the error range at the bottom of the schematic drawing, or in the accompanying literature. If the readings obtained are within this known error range, (called the component tolerance), then you can tentatively assume that this part of the circuit is working properly. However, if the readings obtained are zero or well outside of the tolerance range, then you have pretty good reason to suspect a problem with the associated circuit portion, or possibly with other circuits that feed it.

Many schematic drawings that accompany electronic equipment, especially “projects” that you build from a kit containing individual components, include information that aids not only in troubleshooting but also in the initial testing and alignment procedures that you must follow as soon as you’ve completed the physical assembly process. As a further aid, the literature might include pictorial diagrams that show you where each part belongs on the circuit board or chassis. That way, you can follow the circuit not only according to its electrical details, but along the physical pathways as they actually look.

According to standard schematic drawing practice, every component should bear a unique alphabetic numeric label to designate it, as you see here. However, a few alternative labeling forms are also acceptable.

 This shows the same circuit as the previous one, but the parts list has been eliminated and the diagram contains no alphabetic numeric designations. Instead, the components are identified only by their schematic symbols along with value designations or industry standard part designations. Using this example shown, we know that the transistor is a 2N2222 type and that the resistors have values of 470 Ω, 33,000 Ω, 330,000 Ω, and 680 ohms. The input capacitor has a value of 0.01 microfarads and the output capacitor has a value of 0.1 microfarads. The emitter capacitor, which goes across the 470-ohm resistor, has a value of 4.7 microfarads.

In situations like this one, you’ll usually see a statement at the bottom of a schematic diagram that includes information about the units for the value designations. Such a statement might read “All capacitors are rated in microfarads. All resistances are given in ohms, where k = 1000 and M = 1,000,000.”

Once in a while, you’ll encounter a “hybrid” drawing that consists of a block diagram and a schematic diagram combined. This works well when you want to highlight and explain a particular circuit, and clarify its relationship to other circuits in a system.

The detailed schematic portion of this circuit shows a buffer amplifier of the sort you’ll find in a radio transmitter. An oscillator, (which precedes the buffer) and an amplifier, (which follows the buffer) are represented as blocks with labels inside.

This diagram serves two purposes. First, as you read the schematic portion, you can study the actual component makeup of the buffer circuit. Second, you get a good idea as to the buffer’s place in the overall system relative to the other circuits. The block schematic representation here clearly shows you that the buffer receives its input from a crystal-controlled oscillator, and also that the buffer sends its output to an amplifier. Another schematic diagram and block diagram combination might describe another portion of this same device, which you might recognize as a simple radio transmitter.

Here is another schematic block “hybrid” diagram in which the oscillator is portrayed in schematic detail, but the buffer and the amplifier are represented only as blocks. This figure tells you that the oscillator output goes to the buffer, which then sends the signal along to the amplifier. The only new information obtained here is contained in the schematic representation of the oscillator.

If you look at these two  schematic block “hybrids” together, however, you can picture in your mind, a diagram in which all of the system’s detail is included up to the point where the signal enters the amplifier. The oscillator portion of the bottom figure also tells you that the oscillator is designed to generate Morse code signals because it contains a telegraph key!

Now let’s “shift gears” and recall the block diagram of the AC-to-DC converter that you saw before. This shows the schematic representation of that device, As you compare the two diagrams, note that in this schematic (A), all of the actual components are shown, rather than merely portraying the stages as labeled blocks (at B). 

This also shows a more comprehensive diagram that shows how the block diagram of this same device relates to the schematic diagram while revealing all of the details originally present in both diagrams.

To briefly explain the AC-to-DC converter circuit, more commonly called a power supply, the input electricity enters at the extreme left-hand end of all versions of the diagram. The AC goes to the transformer to set up the proper ratio for conversion to DC. The four diodes in the diamond configuration constitute the rectifier, which converts the AC to pulsating DC, making use of both halves of the cycle. The final stage, the ripple filter, acts to smooth out the pulsations in the DC after the conversion. The “smoothed-out” or “pure” DC then goes along to the output terminals at the right-hand end, where it appears as a voltage just like the electricity that you’d find at the terminals of a 12-Volt battery.