78xx Voltage Regulators

Posted on Feb 6, 2014

The voltage regulators from the 78xx-series are found in many analogue power supplies. It seems, then, somewhat super‚uous to say much more about them. But on the other hand, it can be very useful to highlight the important points, just because they are so ubiquitous. The 78xx is almost always used bare`, because additional components are almost

78xx Voltage Regulators
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unnecessary. In fact, only one additional component is required, and that is capacitor C2. Based on the manufacturer`s recommendation, this capacitor should be 220nF in order to prevent oscillatory behavior. In practice, you will almost always see that a 100nF capacitor used here. This is a value that does not cause problems. C1 is the smoothing (reservoir) capacitor, its purpose is to reduce the ripple of the rectified AC voltage and is not actually related to voltage regulation. If the DC voltage is provided by a mains adapter, then this electrolytic capacitor is usually already part of the adapter, although the value is rather small sometimes. C2 may only be omitted if C1 is close to the 78xx and C1 is of good quality (low ESR). But there is nothing wrong by playing it safe and alwaystting C2. Rule of thumb: always place a 100 nF capacitor on the input as close to the regulator as is practicable. Strictly speaking, there is no need for a capacitor on the output. However, a capacitor of at least 100 nF (C3) ensures much-improved regulation with fast (several microseconds) changes in load current. In practice, a decoupling-capacitor is placed close to the power supply pins of many ICs. These can provide the same function, provided they are placed not too far away. An electrolytic capacitor (C4) can be added for similar reasons: to catch slow (and fast, if it is a good capacitor) variations in load current. There is...

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