Modern solid-state devices are remarkably well built, considering their smallness and complexity, and many of them are surprisingly inexpensive. Moreover, their performance is consistent. However, their electrical and mechanical ruggedness, easy availability, and reproducible performance should not be taken for granted. Careless handling, installation, and operation of these devices and misguided application of them can still cause damage or poor performance.
The following paragraphs offer guidance in working with semiconductor devices, and these hints and precautions are offered for the benefit of all readers. Observance of good engineering practice here will pay off in increased success with semiconductor devices.
This material applies directly or indirectly to all of the circuits in this guide. By presenting it in this one place, we avoid taking space and time to repeat hints and precautions with individual circuits.
Avoid Rough Handling. This includes dropping, hammering, vibrating, or forcibly squeezing or tensing the device; pulling, twisting, or repeatedly flexing its pigtails or lugs; and mounting the device under severe pressure or tension.
Mechanical Assembly. The reader is free to use his favorite method of construction: simple breadboard, perforated board, open chassis, metal box, printed circuit. Employ the same techniques and precautions that would apply to any other electronic circuit.
Install The Semiconductor Last. In this way, the device will not be subjected to repeated heatings caused by soldering in the circuit. It is important to check the circuit wiring thoroughly before installing the semiconductor device, to insure that the device will not be heated by unsoldering and resoldering to correct mistakes in the wiring. When the device is a standard MOSFET. keep the short-circuiting ring (that comes attached to this device) in place until the MOSFET is completely installed in the completed circuit: then remove it. The gate-protected MOSFET needs no such protecting ring.
Remove Semiconductor First. When dismantling a circuit, remove the semiconductor device first. This procedure, which protects the device from heat caused by unsoldering, is the opposite of that recommended when building the circuit (see item above).
Semiconductor Mounting. All methods of device mounting are permitted, as long as the mounting is solid. These include use of socket, soldering or welding directly into circuit, use of clips, use of mounting screws, and use of terminal studs. The pigtails or lugs of the mounted device must be under no stress.
Soldering Or Welding. To prevent internal damage to the semiconductor device, use a suitable heat sink in the regular manner when soldering or welding the device into the circuit. Keep the heat sink in place for a reasonable time after the joint has cooled.
Keep Leads Straight. If a semiconductor device is to be inserted into a socket, straighten its pigtails or lugs beforehand. To drive the device home into the socket, push firmly, but gently and straight down, on the top of the case. When leads must be bent for a particular installation, avoid a sharp right-angle bend, since such a bend tends to break easily. Use care when removing a semiconductor device from its socket.
Hot Case. The metal case of some semiconductors is "hot;" that is. the case is internally connected to one of the electrodes of the device. If the manufacturer's literature or the circuit diagram and/or the text in this guide indicates this condition, keep the case free from contact with other components, metal chassis, and wiring.
Use Specified Components. Employ the exact values of components specified in the circuit diagrams. If you wish to experiment, start first with the specified value. Make adjustments exactly as instructed in the text. Where the manufacturer and model number are given for a component, this is either the only such component available or is the one that at the time of the writing was the one easily available to experimenters on a single-unit retail basis.
Wiring And Isolation. Use the shortest and most direct leads practicable; this will minimize stray pickup, undesired coupling, and undesired feedback. When long leads are unavoidable, lead dress and adequate separation are important. With high-gain devices, such as the FET. MOSFET. and IC. the input and output circuits sometimes must be shielded from each other, especially if they employ inductors or transformers. In all RF and high-gain AF circuits, shield and bypass all susceptible parts of the circuit in the same way that tube circuits are safeguarded.
Grounds. In some of the circuit diagrams, a dashed line runs to the ground symbol. This means that the connection to chassis or to earth is optional and depends upon how the circuit will be used by the reader. When, instead, a solid line runs to the ground symbol, the connection must be made.
Connect Power Last. Connect the power supply last, whether it is AC or DC. and then only after the circuit wiring has been double checked and verified as correct. In an experimental application-where voltages are not specified as they are in this guide-start with a low voltage and gradually increase it.
Type of Power Supply. In many of the circuits, batteries are shown for DC supply, as a matter of circuits simplicity; however, a well-filtered transformer' rectifier type of supply also may be used.
Complexity of Power Supply. Some IC function best with a dual power supply and accordingly are shown with two batteries; others get along with a single battery. Readers who already are expert with ICs may have favorite schemes for converting the two-battery circuit to single battery; however, it is best for all others to wire and test the circuit first as it is given in the guide, and then to experiment later.
Use Specified Voltages. Employ the voltages specified in the circuit diagrams. Although a variation of a few percent, plus or minus, should not drastically affect operation of a circuit, a very large change in voltage can alter performance markedly from that reported in the text.
DC Polarity. Reversing the DC supply voltage can damage some semiconductor devices and cause others to switch off. Carefully observe polarity.
Avoid Excessive Supply. Do not subject the semiconductor device to excessive currents or voltages. Be guided by the values given in the circuit diagrams and in the text. Never allow the combined current or voltage (that is. DC plus peak AC) to exceed the maximum value given in the manufacturer's literature or warned against in the text.
Avoid Heat. Protect the semiconductor device from excessive heat, either external or internal. Where a heat sink is specified, it must be used. Keep the device clear of hot tubes, rectifiers, and other such components.
Avoid External Fields. The semiconductor device and its circuit must be protected from strong, external magnetic fields. Common sources of such fields are transformers, chokes, motors, generators, relays, circuit breakers, and loudspeakers.
Avoid Overdriving. Excessive signal amplitude can degrade the performance of some semiconductor devices and in some instances may even damage them. The maximum signal voltage permissible in a susceptible circuit is given in the circuit diagram or in the text, and should not be exceeded.
Testing And Troubleshooting. Employ standard test procedures when checking circuits or shooting trouble:
1-Use high-resistance (preferably electronic) voltmeter
2-Use low-resistance current meter
3-Monitor with an oscilloscope any AC circuit
4-Use low-distortion AC test signals
5-Be certain that your test procedure does not itself introduce excessive voltages, or transients
6-Test semiconductor devices with an appropriate instrument (transistor tester, IC tester, etc.) or with a foolproof laboratory setup (that is. one in which the test itself does not damage the semiconductor).
In most instances, a service-type ohmmeter will be accurate enough for testing resistors and making leakage or continuity tests. Ohmmeter tests of semiconductor devices, however, are not recommended, since some ohmmeters can apply a damaging voltage to these devices.
SAFETY NOTICE. Some semiconductor circuits--especially those employing diacs, triacs. and silicon controlled rectifiers--are operated from the AC power line and can be dangerous if not carefully built and installed and cautiously handled. To avoid electric shock or damage to equipment when working with such circuits, observe all of the usual precautions that apply to high voltages. For maximum safety with some circuits, a 1:1 isolating transformer should be inserted between the power line and the circuit (as recommended, for example, in Figures 10-6 and 10-7. section 10): this will prevent the circuit from being connected directly to the power line. In other instances, the isolating transformer is desirable, but not mandatory.
