APPLICATIONS OF SOLID-STATE DEVICES

The application of solid-state devices is virtually unlimited. Because of the rugged construction and small size of these devices, you may encounter them in every facet of your job. The scope of solid-state device application ranges from devices as simple as room thermostats to the more complicated ICs in fire alarm systems and voltage regulators. The following paragraphs are just a few examples of the ways in which solidstate devices are being used in NCF equipment.

DIRECT CURRENT POWER SUPPLIES

Diodes are solid-state devices used chiefly as rectifiers, converting alternating current (ac) to direct current (dc). Diode rectifiers are used in fire alarm power supplies, battery chargers, and voltage regulators. A typical power supply circuit that uses a diode bridge or full-wave bridge is shown in figure 4-8.

Figure 4-8.\Typical bridge rectifier circuit.

When the ac input is applied across the secondary winding of Tl, it will forward-bias diodes CR1 and CR3, or CR2 and CR4. When the top of the transformer is positive with respect to the bottom, as shown in figure 4-8 by the designation number 1, both CR1 and CR2 will feel this positive voltage. CR1 will have a positive voltage on its cathode, a reverse-bias condition; and CR2 will have a positive voltage on its anode, a forward-bias condition. At this same time, the bottom of the secondary winding will be negative with respect to the top; that is, CR4 will be in a forward-bias condition and CR3, in a reverse-bias condition.

During the half cycle of the input designated by the number 1 in figure 4-8, we find that CR2 and CR4 are forward biased and will therefore conduct heavily. The conducting path is shown by the solid arrows from the source (the secondary winding of T1) through CR4 to ground, up through RL, making the top of RL positive with respect to the grounded end, to the junction of CR2 and CR3. CR2, being forward biased, offers the path of least resistance to current flow, and this is the path current will take to get back to the source.

During the alternation designated by the number 2 in figure 4-8, shown by the dashed arrows, the top of the secondary winding is going negative while the bottom is going positive. The negative voltage at the top is felt by both CR1 and CR2, forward-biasing CR1 and reverse-biasing CR2. The positive voltage on the bottom of the T1 secondary is felt by CR3 and CR4, forwardbiasing CR3 and reverse-biasing CR4. Current flow-starting at the source (T1 secondary winding)-is through CR1 to ground, up through R L (this is the same direction as it was when CR2 and CR4 were conducting, making the top of RL positive with respect to its grounded end), to the junction of CR2 and CR3. This time CR3 is forward biased and offers the least opposition to current flow, and current takes this path to return to its source.

As can be seen, the diodes in the bridge circuit operate in pairs; first one pair\CR1 and CR3\conducts heavily, and then the other pair\CR2 and CR4\conducts heavily. As shown in the output waveform, we get one pulse out for every half cycle of the input-or two pulses out for every cycle in.

The bridge circuit will also indicate a malfunction in one of two ways-it has no output or a low output. If any one of the diodes opens, the circuit will act as a half-wave rectifier with a resultant lower output voltage.