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1. Solid - State Devices
With the advent of inexpensive, physically small electronic control devices, more and more small appliance manufacturers can be expected to use them in their designs. You have already seen how one such device, a diode, is used to convert ac into dc. There are many other small electronic components which can be used in the control circuitry of appliances and tools. Silicon Controlled Rectifier (SCR). Like the more conventional rectifier diode, this device will permit electrons to flow in only one direction—from the cathode to the anode, against the arrow. However, an SCR also has a third connection to it, the gate. Unless the gate is activated by a small voltage, no current will flow in the circuit. No current will flow because the SCR is not receiving any gate voltage. However, when S2 is closed electrons will flow through the diode in the direction of the arrows. Even though S2 is opened again, current will still continue to flow through the SCR. Once enabled by the gate voltage, the diode remains "latched" open, until switch SI is opened momentarily. Then current will not flow in the circuit until SI is again closed and S2 is closed momentarily. An SCR takes up little space and has no moving parts. However, it can only be used with dc-operated appliances, since it permits current to flow in only one direction. Triac. A more useful device from the standpoint of the modern appliance designer is the triac. This device is really two diodes, back-to-back. When enabled by a small gate voltage, a triac will permit current to flow in both directions (hence the back-to-back diode configuration in the diagram), making it suitable for use in ac circuits. Unlike the SCR, however, the triac cannot be latched—switch S must be closed and gate voltage applied to the gate at all times for the triac to conduct. If there is no gate voltage, the triac should not permit current to flow through it. The advantage of using SCRs and triacs, besides their small size and the fact that they have no moving parts, is that a light-duty, low-current switch (S) can be used to handle heavy current. This eliminates burned and pitted switch and relay contacts and the need for large, heavy switches. Diac. A diac is similar to a triac, but does not have a gate. At low voltages, the diac will not conduct, but when the ac voltage exceeds its breakdown voltage (usually about 35 volts) it will conduct and pass ac. Test Equipment For The Home Appliance Workshop Troubleshooting the cause of appliance failures is basically a matter of using common sense and step-by-step procedure. Through an understanding of how your appliances operate electrically and mechanically, you can usually detect the cause of failure or at least isolate the problem area with little trouble and a minimum of test equipment. Occasionally the problem will involve more sophisticated troubleshooting techniques which require test instruments capable of measuring wattage, voltage, and resistance. The purpose of this chapter is to generally describe the types of equipment which can make such measurements, and suggest certain specialized checking devices which can be built easily and inexpensively to make your workshop more efficient.
http://www.fun-home-projects.com/electrical_projects/​tools/​solid_state_devices.html

2. Rfi Filter For Thyristor Controls
Appliances employing an SCR or a triac as a current, voltage, or speed control may cause radio-frequency interference. This kind of interference can often be eliminated by utilizing an external filter using the circuit. Inductor L, which may be a 100 uH choke coil (such as Miller No, 5250 which will handle 2A) and capacitor C, a 0.01 uF ceramic disc capacitor, form a low-pass filter which passes 60 Hz ac but resists passage of high-frequency transients. These components can be mounted inside an electric outlet box to which a single female ac receptacle is mounted. In the diagram, P represents the power-lie cord arid plug. The interfering appliance is plugged into ac receptacle J.
http://www.fun-home-projects.com/electrical_projects/​tools/​rfi_filter_for_thyristor_controls.html

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