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Regulated Dual Power Supply

Posted December 1st, 2013 by

The first project I’ve decided to publish here is a simple dual power supply, which I use for powering audio circuits requiring such a supply. It’s pretty much a standard design, and should be suitable for low power devices (ie. don’t use it for a power amplifier or a toaster or anything).

A ±15V PSU

As shown, it is configured for ±15V output, but the basic design can be used for other voltages too. Just substitute the 7815 and 7915 with the desired regulators, and use a transformer that supplies sufficient voltage (and make sure your capacitors can handle the voltages). To figure out what transformer is needed, take the desired output voltage, for example 9V to get ±9V output, add the drop-out voltage of the regulator (usually about 2.5V, but we’ll use 3V to be safe), bringing us to 12V, then divide that by the square root of two, giving us roughly 8.5V, which means we’ll need at least a 8.5-0-8.5 transformer.

Such a transformer will probably prove hard to find, and so a 9-0-9 transformer will most likely be chosen. When using regulators such as those shown here, be sure that the output from the transformer never exceeds the maximum input voltage specified by the datasheet (probably ±35V). In other words, don’t use a transformer with secondaries greater than 24-0-24, and even that is cutting it fine as if the mains voltage goes up, the rectified voltage may well exceed the limits of the regulators.

The switch must be one capable of mains usage, so something huge and heavy duty won’t hurt. The fuse is there for protection in case the transformer primary winding short circuits or something else horrible happens.

The diodes are stock standard 1N4004s, but anything in that series should work fine.

C1 and C2 should be rated higher than the rectified DC (as shown, they should be rated to 35V, with 25V as the absolute minimum). The transformer output multiplied by the square root of two gives the rectified DC voltage, so 15 multiplied by the square root of two is roughly 21V, so the capacitors should be able to withstand voltages higher than that (you don’t want them to explode; trust me).

C3, C4, C5 and C6 serve to stop the regulators from oscillating, and any type will do. (I like ceramics, but polyesters work fine too.) They’ll probably be rated to 50V, so you won’t have to worry about them exploding or anything.

C7 and C8 must be rated to voltages higher than the output and in this case, 16V will probably do (since the regulators are accurate to ±4%) but higher (25V) is recommended.

Remember to earth the chassis of the power supply and the transformer (if it has a chassis) to prevent electric shock.

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