Center tap

In single-phase bridge circuits for ac connections and for very low ac output voltages below 5 V, single-phase center tap circuits are used as rectifier circuits for CP transformer-rectifiers. They have an efficiency of 60 to 15% and a residual ripple of 48% with a frequency of 100 Hz. A three-phase bridge circuit for three-phase alternating current is more economical for outputs of about 2 kW. It has an efficiency of about 80 to 90% and a residual ripple of 4% with a frequency of 300 Hz. The residual ripple is not significant in the electrochemical effect of the protection current so that both circuits are equally valid. 

Figure 3-18 Forward-mode secondary winding arrangements (a) center-tapped secondaries (b) full-wave secondaries (c) isolated secondaries.

Figure 3-28 Output stages for forward and boost-mode converters (a) balf-wave forwardmode (b) center-tapped forward-mode (c) full-wave bridge forward-mode (d) boost-mode.

Portable electrical hand tools and equipment shall be properly grounded and wound to operate on llOV a.c. center tapped to earth supply, and shall only be connected to the system by permanent joints or proper connections. 

To model the transformer, an ideal center-tapped transformer is combined with a nonlinear core model for the F material. As this circuit counts on the saturation of the core, a SPICE primitive inductor will not work. By adding this nonlinear core model across the input of the center-tapped transformer, the magnetizing inductance and saturation characteristics of the core are realized. 

The inductance-type transducer consists of three parts a coil, a movable magnetic core, and a pressure sensing element. The element is attached to the core, and, as pressure varies, the element causes the core to move inside the coil. An AC voltage is applied to the coil, and, as the core moves, the inductance of the coil changes. The current through the coil will increase as the inductance decreases. For increased sensitivity, the coil can be separated into two coils by utilizing a center tap, as shown in Figure 7. As the core moves within the coils, the inductance of one coil will increase, while the other will decrease. 

Position sensors convert the position into an electrically measurable signal such as resistance, voltage, current, inductance, pulses, or capacitance. The simplest and most widely used position sensor is the potentiometer. The potentiometer has three terminals, one for each end of the resistive element and one for the brush. As the brush moves, the resistance between the center tap and end terminal changes (Figure 3.130). 

The position can also be determined in a resistive or voltage mode. In the resistive mode, a current is sent to the center tap and to one of the end terminals, whereas the resulting voltage drop is measured by the transmitter. A potentiometer has an infinite resolution. Linearity for a precision rotary potentiometer can be as good as 0.25%. However, due to linkages and gears, the linearity for the entire assembly usually is about 0.5-1%. A linear variable differential transformer (LVDT) is mostly used in linear motion applications and also inside some pressure transmitters (Figure 3.131). 

Figure 1. Improvement in part cleanliness after switching from an oil-base to a water-base coolant in a center-tapping application.

FIGURE 7.52 The graphical solution to the push pull Class B amphfier configuration is shown. The quiescent point is midway between the ends of the loadline as in Class A, but unlike Class A, the quiescent current is zero. Each transistor output sees Ri at the collector since the center tapped primary transformer has a 1 1 turns ratio from Ri to each transistor even though it has a 2 1 primary (end to end) to secondary turns ratio. 

Because the phase of one transistor is of the opposite phase of the other, a center tapped transformer is needed to combine the two halves properly. This maybe done without the use of a transformer if complementary pairs of transistors are available (one NPN and one PNP). Both cases are presented in Fig. 7.53. 

The center tap current of the transformer is half-wave DC at twice the frequency of the drive signal so that... 

The fuU-wave rectifier utilizes both halves of the input sinusoid one possible implementation is shown in Fig. 10.15(a). The fuU-wave rectifier consists of two half-wave rectifiers connected to a load R. The secondary transformer winding is center tapped to provide two equal voltages Vi for each half-wave rectifier. When the node A is at positive polarity V with respect to node B, D1 will be forward biased and D2 will be reverse biased. Therefore, diode D1 will conduct and the current will flow through R back to the center tap of the transformer. When the node B is at positive polarity Vi with respect to node A, D2 will be forward biased and diode D1 will be reverse biased. The current conducted byD2 will flow through R and back to the center tap. The current through R is always in the same direction... 

The full-wave rectifier just discussed requires a center-tapped transformer. An alternative implementation of the full-wave rectifier, as shown in Fig. 10.16 (a), is the bridge rectifier. This rectifier has four diodes arranged similar to Wheatstones bridge and does not require a center tapped transformer. 

The PIV is about half the value for the fuU-wave rectifier with a center tapped transformer, which is an... 

L = winding inductance N = number of turns (center tapped)... 

Some of the disadvantages of this configuration are the transistors must block twice the supply voltage flux symmetry imbalance can cause transformer saturation and special control circuitry is required to avoid this problem and the use of a center-tap transformer requires extra copper resulting in a higher VA rating. 

Two smoothing capacitors (Qn) are used to create a center tapped DC source, and this configuration (Fig. 10.85(b)) utilizes the transformer core efficiently. The voltage across each transistor is equal to the supply voltage (half of push-pull) and, therefore, is suitable for high-voltage inputs. 

Center tap A connection made at the electrical center of a coil. 

Transformer, 120V to 12V, 450 ma, center tapped Portable AM/FM radio Multimeter (or "multitester") t Lamp bulb, tungsten, 12 volt dc, green or blue Fuse, fast-action, 315 ma, 5 x 20mm t Phillips head screwdriver, for replacing fuse Color code slide rule Resistor, 100 ohm, 10 or 1 watt Resistors, 150, 180, (1/2 or 1/4 watt)... 

Connect the other end of that same clip lead to either one of the two secondary wires on the transformer, which both have thin yellow plastic insulation on them. Do not use any of the black wires of this transformer, either the thinly insulated "center tap" of the secondary coil or the two thickly insulated black wires of the "primary" coil. (This experiment can be done either with or without a long "power cord" and plug attached to the primary. )... 

Transformer, 120V to 12V, 450 ma, center tapped Portable AM/FM radio... 

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