It works!

Yay, I received my Pi at last!

Following John's good precedent, I measured my polyfuse resistances to be:

F1 = 4.0 ohms

F2 = 3.2 ohms

F3 = 0.3 ohms (but maybe close to 0.2, occasional flicker), LS digit questionable

These values are a lot higher than John's.  I used two different DVMs, one of them a costly Fluke (although not calibrated), but neither has probe lead resistance nulling (I subtracted the 0.2 and 0.3 ohms indicated when the probes are shorted).

Allowing for the lead offsets, the two DVMs track each other perfectly across this range, so I have confidence in the results.  I don't regard the F3 reading as reliable, since it's on the least significant digit displayed.  It's too high for comfort though (0.5 and 0.6 ohm readings on the DVMs) --- in effect the board's +5V is unregulated because of it.

F1 and F2 are outrageously high of course, and do more harm than good by deregulating the supply even further.  This is a plain and simple design fault.  Also, the fact that F1 and F2 have so greatly differing resistances is a worry --- clearly this component does not have a tightly defined resistance parameter, even worse when compared with John's.

I haven't powered the board up yet, as I must be the only person on the planet without a micro-USB charger.

I'll pop out and buy one today, or an adapter for a barrel connector or something.

Morgaine.

Haha! so much time on the forums and then didn't prepare for the coming of the 'pi.

There are way to many people who say things like: this won't work. just from a theoretical viewpoint.

0.3 ohms at max 0.7A will give you 0.2V maximum drop.

With a proper regulated 5.0V powersupply, that will give you 4.8 to 5.0V on the line labeled "+5V" on the raspberry pi.

Why the F*** do you think that won't work? The 5V is connected to the 3.3V LDO. That will work downto 4.3V AT LEAST (i.e. at maximum load on the regulator, but the 'pi is using less). The 5V is connected to the line labeled "VBAT" on the BCM2835. That leads me to suspect that this is intended to work on a single cell LIPO. So it should work downto about 3.0V, but AT LEAST to 3.3V.

Next it goes to the USB ports. If you read the USB specs, devices are required to work down to somewhere in the low 4V. Not all do, but they are not USB compliant. They migth work on lots of other USB ports that do deliver a higher voltage, but the remain out of spec.

So with a worst-case 4.8V (4.9 in practise) at the top of F1 and F2, and a 100mA max powerdraw, we'd loose another 0.4V with the 4 ohms on those fuses. That's indeed close to spec.

If an USB device doesn't work, you have several options.

* Get a different device that does work.

* Get a powered hub in between.

* short out F1 and/or F2.

* if that doesn't help enough, short out F3 as well.

When you start shorting out those fuses, you'll be relying on the current limiting on the power supply. No big deal. Connect it to a PC powersupply and start shorting out things on the GPIO port and you'll be able to start a fire. Don't do that then!

Roger Wolff wrote:

 

With a proper regulated 5.0V powersupply, that will give you 4.8 to 5.0V on the line labeled "+5V" on the raspberry pi.

And what exactly does a "proper regulated 5.0V powersupply" have to do with a USB cellphone charger?  Virtually nothing in practice.  It's no use talking theory, or saying there's a proper spec for USB chargers, when massive experience in the Pi community has shown that cellphone chargers are very commonly unadulterated junk, and very frequently you have to try several before you find one that works at all even when they claim adequate amperage.

It's wrong to think of it as a "power supply", which conjures up ideas of stability, regulation, protection, and even some reasonable headroom.  It's just a charger, and it's made to the lowest possible spec and with the worst quality materials they can get away with.  The cellphone market is totally cutthroat and cellphones are given away free with phone contracts, so it doesn't surprise me in the slightest that the situation is as bad as it is.

The end result is not a supply voltage you can rely on, and making the situation worse with extra drop across a 0.3 ohm polyfuse is not clever when the Pi is specifically targetted at using cellphone chargers as its power source owing to the micro-USB connector.

Why the F*** do you think that won't work?

FUSE?

To answer seriously though, it will work in many cases, but it's marginal ,and it's poor design because it causes substantial loss of regulation.

The worst part of this though is the coupling together of distinct power paths, totally unnecessarily.  Nothing you connect to the Pi can expect a stable voltage source even if you provide one at the micro-USB input, because F3 deregulates it for all sinks on the board, including the two USBs and the P1 header.  Under no circumstances can the Pi's power design be considered satisfactory when it turns a regulated supply into a highly unregulated one.

Just because the nominal +5V for the two USBs might stay within USB spec (if you're lucky), this doesn't mean that it's good power design for something that is powered by P1-pin2 to experience a sizeable voltage drop whenever USB usage changes.  There's nothing wrong with using cheap polyfuses, but the power paths should not be coupled in this way.  F3 should control the SoC alone, F1 and F2 should tap off the power input connector, and P1-pin2 should have an independent polyfuse also tapping directly off the input connector.  F3 as it stands is a misdesign, and all the trip values are too low.

Morgaine.

Roger Wolff wrote:

 

With a proper regulated 5.0V powersupply, that will give you 4.8 to 5.0V on the line labeled "+5V" on the raspberry pi.

And what exactly does a "proper regulated 5.0V powersupply" have to do with a USB cellphone charger?  Virtually nothing in practice.  It's no use talking theory, or saying there's a proper spec for USB chargers, when massive experience in the Pi community has shown that cellphone chargers are very commonly unadulterated junk, and very frequently you have to try several before you find one that works at all even when they claim adequate amperage.

It's wrong to think of it as a "power supply", which conjures up ideas of stability, regulation, protection, and even some reasonable headroom.  It's just a charger, and it's made to the lowest possible spec and with the worst quality materials they can get away with.  The cellphone market is totally cutthroat and cellphones are given away free with phone contracts, so it doesn't surprise me in the slightest that the situation is as bad as it is.

The end result is not a supply voltage you can rely on, and making the situation worse with extra drop across a 0.3 ohm polyfuse is not clever when the Pi is specifically targetted at using cellphone chargers as its power source owing to the micro-USB connector.

Why the F*** do you think that won't work?

FUSE?

To answer seriously though, it will work in many cases, but it's marginal ,and it's poor design because it causes substantial loss of regulation.

The worst part of this though is the coupling together of distinct power paths, totally unnecessarily.  Nothing you connect to the Pi can expect a stable voltage source even if you provide one at the micro-USB input, because F3 deregulates it for all sinks on the board, including the two USBs and the P1 header.  Under no circumstances can the Pi's power design be considered satisfactory when it turns a regulated supply into a highly unregulated one.

Just because the nominal +5V for the two USBs might stay within USB spec (if you're lucky), this doesn't mean that it's good power design for something that is powered by P1-pin2 to experience a sizeable voltage drop whenever USB usage changes.  There's nothing wrong with using cheap polyfuses, but the power paths should not be coupled in this way.  F3 should control the SoC alone, F1 and F2 should tap off the power input connector, and P1-pin2 should have an independent polyfuse also tapping directly off the input connector.  F3 as it stands is a misdesign, and all the trip values are too low.

Morgaine.

Morgaine Dinova wrote:

 

And what exactly does a "proper regulated 5.0V powersupply" have to do with a USB cellphone charger?  Virtually nothing in practice.  It's no use talking theory, or saying there's a proper spec for USB chargers, when massive experience in the Pi community has shown that cellphone chargers are very commonly unadulterated junk, and very frequently you have to try several before you find one that works at all even when they claim adequate amperage.

Nobody has convinced me of such a thing. Before the first raspberry pi's were being delivered, I started making noises that even though the power supplies might be up to spec, most cables are not.

We have measured whole ohms in cables we had lying around.

Of course, people have reported that when they got a better USB-charger the pi suddenly started working. But with the high quality charger, they also got a better cable. So it's quite possible they used the new cable as well. And of course they stopped testing the moment the 'pi started working. So it's the new usb charger that did the job.

Pete Lomas who, as far as I know, worked on both the schematics and the layout of the 'pi. Has his habits. He's overly cautios about overvoltages, decoupling capacitors, and fuses. So there are a bunch of "extra" capacitors on the board that wouldn't have been necessary. There are a bunch of clamping diodes that 99% of the users will never use. And there are a bunch of fuses that some (including me) think are redundant.

And a 0.14V voltage drop in practice is the consequence of the fuse. So what?