Ac Measurement

Topics: Rectifier, Root mean square, Voltage Pages: 12 (3280 words) Published: June 24, 2013
Chapter 13


This chapter explains rectification for measurement and rectifying instruments. Knowledge of the circuit principles will help you use these instruments properly and measurements alternating current correctly. One of the most common and economical methods for measuring alternating currents is to rectify these currents and read the resultant DC on an analog or digital volt-ohm meter (VOM). You need to take many considerations into account when using rectification: what type of rectification you use, what scale conversion you will need, and the sensitivity of the meter.

Alternating current periodically changes direction, which is why it is called alternating current. Direct current, on the other hand, maintains one direction or polarity of current. Rectification is the process of changing an alternating current into a direct current. Whether we use one direction (half cycle or half wave) or both directions (full cycle or full wave) is determined by the circuitry.

In half-wave rectification only one diode is used. As you will recall from previous chapters, a diode is a check valve for electrical current. Current may flow only in one direction through a diode. It obtains its name from di-, meaning two, and -ode for electrodes. The electrodes are named the anode (positive electrode) and cathode (negative electrode). There are many different kinds of diodes. We will restrict ourselves here to just two types, germanium and silicon. The only differences between the two that are significant to us will be the: 1. Voltage necessary to maintain current flow in the positive direction—approx. 0.1 to 0.3 for germanium, approx. 0.5 to 0.9 for silicon.


216 Chapter 13—AC MEASUREMENT

2. Peak inverse voltage rating (explained in the text). 3. Power-handling capability (silicon is far more capable). Figure 13–1 represents a diode schematically. Current flow is shown as electronic (negative to positive). The cathode is the bar, and the arrowhead is the anode. This schematic diagram came about because the original diode was as shown in Figure 13–2. This was the original diode used in the “crystal radio.” The operator would scratch on the galenium crystal until a rectifying junction was found and then, all other things being proper, the operator would hear an AM radio station. Note the similarity between the actual physical cat whisker device and the schematic of a diode.

Figure 13–1

Schematic of diode in circuit.

Figure 13–2

Cat whisker and crystal.

One of the more important parameters for a diode is how much current it can safely pass before it is damaged or destroyed. Diodes are generally given one of two ratings: average and peak. Typically, a diode with a 1-amp average current capability can pass 10 amps for a cycle or two, no more. This capability is called forward current.


Forward voltage drop is the amount of voltage required to maintain current through the diode. In a mechanical check valve the pressure of the fluid in the correct direction must be high enough to overcome the pressure on the reverse side of the check valve, and a bit more to overcome the spring of the check valve. So too, a diode must have more than just equal voltages to conduct. In germanium diodes, this is anywhere from 0.1V to 0.3V for germanium and 0.5V to 0.9V for silicon diodes.

Just as in the mechanical check valve, if pressure on the reverse side becomes high enough (in relation to the forward side), the valve will be destroyed. There is always a limit. Since voltage is pressure, diodes have this limit as well. The specifications always give the amount of voltage in the nonconducting direction that the diode can withstand. This is the reverse potential across the diode. If it is exceeded, a rectifying diode will be destroyed.

The process of converting AC to DC will...
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