Thank you for the answer.Burngate wrote:Yes you can. But ...
If you connect to the 5v and ground pins on the GPIO connector you will be bypassing the poly-fuse. Also there is the risk of connecting it the wrong way round. But it will work
Since the Pi uses about 3/4 A and the LM7805 is a linear regulator, the same current passes through the regulator from the 9v source
This means about 3W will be dissipated in the regulator - it will get hot without a heatsink. Also, if the 9v is coming from a battery, it will go flat that much faster.
Use a switching regulator
Oh, thank you very much.mahjongg wrote:Yes, you could build your own switcher, but why not simply buy a cigar plug to USB port adapter. Used to power your USB device (like a iPod) from the cigar lighter of a car. These often can be bought for less than $5, and already contain such a switcher, plus they have an USB output port so you can directly plug in a standard USB to micro-USB cable, no fussing around with other power entry problems which bypass the polyfuse. By the way, a normal fuse would have too much resistance to be used, or they would have such a high current rating that they would make no sense to use.
the adapter is often easy to disassemble, try unscrewing the top. After that you are left with a board, and wires running to the top contact, which is the + input, and side blades, which is GND (the - input).
That's probably why it couldn't use Ethernet. And could only slowly boot some initial stages from memory card.gordon77 wrote:and interesting the datasheet says maxm discharge is only 120mA, not much use for a Pi.
Thank you. I'll look into it. But also I'm going to make some more measurements with my current circuit.[email protected] wrote:Old thread resurecting
I've been looking at some "car charger" units as of late for another project - ie. take 12v turn it into 5v (or 19v for laptops!) and I'm not convinced some of the off-the-shelf ones are quite as efficient as they could be - and they probably don't have to be either - afterall, the car will be running most of the time, thus charging the main car battery, and even when its not running, the car battery is big enough to run most 5v USB power stuff for some time anyway. The one I currently have in my car to charge my phone runs very hot - my bet is that there is nothing more than a simple 7805 in it ...
So if you're a dab-hand with electronics then using one of the SMPS regulator chips and building up a circuit around it isn't too hard.... Alternatively I've used this in the past:
http://uk.farnell.com/xp-power/sr10s05/ ... tt=sr10s05
it's a drop-in 7805 equivalent, but uses SMPS technology.
There are others just like it - a bit more expensive than a bog-standard 7805, but personally I think they're worth the extra just because you don't have to make your own from components and it really is a drop-in replacement.
There seems to be a few alternatives out there - e.g.
http://www.rapidonline.com/Electrical-P ... ries-78072
as well as the multitude of ready built ones on ebay with adjustable outputs too, but they're typically not a drop-in for the 7805.
The "UBEC" devices have been used in the (electric powered) Radio Control world for a long time - they're just another SMPS device - UBEC meaning Universal Battery Eliminator Circuit - designed so you can power the electronics off the main power drive battery - usually 11-14 volts (ish) when you need 5 for the electronics, rather than having a separate battery for the electronics - hence the term battery eliminator...micedwards wrote:These step-down converters look like a much better choice than traditional 7805s ...
I have no experience with them, but their efficiency is very high (~95%) and they look like exactly what you need.
Now that beats my ipadSonny_Jim wrote:I've used a cheap UBEC along with an 18V battery from a cordless drill, worked for about 12-14hrs as an wireless AP.
Change that clunky old 7805 for a cheap UBEC circuit. It won't get hot and your battery will last longerAlek wrote:I am using a 7805 with a lithium batterie that I found in my house and it seems to be working just fine for me, but the 7805 chip does get a little hot when powering the raspberry pi. I will probably have to add a heat sink onto to the 7805 to keep it from getting to hot!