Electronics Blog

The schematic symbol for a capacitor

Q. In the schematic symbol for a Capacitor, one plate is straight and the other is curved. Today, the curved plate in the symbol for a non-polarized Capacitor has no significance. But at one time, it did mean something. Can anyone tell us what it was, and how you could tell which plate it was by looking at the actual capacitor?

A. The curved plate was the outside foil. On the capacitor, it was the end marked with a black band.Refer to figure 1. (attached)

A paper capacitor was made by taking a sheet of (very thin) aluminum foil, a sheet of paper for the dielectric, another sheet of foil, and rolling them up into a tube. A lead was attached to each plate, and the roll was covered with wax-impregnated paper. They were popular in tube equipment high working voltage and low Capacitance.

Today, a replacement would be the Film capacitors. These capacitors, also known as plastic film capacitors, film dielectric capacitors, or polymer film capacitors, are generically called “film caps.” This type, as well as power film capacitors, are electrical capacitors with an insulating plastic film as the dielectric, and sometimes are combined with paper (like mentioned above) as carrier of the electrodes. To illustrate the difference, please see figure 2. (attached)

Capacitor Diagram


Trimming your antenna

I recently replaced the antenna on my car. It’s for an AM/FM broadcast-band receiver. The radio doesn't seem to be receiving as well as it used to. Is something wrong with the antenna?

A receiving antenna has to be “tuned” for the same reasons a transmitting antenna has to be tuned for minimum SWR. Somewhere on the back or side of the radio is an access hole for a small variable “trimmer” capacity. It will probably be near the antenna jack. Using an insulated screwdriver or plastic alignment tool, adjust the trimmer for best reception at the upper and lower ends of the band. There may be separate trimmers for AM and FM.

How Photocopiers and Laser Printers Work

How Photocopiers and Laser Printers work

By Louis E. Frenzel

Everyone talks about how the personal computer has changed the way we do work in the office-place. While computers have indeed affected not only the way we work but our productivity, there is another electronic device that has had an even greater impact. That device is the photocopier. Before the photocopier, we had to use carbon paper to make copies of letters or other typed documents. Today carbon paper is rarely used except in multipart forms.

All typed documents are duplicated by putting them through a photocopy machine. And photocopiers allow the duplication of any printed material, making it possible to copy and distribute vast amounts of printed material with speed and ease.

The photocopier is also a part of the one of the most popular types of computer printers, the laser printer. The laser printer is basically a photocopier with a laser that traces out the material to be copied.


The basic photocopying process was developed by Chester Carlson in the 1930s. The technique is generally known as electro-photography or xerography, which is Greek for “dry writing.” This technique uses static electricity and photoconductive materials to transfer an image from a source to a blank sheet of paper.

Figure 1 shows a diagram of a basic photocopy machine. It consists of a drum made of aluminum whose surface is coated with a material such as selenium. The selenium is very light sensitive. The photocopy process begins by giving the drum a positive electrical charge. This is done by rotating the drum adjacent to a fine wire closely spaced from the drum’s surface. A high voltage of 6000 to 7000 volts is applied to the wire.

Photocopier Figure 1

The wire, Called a corona, ionizes the air around it and thereby produces an electric charge which is transferred to the drum. A positive charge is evenly distributed across the drum surface. As long as the drum is kept in the dark, it acts as an insulator and therefore retains the positive charge.

However, if the drum should be exposed to light, it becomes a good conductor and the electrical charge is transferred to the aluminum base of the drum and conducted away, thereby neutralizing the surface. Once the drum is positively charged by the corona, the page being copied is scanned by a light and an optical system which focuses the reflected light on the drum as it rotates.

This distributes the information to be copied across the drum surface. The electrical charge on the drum is dissipated wherever light strikes it. The dark parts of the image, namely the black type, do not discharge the drum. That portion of the drum retains the positive charge. The charged areas of the drum accurately represent the type, graphics or other information being copied.

Next, a powdered dry ink called toner is applied to the drum. The toner is given a negative charge. Since opposite charges attract, the negative toner is picked up by the positively charged areas on the drum. The image to be copied therefore is now present in toner form on the drum. Plain white paper is pulled into the copier mechanism by rollers and given a positive charge by a corona.

The paper is moved past the drum where the toner is attracted to the paper. The paper then passes through heated rollers that melt the toner ink and fuse the image to the paper. A very high quality copy of the original emerges from the machine. The cycle can repeat. The drum is exposed to light to erase any residual image and is again precharged before being exposed to the next item to be copied.

The basic photocopy mechanism, which consists of the photo-sensitivedrum, corona, toner and, associated rollers and other mechanics is generally referred to as the photocopier “engine.” This engine is the heart of the popular laser printer. The light source and optical mechanism in the photocopier is replaced with a sophisticated laser raster scanning system that is used to expose the drum in the pattern to be printed.

Laser printers

The basic laser printer mechanism is shown in Figure 2. It consists of a semiconductor laser diode which is turned off or on by the electronics to produce a dot matrix image of the material to be printed. The laser beam is shined on a multi-surface or polygonal rotating mirror. The mirror, which rotates at high speed, causes the laser beam to be scanned across the drum as the drum rotates. The rotating mirror and drum move in Synchronism so that a raster scan is produced by the laser beam on the drum. The static charge on the drum is neutralized where the laser light strikes.

A laser printer is really a high density dot matrix printer. Because the laser beam can be focused to an extremely small pinpoint of light, the dot density is extremely high. Most common laser printers have a density of 300 dots per inch. All type fonts or graphics symbols to be printed are created with multiple dots. Because so many dots are used to form a character, it is difficult to distinguish it from a continuously formed letter quality character typical of that printed by a daisywheel printer or typewriter.

Figure 3 shows a block diagram of the electronics in a laser printer. Information to be printed is first sent to the printer by way of a serial or parallel I/O interface. The data is usually text in ASCII code format, this is stored in the printer RAM. A microprocessor controller inside the printer examines the data to be printed and builds a bit map from that information. A bit map is a dot pattern representing a character in RAM.

Essentially what happens is that the ASCII Codes are translated into bit maps of the characters they represent. As a result, the microprocessor builds up the entire page by assembling bit maps for the characters to be printed. Because the dot density is very high, a large RAM is required to store a Single page of information. Most laser printers Contain one to two megabits of RAM.

When the entire full page image has been stored in RAM, the individual bits are scanned out sequentially and fed to the laser beam, turning it off and on. The laser beam scans across the drum, discharging some areas but leaving other areas charged. The charged areas represent the image to be printed.

Once the entire page has been scanned and the drum appropriately discharged, the printing process becomes the same as that used in copying. Toner is applied to the drum and adheres to the charged areas. The blank paper is charged by the corona and passed adjacent to the drum where it picks up the toner image. The paper is then passed through heated rollers that fix the image to the paper. The result is an extremely high quality printed image.

The laser printer offers numerous advantages over the older and more common impact-type dot matrix and daisywheel printers. One of its best features is its silent operation. Impact printers make a considerable amount of noise, which is disturbing in an office environment. Laser printers, like photocopiers, are almost totally silent in their operation.

Figure 3: Block diagram of the electronics in a laser printer.

In addition, laser printers are extremely fast. The average laser printer can produce approximately 12 pages per minute and the fastest models can print over 200 monochrome pages per minute.

Finally, the printing quality is excellent. In fact, laser printers are commonly used in desktop publishing systems for creating the camera-ready copy of documents to be printed.

Perhaps the greatest disadvantages of the laser printer is its complexity and high cost. However, as the printer has become more popular and volume has increased, prices have dropped considerably. New technological advances are permitting even higher printing speeds as well as improved quality through greater dot density. Laser printers with a dot density of up to 1200 dots per inch are available for extremely high-resolution printing.

Louis E. Frenzel is president of Teknowledgy Sources, Inc., an Oakton, Virginia-based venture involved in the development of technical training materials in computer and electronics subjects.


Electronics Instructor Q & A


Q. I have the analog meter that comes standard with my course. In Lesson 1408 it says that on the ohms ranges, the + lead is the black one and the – lead is the red one. Isn’t this backwards? I thought red was always + and black was always - .

A. To make the red lead + and the black lead – on the ohmmeter ranges would require an extra set of contacts on the range selector switch. This would make the meter more expensive than necessary and the extra cost would be reflected in the cost of our course. The extra contacts would also make the switch more complicated, which would reduce the reliability of the meter. When we chose this meter, we felt that our students would be able to understand this difference. If you wish, you can simply unplug the leads from the meter and swap them around when you are using a resistance range.


No Negative Solution

Q. In my lessons, the op amps are shown with both a positive and a negative power supply. What do you do if you only have a single power supply available, such as a single battery?

A. There is several ways around this problem. The simplest is to use a power supply “splitter.” This consists of two equal-valued resistors, which are connected, in series across the power supply. The resistors form a voltage divider, so that half the supply voltage is dropped across each resistor. The common “ground” for the signal is the point where the two resistors are connected together. This works only if the current is relatively low.

For higher currents, a transistor can be used. Its bases are connected to the output of an op amp. The inverting input of this op amp is connected to the emitter of the transistor. The non-inverting input is connected to a resistor “splitter.” The transistor acts as a current amplifier. A third way to solve the problem is to use an oscillator, and then rectify and filter its output to provide a separate negative supply. There are op amps that are designed to operate on a single power supply. The LM 324 is an example of this. The IC has four independent op-amps in a singlepackage. We use this IC in our microprocessor lab lessons on Digital-to-Analog converters.


Functional Junction

Q. I have heard that there are solid-state cooling devices. What are they and how do they work?

A. These are called “Peltier Junctions.” Unlike conventional cooling devices, they do not use mechanical pumps and refrigerants. There are no moving parts. Basically, certain materials are used to make a PN junction. The junction is forward biased. The current carries heat away from one side, which becomes cooler, and transfers to the other side which becomes warmer.

Peltier devices are not at all as energy efficient as conventional mechanical cooling devices and large amounts of current are needed to make them work. They are also more expensive than mechanical systems, for the same amount of cooling power. But their reliability and lack of moving parts makes them useful in some applications. For example, they are used in nuclear submarines, where electrical power is abundant, and the fact that they make no sound is a great advantage.


Delaying Action

Q. I do not understand what “propagation delay” is.

A. When the input(s) to a gate or flip flop change, it takes a certain amount of time, usually measured in nanoseconds, for the output to respond to the change. This is called the gate or flip-flop’s propagation delay because the input - signal is delayed in going through the gate. Most of the propagation delay is caused by the internal capacitance of the transistors inside the IC. It takes a little time for them to charge or discharge in response to the changing voltage on the input of the device. The propagation - delays of the gates and/or flip-flops in a digital circuit determine its maximum operating speed.


PINning it Down

Q. I was reading a magazine article the other day and it mentioned PIN diodes. What are PIN diodes?

A. PIN diodes have a P-type semiconductor and an N-type semiconductor, as do ordinary diodes. But between the P and N material, there is a very thin layer of undoped or “intrinsic” semiconductor. This is what the letter I stands for in “PIN”. Undoped semiconductor is a good insulator. The I layer is put between the P and N layers to reduce the capacitance the diode. The reduced capacitance makes the diode start to conduct and stop conducting faster than regular diodes. PIN diodes are used in circuits that handle radio frequency and high-speed digital signals.



Lesson 2342B-6 seaming contradiction question
Q. In lesson 2342B-6 I am confused by a seaming contradiction.
Paragraph 2 of Topic 17 describes the leads as shown in fig. 36a & fig 38a. However fig.38d shows what I believe to be a different configuration.
Is the flat sided transistor from fig38b a different configuration that I need to memorize next to that of fig36a or am I missing something? Do I then also need to memorize fig 38 b, c & e?
A. Memorization is such a double edged sword. While it does help in the short term, it defeats the purpose when we are exposed
to the information applied differently than we memorized.
Since it does not fit the same way, we typically cannot see how
to apply it to the new situation. In terms of the different lead arrangements, we have illustrated them using typical pictorial drawings.
If you are actually working with transistors, I would highly recommend that you refer to the specification sheet for the actual lead arrangement.
I tried doing what you are suggesting (memorizing), and blew up several until I realized that what I memorized as connected ECB was actually EBC.
It would be better for you to understand that different types may or may not have the same lead arrangement and to work with them as they come.


How to get the proper voltage reading in Lesson 2324B-8

Q. In Lesson 2324B-8 on page 39 the book shows that connecting a wire between the circuits gives a voltage reading of 9v between points A and B. I’ve been racking my brain trying to figure out how we came up with 9v. Can you please explain?

A. Let's see how the value of 9V was calculated.

The first portion of that figure shows there are two separate circuits involved. The one on the left is a series circuit of two resistors, while the one on the right only has one resistor being used as the one that is not connected on the left hand side cannot and will not have a current flow through it due to the fact there is no complete path through it.

Now if we analyze those two circuits based upon that information, there will be a 15V drop across R2, and a 6V drop across R4. We know those facts from the basis of 20V/20Ω = 1A and 1A x 15Ω = 15V, while the other is just 6V across the only component of 60Ω.

The meter connected between points A and B does not have a potential measurement due to the fact that there is nothing to make a common connection between those two circuits. Granted, you could try to say the meter is the common element, but as it is measuring volts and has a very high resistance when in that mode, it becomes less of a possibility.

Now when we do connect the common line as shown in (b), those two voltages we calculated before are still there, but now we do have the two playing nicely together in the same sandbox.

Therefore, when we measure from point A which is at the 15V potential, and point B which is at the 6V potential, we arrive at the potential difference between them of 9V. That is how we got to the answer.

Hopefully this answers your question, but if not or if there are other questions/concerns that come along as you progress through your lessons, please feel free to contact the instruction staff using the information below.

In addition, please make sure to include your student number in any and all communication with us, as well as putting that information on all exams submitted.

What are the three different transistor configurations?

Q. In my lessons, the three different transistor configurations are described. I am having trouble telling them apart.

A. A transistor has three elements: collector, base, and emitter. The input signal is applied to one element, and the output is taken off a second element. The one left over is the common element.

For example, say the input signal is applied to the emitter and taken off the collector. That leaves the base, so this describes the common base amplifier. The signal is AC. One side of the input signal source must go to AC ground. The other side must go to the input element of the transistor.

The output signal comes off another element. One side of the load to which the output signal is sent must be at AC ground, too. The remaining element of the transistor must also be connected to the AC ground. This is because current must flow from the input source, through the transistor, and back to the input source.

Output current must also from the transistor, through the load, and back to the transistor through the ground. Both of these currents, the input and output, must flow through the common transistor element. In these circuits, the ground acts as a common conducting path for the AC input current, the transistor, and the AC output current.

This is where the “common” comes in. Since the remaining transistor element is connected to AC common ground, we call the configurations common emitter, common base, and common collector.

Students are often puzzled by the common collector configuration. The collector goes to the power source, Vcc, instead of to ground. While Vcc is the source of the DC voltage for the circuit, Vcc is a short to ground for AC signals. In a power supply operated off the AC power lines, there are filter capacitors with high capacitance values. Their reactances are very low at all the signal frequencies.

Even in battery operated amplifiers there are usually electrolytic capacitors connected across the battery, since as the batteries discharge, their internal resistances rise, and can interfere with the operation of the amplifiers they power. The result is that these capacitors act as short circuits to ground for any AC signal current that flows to Vcc. This is why the common collector has its common element (the collector) at AC ground, even though it is connected to Vcc.

Why is my PC clock not accurate?

Q. I have noticed that the clock in my computer isn't very accurate. It seems to lose time at several minutes a week. Do I have a computer virus?

A. Not likely. The clocks in computers are not very accurate. This function is done by a special clock / calendar chip on the motherboard that runs off a battery. The time base for this chip is a crystal controlled oscillator that would not be affected by a virus. The software merely reads in the output from the chip at regular intervals. The crystals drift a bit, which throws the clocks off.

You can change the date and time by double-clicking on the My Computericon on your desktop. When the window opens, click on Control Panel,” and when the Control Panel window opens, click on Date / Time.” The window for this will then open, and you will see a calendar with the date highlighted and a clock on the right. Below the clock is a box you can use to set the time. You can use your mouse to set the date and the time zone you are in, too.

Q. I have learned about Class A, B and C amplifiers, but have heard that there is also a kind called Class D. What is a Class D amplifier, and how does it differ from the first three classes?

A. In a Class D amplifier, the power transistors are operated as switches. They are driven by pules so that they turn on and offvery rapidly. The duty cycles of their control pulses are made to follow variations in the signal voltage.

In other words, the on” and “off” times are varied, so that the output from the power transistors is a series of pulses of varying widths. The pulses are smoothed into a continuous waveform in the output by LC filter circuits.

The advantage of Class D is that the power transistors are either on or off. When they are on, the current through them is high, but the voltage across them is low.

Since power is the product of current times voltage, the power dissipated in the output transistors is low. The same is true when the transistors are off. Although the voltage across them is high, the current through them is low, so the power they must handle is also low.

The transistors spend very little time in the range where both the current through them and the voltage across them are substantial, and the power is high. Only during the relatively short rise and fall times of the pulses are they in this range.

(One instructor likens this to someone running quickly back and forth over hot coals. He spends most of his time at one end or the other of the coals, where the ground is cool, and little of the time actually over the hot coals.)

Consequently, these amplifiers can produce a powerful output signal without having to dissipate much power. Class D amplifiers are very efficient. Unlike Class C amplifiers, they do not require a tuned load. They are often used in the power output stages of audio amplifiers and radio transmitters.



Q. My lessons say that the current through a capacitor is 90 degrees ahead of the voltage. Why is this?

A. A capacitor draws current whenever the voltage across it changes. The faster the voltage changes, the more current it will draw. Now think about the shape of the sine wave for the voltage across the capacitor. It is steepest at the point where it goes through zero and changes polarity.

In other words the voltage changes fastest in this part of the AC cycle. This occurs at 180° and 360° in the sine wave for the voltage. So the current will peak at these points. The voltage change slows down and stops momentarily when its sine wave reaches 90° and 270°. The current reaches zero at these points. So the sine wave for the current peaks at 180° and 360°. It goes through zero at 90° and 270°. This describes a sine wave that is 90 degrees ahead of the sine wave for the voltage.