Electronics

The Four Layer Diode

If we add a junction to a transistor, we get a four-layer device. At first glance, this may seem either useless or counter-productive, but this is not the case.

The diagram to the right shows the structure and nomenclature of the four-layer diode, together with its equivalent structure, showing its logical behavior and its schematic symbol. Rather than acting simply as an odd sort of diode, this device actually behaves as two transistors, interconnected back-to-back as shown. The external connections are designated the cathode (n-type connection) and anode (p-type connection). This keeps us from having to distinguish between the n-emitter and the p-emitter.

In operation, the device is placed in a circuit so that the anode is held positive with respect to the cathode. If this voltage polarity is reversed, the four-layer diode exhibits two reverse-biased junctions and will not conduct current.

The applied voltage appears mainly across the reverse-biased center junction of the device, and the quiescent current flowing through the device is quite small, consisting essentially of a leakage current. As the applied voltage increases slowly, the leakage curent increases slowly as well, until the applied voltage reaches a certain level. At this voltage, known as the breakover voltage or firing voltage of the device, the two logical transistors both turn on and current increases dramatically. At the same time, the voltage across the diode decreases to almost zero, and the applied voltage now appears across whatever load circuit or device is connected in series with the diode. The diode has just switched from its off (or blocking) state to its on state.

Once the four-layer diode is conducting, it will continue to conduct as long as current continues to flow. It can only be turned off by reducing the circuit voltage and/or current to below the levels required to sustain conduction. Typically, this means either removing power totally from the circuit, or else using the diode as a device to periodically discharge a capacitor.

The breakover voltage and current-handling capacity of a given four-layer diode are determined by the exact details of its manufacture. Four-layer diodes may be manufactured with a wide range of voltage and current ratings.

There is one problem which can appear with the four-layer diode: if the applied voltage rises rapidly, the inherent capacitance associated with the reverse-biased middle junction will transmit the applied voltage more directly, and cause the diode to switch on at a voltage well below its designed breakover voltage. This phenomenon is known as the rate effect.

Applications for the four-layer diode fall into two categories. In active cirucits, they can be used as the active element in a sawtooth waveform generator or triggered pulse generator. Or, they can be used as a "crowbar" element in a power supply. In this last application, the four-layer diode is placed directly across the output terminals of the power supply, used to power delicate circuitry. If the supply voltage should rise for any reason to a level that might damage the circuitry, the four-layer diode breaks over and draws a heavy current from the supply. This overloads the supply and causes the fuse or circuit breaker to blow. It's a drastic measure, but still much better than allowing expensive circuit components to be damaged or destroyed.

The Diac and Triac

One of the drawbacks of all of the four-layer diodes is that they all require a dc voltage of the correct polarity in order to operate. It would be nice if we could have some sort of SCR that works for either polarity, so it can be used with an applied ac voltage.

Now, we created the four-layer devices by essentially connecting two transistors back to back in a single silicon crystal. Can we extend this concept and connect two SCRs back to back?

The diagram to the right shows the resulting five-layer device, which is known as a diac. At first glance, it seems unreasonable or even impossible, considering that each connection to the semiconductor crystal overlaps a pn junction. However, the device does work, and indeed works well.

The terms anode and cathode no longer apply, so the connections are simply named terminal 1 (T1) and terminal 2 (T2). Each terminal can serve as either anode or cathode, according to the polarity of the applied voltage.

That same applied polarity also determines which of the end junctions is active, and which one is bypassed. Thus, if T1 is positive with respect to T2, T1's N-type region is ignored (electrons are pulled away from that junction) and its P-type region serves as the anode. At the same time, the relative negative voltage at T2 pulls holes from the P-type region towards the terminal (removing them from the next junction), but tends to push electrons from its N-type region across that junction into the P-type region, thus making them available for conduction.

The diac, like the four-layer diode, remains non-conducting until its breakover voltage is reached, at which point it turns on fully and remains on until the applied voltage or circuit current are reduced below the holding values at which conduction can be maintained. Since the diac is normally used in ac circuits, operating as part of the control circuit for devices powered from a household wall socket or similar source, this is not a problem. In such applications, the diac is triggered each half-cycle of ac power, and then turns off at the end of the half-cycle when the line voltage reverses polarity.

The drawback of the diac is the same as it was for the four-layer diode: it cannot be triggered at just any point in the ac power cycle; it triggers at its preset breakover voltage only. If we could add a gate to the diac, we could have variable control of the trigger point, and therefore a greater degree of control over just how much power will be applied to the line-powered device.

The figure to the right shows the result. This device is known as a triac. Here, the main connections are simply named main terminal 1 (MT1) and main terminal 2 (MT2). The gate designation still applies, and is still used as it was with the SCR.

The useful feature of the triac is that it not only carries current in either direction, but the gate trigger pulse can be either polarity regardless of the polarity of the main applied voltage. The gate can inject either free electrons or holes into the body of the triac to trigger conduction either way. For this reason, you may see the triac referred to as a "four-quadrant" device.

As with the diac, the triac is used in an ac environment, so it will always turn off when the applied voltage reaches zero at the end of the current half-cycle. If we apply a turn-on pulse at some controllable point after the start of each half cycle, we can directly control what percentage of that half-cycle gets applied to the load, which is typically connected in series with MT2. This makes the triac an ideal candidate for light dimmer controls and motor speed controls. This is a common application for triacs.

What Is GPS

The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense. GPS was originally intended for military applications, but in the 1980s, the government made the system available for civilian use. GPS works in any weather conditions, anywhere in the world, 24 hours a day. There are no subscription fees or setup charges to use GPS.

How it works

GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the user's exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user's position and display it on the unit's electronic map.

A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude). Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more.

How accurate is GPS?

Today's GPS receivers are extremely accurate, thanks to their parallel multi-channel design. Garmin's 12 parallel channel receivers are quick to lock onto satellites when first turned on and they maintain strong locks, even in dense foliage or urban settings with tall buildings. Certain atmospheric factors and other sources of error can affect the accuracy of GPS receivers. Garmin® GPS receivers are accurate to within 15 meters on average.

Newer Garmin GPS receivers with WAAS (Wide Area Augmentation System) capability can improve accuracy to less than three meters on average. No additional equipment or fees are required to take advantage of WAAS. Users can also get better accuracy with Differential GPS (DGPS), which corrects GPS signals to within an average of three to five meters. The U.S. Coast Guard operates the most common DGPS correction service. This system consists of a network of towers that receive GPS signals and transmit a corrected signal by beacon transmitters. In order to get the corrected signal, users must have a differential beacon receiver and beacon antenna in addition to their GPS.

The GPS satellite system

The 24 satellites that make up the GPS space segment are orbiting the earth about 12,000 miles above us. They are constantly moving, making two complete orbits in less than 24 hours. These satellites are travelling at speeds of roughly 7,000 miles an hour.

GPS satellites are powered by solar energy. They have backup batteries onboard to keep them running in the event of a solar eclipse, when there's no solar power. Small rocket boosters on each satellite keep them flying in the correct path.

Here are some other interesting facts about the GPS satellites (also called NAVSTAR, the official U.S. Department of Defense name for GPS):

The first GPS satellite was launched in 1978.
A full constellation of 24 satellites was achieved in 1994.
Each satellite is built to last about 10 years. Replacements are constantly being built and launched into orbit.
A GPS satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended.
Transmitter power is only 50 watts or less.

What's the signal?

GPS satellites transmit two low power radio signals, designated L1 and L2. Civilian GPS uses the L1 frequency of 1575.42 MHz in the UHF band. The signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go through most solid objects such as buildings and mountains.

A GPS signal contains three different bits of information - a pseudorandom code, ephemeris data and almanac data. The pseudorandom code is simply an I.D. code that identifies which satellite is transmitting information. You can view this number on your Garmin GPS unit's satellite page, as it identifies which satellites it's receiving.

Ephemeris data, which is constantly transmitted by each satellite, contains important information about the status of the satellite (healthy or unhealthy), current date and time. This part of the signal is essential for determining a position.

The almanac data tells the GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite transmits almanac data showing the orbital information for that satellite and for every other satellite in the system.

Sources of GPS signal errors

Factors that can degrade the GPS signal and thus affect accuracy include the following:

Ionosphere and troposphere delays - The satellite signal slows as it passes through the atmosphere. The GPS system uses a built-in model that calculates an average amount of delay to partially correct for this type of error.
Signal multipath - This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors.
Receiver clock errors - A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.
Orbital errors - Also known as ephemeris errors, these are inaccuracies of the satellite's reported location.
Number of satellites visible - The more satellites a GPS receiver can "see," the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground.
Satellite geometry/shading - This refers to the relative position of the satellites at any given time. Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping.
Intentional degradation of the satellite signal - Selective Availability (SA) is an intentional degradation of the signal once imposed by the U.S. Department of Defense. SA was intended to prevent military adversaries from using the highly accurate GPS signals. The government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers.