How to make a stabilized voltage with the 78XX/79XX-series.

A common way to stabilize a DC voltage is to use those clever three pin IC's in the 78XX/79XX-series. They have a preset output voltage. XX is the value of the stabilized output voltage. 78 means positive and 79 negative voltage. The IC's have one input, one output and one reference terminal. The latter is set to ground potential for a normal use.

The picture below shows a normal use of the 78XX.

And this shows an equivalent circuit with the 79XX.

The pin configuration is for a TO220. This is the most common package.

The IC is protected against short-circuit and too high current. The current limitation is for TO220; 0,5, 1, 2 or 3 ampere

It is important to notice that the maximum difference between input and output voltage must be more than 2V. The capacitors at input and output terminals stabilize the circuit and prevent oscillation. The values are specific for the 78XX and 79XX models, according to the schematics on this page.

Here comes a schematic of a complete typical power supply with dual voltages:

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Robin Tomtlund
My e-mail address: robin2@hem1.passagen.se