Raspberry Pi 3.3 V Power Supply

As well as a power supply for the Raspberry Pi, with a few more components it is possible to create a convenient 3.3 V rail for those low power breadboard projects. However, this part is optional and not necessary.

The TS2950CT is a 3.3 V voltage regulator, which is available from many on-line shops, and is very handy if you have a local Maplin electronics shop near you. This regulator can accept an absolute maximum voltage of 30 V at the input, whilst the LM7805 can accept a maximum of 35 V at the input. However, around 12 V for both is the limit I am placing to be on the safe side.

Smoothing Capacitors

C5 provides filtering for both voltage regulators so a nice large value around 1000 µF will be good enough.

C3 does not need to be very large, but anything between 110 µF and 330 µF will be good enough. These electrolytic capacitors must have a rating above 16 V.

C1 is for noise decoupling and is typically a ceramic disc type. Anything in the range of 47 nF will work to filter out digital switching noise on the voltage planes.

SMD Polyfuse F2

The typical current rating of the TS2950CT-3.3 voltage regulator is 160 mA, and the absolute maximum current rating is 250 mA. A fuse with a hold current of around 160 mA provides a good margin of safety and will provide more than enough current to power a couple of logic chips.

Although both of the voltage regulators have built-in current limiting and thermal limiting circuitry for short-circuit protection, F2 and F3 are there to be on the extra safe side. A polyfuse will often exhibit a voltage drop, and your application may not require them if it is a fixed circuit for a single purpose.

Make a Breadboard Power Supply

An independent 3.3 V power supply for breadboard projects is useful, and ideal thus avoiding the use of the 3.3 V available on the P1 header socket of the Raspberry Pi. This build has some header pins for tapping off the voltage to a breadboard. The grey coloured pads signify header pins. I am using a double row of header pins, as I did not have any single row available. I managed to buy 10 sticks for around a pound from eBay.

The great thing about this power supply circuit is that not only can you power the Raspberry Pi from it but you can also use it to power small breadboard projects. This minimizes the wires and adapters used.

SMD Build

You can also get some cheap SMD voltage regulators on eBay. The AMS1117 costs around a pound for a quantity of ten. If you have never used SMDs then this might be the one to start with because it is a convenient size to solder onto a circuit board. In the following links, I have a power supply build using SMD for those with a little more experience.

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