Wednesday 22 October 2014

Make your Raspberry pi b more power efficient with a cheap dc step down

Modified RasPi B with a buck converter

The raspberry pi b (the original one, not the b+) uses a linear regulator to bring the 5V down to 3.3V. That's a lot of waste: 33% of the power consumed by the pi is dissipated as heat here! A DC step down (buck converter) would definitely do better than 66% of efficiency. This would certainly improve the stability when running on a weak power supply and make it run much longer if on batteries. Without making any precise calculations, using a buck converter would make the raspberry pi consume somewhere between 20 and 30% less power (efficiency of about 90% vs 66%). Here is how I did it in 10mins for 1.29 USD:

Instead of building a dc step down myself or buying one from a recognised electronics dealer, I chose to buy a cheap one from eBay for 1.29 USD:

Here are the specs of the 'Mini-360' buck converter:
  • Input voltage: 4.75V-23V
    GOOD: Our input voltage is 5V 
  • Output voltage: 1.0V-17VOutput voltage: 1.0V-17V
    We want a 3.3V DC as output which is also in the range. We are here 1.7V lower than the input and is no problem with this converter but some of them require a voltage difference of e.g. 1V. We had no point of knowing here as it was not specified but I was pretty confident as 1.7V is already a fairly big difference.
  • Current output: peak 3A, 1.8A long time
    This should be fine as well since my Raspberry Pi runs on a 5V 1A power supply.
  • Transfer efficiency: 96% (highest)
    We should therefore expect something lower than 96%. Too bad we don't get a formula. The good news is, the closer the input is to the output the higher the efficiency.
  • Swtiching frenquency: 340KHz
    Higher frequencies decrease the efficiency for a given input/output ratio but also impacts the transient response (see: reference). 
  • Output ripple: 30mV(no-load)
  • Load regulation: ±0.5%
  • Voltage regulation: ±2.5%
  • Operating temperature: -40℃~+85℃
I then decided not to remove the regulator (unlike this mod) but to cut its pins and solder wires to the dc step down to the pads on the pcb instead. 
Wiring the dc step down converter to the PI

The best way to wire the regulator is to solder GND (black wire) and Vin (yellow wire) first (you don't need to wire both GND on the converter as they are common). Connect the power to the RPi and measure the Vout (blue wire) with a multimeter (put the red test lead of your multimeter on Vout and the black one one GND) and turn the screw of the converter until your multimeter reads 3.3V DC. You can now solder Vout (the blue wire on the picture) to the PI. You're done!

The Raspberry Pi modded to use a DC step down converter
instead of its linear regulator.

This project cost me 1.29 USD + 10mins and works perfectly!!
(it actually took me 30min to wirte this post which is nonsense)