Sky Digibox Sky Analogue Computer Car
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Most power supplies contain two distinct parts, (i) the transformer, rectifiers and smoothing capacitor(s) and (ii) the regulator circuit-as shown in Fig. 1. The action of the regulator is to reduce the potential at its input terminals to the desired output voltage (normally 12 volts for amateur equipment). This reduction has to be achieved irrespective of the ripple or other voltage variations at its input. Since this is a "cutting-down" effect, in order to work correctly the input voltage must be greater than 12 volts for all parts of the cycle. If the regulator supplies a constant load, then the current it passes will also be constant.
Let us now look at the transformer, rectifiers and smoothing part of the supply. Assuming full-wave rectification, as shown in Fig. 1, then if the capacitor (C) was not present a series of consecutive half sine waves would appear at the regulator's input. However, the action of this capacitor is that it charges up to the peak value of the rectified sine waves through the rectifiers and discharges, relatively slowly, through the regulator and thus the load. During this latter part of the cycle the voltage across the capacitor falls linearly at a rate of I/C volts/second. When this linear decay is interrupted by the next rising half sine wave, the current is subsequently governed by the characteristics of the rising half sine wave. The value of this current is C x dV/dt and this can be surprisingly large. The current passing through the conducting rectifier is now the capacitor's charging current plus the load current.
Going back to our 12 volt supply, and assuming it to contain (typically) a 15-0-15 volt transformer and to supply a load current of 5 amps, the rate of fall of the voltage across the capacitor is 5/C volts/second. This is shown in Fig. 2 for a range of values of the smoothing capacitor, from 400OµF to 20 000µF. The associated peak capacitor charging current has been found from the intercept of the decay with the next rising half sine wave and is shown in Fig. 3. As may be seen here, for a 5 amp load current and a typical smoothing capacitor value of 10000µF, the maximum charging current is 38.5 amps and the maximum current in the rectifiers is 435 amps!
It is important therefore that the rectifiers should be chosen so that these high peak currents are within their ratings&emdash;or smoke may be expected!
The high peak-to-average current ratio in this form of circuit has long been underestimated by p.s.u. constructors and has frequently been the cause of disappointment in the reliability of such supplies. The appreciation of the high peak currents to which the rectifiers are subjected enables appropriate diodes to be selected. In conjunction with this, Fig. 3 indicates the wisdom of reducing the value of the smoothing capacitor as much as possible &emdash; consistent with maintaining the voltage at the input to the regulator at least one volt higher than the desired output voltage at all parts of the cycle.
Copyright ©2006 SatCure
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