I found the following web page which describes how to construct a power supply for the PI - http://elinux.org/RPi_5V_PSU_construction
A few things I've noticed that I am slightly puzzled for example there is a resistor network applied to an 7805 to increase the output voltage to 5.27volts.
At the bottom of the page there is also the mention of creating 5.75volts from a 7805 regulator.
My understanding is the PI only needs 5v so why the increase in output voltage??
Furthermore the circuit also has 340ohms (10 + 330) connected in series with the output of the regulator this seems crazy to me because you are wasting a lot of energy as heat.
If you need to increase the output voltage of the 7805 regulator why not use replace the 10ohm resistor with a schottky diode for 5.3v output or a silicon rectifier diode for 5.7v output and do away with the 330ohm resistor??
This circuit also includes a transient suppressor but why is the fuse after and not before surely if the regulator goes bad the fuse needs to blow asap not if you're exceeding the current requirement from the output terminals??
Richard S.
Re: homebrew power supply from eLinux.org??
The 340R across the 5V25 ouput will eat less than a tenth of a Watt if my (admittedly dodgy) mental arithmetic is right.
The 5V25 putput is the top end of the USB voltage spec and provides a bit more headroom for sag caused by those pesky polyfuses.
A transient supressor is just that - it clips momentary voltage spikes and isn't really meant for crowbarring long term overvoltages. I wouldn't bother with one.
Although a maximum input voltage of 35V is quoted, remember that this is only the maximum input voltage of the 7805 and it must be kept within it's dissipation limits with due regard to the total current consumption of the Pi + attached peripherals and the voltage dropped across the regulator, thermal resistance of the heatsink assembly, etc. (plenty about this on Google).
The 7805 is a nominally 1 Amp device (although thay can deliver more) so you'll have at least 500mA spare for the USB ports, I think. Remember that the polyfuses will cry "enough!" before this limit is reached.
7805's are cheap, so a more elegant solution may be to use three - one for the Pi (into it's stock power port will be fine) and one hardwired to each of the USB ports (with the polyfuses removed). Obviously soldering will be required (and Pi warranty voided), but no resistor networks will be needed and the regulators will run cooler and require less heatsinking. IIRC the 7805 has a short circuit current limit of 250 mA so current limiting protection on the USB ports can probably be lived without in almost all cases *chooses words carefully*
Thanks for posting.
The 5V25 putput is the top end of the USB voltage spec and provides a bit more headroom for sag caused by those pesky polyfuses.
A transient supressor is just that - it clips momentary voltage spikes and isn't really meant for crowbarring long term overvoltages. I wouldn't bother with one.
Although a maximum input voltage of 35V is quoted, remember that this is only the maximum input voltage of the 7805 and it must be kept within it's dissipation limits with due regard to the total current consumption of the Pi + attached peripherals and the voltage dropped across the regulator, thermal resistance of the heatsink assembly, etc. (plenty about this on Google).
The 7805 is a nominally 1 Amp device (although thay can deliver more) so you'll have at least 500mA spare for the USB ports, I think. Remember that the polyfuses will cry "enough!" before this limit is reached.
7805's are cheap, so a more elegant solution may be to use three - one for the Pi (into it's stock power port will be fine) and one hardwired to each of the USB ports (with the polyfuses removed). Obviously soldering will be required (and Pi warranty voided), but no resistor networks will be needed and the regulators will run cooler and require less heatsinking. IIRC the 7805 has a short circuit current limit of 250 mA so current limiting protection on the USB ports can probably be lived without in almost all cases *chooses words carefully*
Thanks for posting.