246CHAPTER 9. TRANSFORMERS• Variable transformers can be made by providing a movable arm that sweeps across the lengthof a winding, making contact with the winding at any point along its length. The winding, ofcourse, has to be bare (no insulation) in the area where the arm sweeps.• An autotransformer is a single, tapped inductor coil used to step up or step down voltage likea transformer, except without providing electrical isolation.• A Variac is a variable autotransformer.9.6Voltage regulationAs we saw in a few SPICE analyses earlier in this chapter, the output voltage of a transformer variessome with varying load resistances, even with a constant voltage input. The degree of variance isaﬀected by the primary and secondary winding inductances, among other factors, not the least ofwhich includes winding resistance and the degree of mutual inductance (magnetic coupling) betweenthe primary and secondary windings. For power transformer applications, where the transformer isseen by the load (ideally) as a constant source of voltage, it is good to have the secondary voltagevary as little as possible for wide variances in load current.The measure of how well a power transformer maintains constant secondary voltage over a rangeof load currents is called the transformer’s voltage regulation. It can be calculated from the followingformula:Regulation percentage =Eno-load - Efull-load(100%)Efull-load“Full-load” means the point at which the transformer is operating at maximum permissiblesecondary current. This operating point will be determined primarily by the winding wire size(ampacity) and the method of transformer cooling. Taking our ﬁrst SPICE transformer simulationas an example, let’s compare the output voltage with a 1 kΩ load versus a 200 Ω load (assumingthat the 200 Ω load will be our “full load” condition). Recall if you will that our constant primaryvoltage was 10.00 volts AC:freqv(3,5)i(vi1)6.000E+019.962E+009.962E-03Output with 1k ohm loadfreqv(3,5)i(vi1)6.000E+019.348E+004.674E-02Output with 200 ohm loadNotice how the output voltage decreases as the load gets heavier (more current). Now let’s takethat same transformer circuit and place a load resistance of extremely high magnitude across thesecondary winding to simulate a “no-load” condition: (See ”transformer” spice list”)So, we see that our output (secondary) voltage spans a range of 9.990 volts at (virtually) no loadand 9.348 volts at the point we decided to call “full load.” Calculating voltage regulation with theseﬁgures, we get: