Question concerning power for Raspbery Pi

Cory,

I've used the 7805 for many years to power my TTL projects. They're dead simple to use, provide up to 1.5A, and they're dirt cheap. Feed it 12V and it will give you 5V. (But do read the specs on whatever specific device you get.)

https://www.sparkfun.com/datasheets/Components/LM7805.pdf

--Jeff

The 7805 will work but is lossy (18W input power vs. 7.5W output power) and will need a heat sink to dissipate the 12W difference. Not good for battery operation. Pros are there are multiple sources and been around forever making it a good candidate for new products that have a long life cycle and reliable power source.

At 96% efficiency the KISR33 will have input power requirement of around 8W and will half the energy usage from a battery. The actual performance may be less than 96% because efficiency depends on a number of variables such as input voltage and load current but the efficiency is almost always better than linear regulators.

I thought about using the 7805, but the reason I did not is as you explained. I believe is was Revision 3 that did some videos on making a usb battery pack that I watched, where they used the 7805 and it would not power some devices.

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I went ahead and ordered the KIS3R33S Step-Down Module considering it already has a female USB port and is easy to set up.

Here is a sneak peak at what I currently have finished of my custom power supply.

As you can see its not complete. I am waiting on the KISR33 to arrive and I havnt yet ordered the TFT LCD I am going to use.

To simply run down on whats going on, I used a basic prototyping board to wire everything up. Power goes into the Size M DC Power Jack that can be seen on the left side. The positive wire runs to the switch you can see on the right which then runs to the PCB. The negative wire from the DC Power Jack runs directly to the PCB beside the postive connetion. (No worries, there is no short) The LED has a built in resister which you can kind of make out on its negative wire, this i connected directly to the power supplied to the board. The Black and Yellow wire you see to the left is the wire I will be using for the TFT LCD which is a direct 12v so its also connected to the power supplied to the PCB. The red and black on the bottom will go to the KISR33. Everything is encased in a recycled iPod touch box.

IF anyone can spot any errors in my work or have any suggestion, please let me know. I know I am a beginner, so any critisicm and helpfull recommendations are welcomed.

My only concern right now is the power cord I am currently using, (recycled I.T.E Power Supply I am not sure went to) is your basic 110-120VAC 60hz 190mA input with an output of 12V DC 750mA. I am worried that 750mA would only be enough current to efficiantly power the Pi, not the TFT LCD alongside it. Thoughts?

You are right to be concerned about your 12V supply. You can calculate the power requirement by adding the loads.

LCD Power + Pi Power + USB Power

                    75 %

LCD Poweryou can get from the datasheet

Pi Power = Pi chip max operating  current * 5V / efficiency (I would leave margins and use efficiency of 75%)

USB Power = 2A * 5V (max per 2.0 spec) / efficiency

You will need minimally a 12V/10W power supply just for max USB power requirements. Everything else should be fairly low power.

I would look for a 12V/15W power supply which will be about double what your have.

Thank you for the info, I will look around for a 12v 1500mA - 2000mA supply.

Thank you for the info, I will look around for a 12v 1500mA - 2000mA supply.