RG1 1.8v regulator

Pete is a conservative designer. He likes the safety measures like fuses, decoupling caps, protection diodes etc etc.

In this case, the fuses caused a lot of trouble. If you like to blame something else, fine. I'm going to do other things.

Removing the polyfuses does not change a poor power design which was modified to cut costs from the original design on the Alpha board.

I respect your opinion and if you want to stick to it fine, your opinion, feel free to do other things.

I'm not judging Pete as a designer, I'm judging the final product.

It is not exactly the same thing but you can watch this video to see how poorly some of these power adapters are designed/made.

http://www.youtube.com/watch?v=T88ej64aXUM&feature=player_embedded

BTW I didn't search for the video, it was posted while long ago on the Rpi forum.

-J

selsinork wrote:

In my case, the quality of the usb charger was never a question. The 0.5v drop across the crappy micro usb cable was the real problem. For that reason alone I agree with jamodio that the idea was a bad one.

 

Note that they didn't make any effort to promote better cables or better power supplies. It seemed much more like "everyone has a micro usb charger and it'll work with any old crappy charger and cable you have lying at the back of the cupboard". It's the combination of a cost constrained design with the flippant attitude that makes it a bad idea.

Hindsight is always 20-20, but in hindsight it's difficult to argue that it was anything other than the wrong choice, even when it's understandable why the choice was made.

 

Thing is that a crap charger and even worse cable probably works just fine as a phone charger simply down to them spending a few pennies extra on a decent power system... The same would likely be true for the Pi...

My opinion is that the RasPi's power design "seemed like a good idea at the time", and given the official USB specs for power supplies and (possibly) cables there was every reason to believe it would work.  The same is mostly true for polyfuses as well.  However, the reality is that a lot of imported USB devices and cables and polyfuses do not meet specs, and it is usually impossible to tell when you order the devices.  The Micro USB cable is a prime example: you have no idea what wire gauge the cable uses for power and ground when you order it. That cable should certainly be included -- I don't remember how many times I've advised people here and at the RasPi forum that when multiple power supplies fail to produce enough RasPi voltage, check the Micro USB cable.

My own favorite example of cables not meeting specs is cheap HDMI cables that don't connect the individual ground pins.  Instead they rely on a foil shield connected to the connector shells on both ends.  This works fine in most HDMI applications, but if your application needs the ground pins you're in trouble.

Ok, agreed, that's one crappy USB powersupply that won't power a 'pi.

selsinork wrote:

 

Tooms wrote:

 

Now i just need to build an 1.8V and 2.5V board and then do testing with the RPI board.

personally I'd not bother with the 2.5v one, going by the schematics it's only used for the composite video and doesn't look like it'll ever have significant current requirements, so an LDO will be fine.

yes your right, there is no point in the 2.5V

Tooms

However, the reality is

  there's a universal truth in there somewhere that reality never meets the spec.

I wonder how much of the conservative engineers outlook of working out what's required, doubling it and then adding a bit of extra margin is because he knows that somewhere there's a beancounter who'll force a reduction for 'cost' reasons and to hell with the spec.

The Pi being quite up front about everything being to meet the $25 price point probably suffers in that there was never any margin for the beancounter to cut...

Ok, agreed, that's one crappy USB powersupply that won't power a 'pi.

That case design is quite common for UK plugtop supplies for cheap electronic stuff. I have had several just like it, most of which died fairly quickly. Not hard to see why.

Guys. Just FYI, my raspberry pi boots just fine on 3.7V.

While slowly turning the voltage down, it crashed somewhere between 3.7 and 3.5V. That's when the "link" leds went off. It didn't come back up when I slowly turned the voltage back to the "last known working"  voltage of 3.7 and it DID come back up when I powercycled it from there.

My 3.3V has dropped down to 2.7V. So the dropout of the LM1117 RG2, is about 1V. That's not very impressive for an LDO.

So, if you want to keep your 'pi running, you have to keep your 5V above 3.7V. This is "actual device measurements". For "nominal operation" (e.g. to have the 3.3V within spec), you need about 4.3V, to have some margin, good engineering practise would set the  require met at "at least 4.5V" at the input.

But notice that you have to go almost a whole volt below that before the 'pi stops working. That's proper engineering.

The things that eat away at this margin are:

* crappy powersupplies that don't deliver 5V at 400mA.

* crappy powercables.

* the polyfuse.

Roger Wolff wrote:

 

Guys. Just FYI, my raspberry pi boots just fine on 3.7V.

My RasPi system works down to 4.65V.  Below that my DVI-D monitor (connected through an HDMI Switcher) starts to misbehave.  I guess your peripherals are a lot more tolerant of low 5V than mine    LM1117 dropout is indeed about 1V according to its spec sheet.

Most 3.3V parts allow 5% - 10% tolerance, so I wouldn't let it go too low.  The previous low voltage I've heard of was 4.2V, which reduces 3.3V to 3.2V.  Below 3.3V you'll start losing Ethernet drive, but if you're only driving a 2m cable instead of 50m you'll be fine.  I would think you'd want to have at least 4.3V so RG2 is producing a precisely regulated 3.3V instead of propagating any misbehavior on the 5V0 net.

Proper engineering would require you to do the test at both end's of the Pi temperature range.

It would also require to repeat the CE testing at the lower voltage, as it will likely make it more sensitive to radiation.

I really don't see what the test is supposed to prove?

The second test was usefull to prove that it isn't representative for all Pi's.