Power supply WARNING

[BlueClogger]

Burngate said:


Ohm's Law: V=IR, or R=V/I
The implication is that R is a constant for a given object. Turns out to be not neccessarily true for all objects.
So we redefine Ohm's Law: r=dV/dI
r is now a variable, and is calculable. Ohm's Law is now not a law, just a definition of resistance, which can change with circumstances.
Power supplies: for simplicity, we assume it can be modelled as a constant voltage source with a built-in (constant) source impedance.
Then measuring its open-circuit (no load) voltage gives us one parameter, and measuring its short-circuit current gives us the other.
But if we have something - a length of naff cable for example - between the PSU and our meters, we are going to get a different (wrong?) answer. Having another meter gives us the opportunity to characterise that naff cable as well.
I buy a PSU complete with cable, and it says "5V 1A", I expect to get that out of the end of the cable. If I find I can't, I can look into it. Maybe it's just the naff cable, maybe the PSU before the cable, or maybe both. Interesting, either way, but at the end of the day, not useful - I can't ask the supplier for just a new cable or just a new PSU, he'll just replace them as one item.
More to the point, even if that PSU had done what the tin said - give out 1A @ 5V - by the look of it it could roast my Pi, kill me, or burn my house down. Or someone else. Not only should we keep well away from these things, we should make a song and dance to get them taken off the market.
Like nuke China, perhaps



Don't do that - they're building the Raspis!

[adlambert]

I've found an old 6V 500mA transformer in a drawer and I've desoldered a bridge rectifier out of an old project. Now I'm looking for a 7805 and a nice smoothing electrolytic, I know I've got them somewhere…..

[TorC]

[quote=adlambert]I"ve found an old 6V 500mA transformer in a drawer and I"ve desoldered a bridge rectifier out of an old project. Now I"m looking for a 7805 and a nice smoothing electrolytic, I know I"ve got them somewhere…..

Two things to consider.  First, the B model draws up to 700mA at 5V - a little close there.

Second, I'm no EE, but as I understand it, a diode has a .7V drop, which means that a bridge rectifier fed 6V will put out 4.6V - a little underpowered for the Pi to start with.

Tor

[error404]

If it's a 6VDC supply it will already be rectified (and if it's 6VAC, that's 6V RMS, or ~8.5V peak). Most are unregulated, so it probably puts out somewhat more than 6V when unloaded, dropping to ~6V at full rated load.

A 7805 needs about 2V of drop across itself to stay in regulation, so really you'd want at least a 7V input with that chip. You could use a low-dropout regulator instead, which would probably be okay enough to work, but really this is not ideal. It will also generate quite a bit of heat.

[Jaseman]

I can see a sudden demand for 10,000 power supply units at the end of January.  Can the Pi Store handle that?

[plugwash]

Burngate said:


Power supplies: for simplicity, we assume it can be modelled as a constant voltage source with a built-in (constant) source impedance.


However this is an oversimplified model, a decent regulated power supply will change very little in output voltage across it's defined load range then will start to drop off after that.



Then measuring its open-circuit (no load) voltage gives us one parameter, and measuring its short-circuit current gives us the other.


For characterising a regulated PSU you are better off measuring no load voltage and full load voltage.



But if we have something - a length of naff cable for example - between the PSU and our meters, we are going to get a different (wrong?) answer.


It's not wrong per-se, it's just measuring the combined performance of the PSU with cable rather than the performance of the PSU alone.


Having another meter gives us the opportunity to characterise that naff cable as well.


Indeed


I buy a PSU complete with cable, and it says "5V 1A", I expect to get that out of the end of the cable.


Indeed to within reasonable tolerances, some voltage variation is normal and acceptable


If I find I can't, I can look into it. Maybe it's just the naff cable, maybe the PSU before the cable, or maybe both. Interesting, either way, but at the end of the day, not useful - I can't ask the supplier for just a new cable or just a new PSU, he'll just replace them as one item.


Indeed.


More to the point, even if that PSU had done what the tin said - give out 1A @ 5V - by the look of it it could roast my Pi, kill me, or burn my house down. Or someone else. Not only should we keep well away from these things, we should make a song and dance to get them taken off the market.


Unfortunately it's like whack-a-mole, you can get one vendor shut down and a recall notice put out but there are plenty more to fill the gap left by doing so.

The only way to solve this is to either solve the problem at source in china (good f*cking luck) or to heavilly clamp down on the likes of ebay and amazon (and I think this cure may be worse than the disease)

[lAlarik]

Do you guys reckon an iPad or iPhone wall charger will work with the Pi?

Otherwise, plugging the USB cable into a running pc port would make it?

[Chromatix]

No reason why an iDevice wall charger shouldn't work, as long as you have the right cable to run from it to the R-Pi.  Mine came with a cable that would only fit the iPhone dock.

[lAlarik]

Yeah i have a normal USB to USB mini B plug from a Nokia phone so that should make the trick

[Chromatix]

It will need to be a micro-B, not a mini-B plug.

[lakyljuk]

And what about this one?

5V 2A 10W Regulated Switching Power Supply

[mgmt_idiot]

It's just personal preference, but I'm planning to use this one:

http://www.maplin.co.uk/compac.....ply-562310

Though I'm also thinking of using the charger that came with my Kindle

http://www.amazon.co.uk/Amazon.....038;sr=1-1

[plugwash]

lakyljuk said:


And what about this one?

5V 2A 10W Regulated Switching Power Supply



The specs are fine for the Pi so if it lives up to those it will work fine.

Trouble is it's from a seller in hong-kong and it's of a brand i've never heard of. Ultimately the only way to tell if a PSU is safely made is to destructively dismantle it, since most people aren't going to do that unless they are buying hundreds buying direct from china can be a risky buisness.

Also that PSU has exposed screw terminals for connection which means you will have to add additional casing to use it safely.

[cheapskate]

As with other things, price is no indication of quality. The fact that you can buy lousy open-loop 50p psus from China on eBay for £3 today, doesn't stop you buying the same thing in a nice blister pack in a shop for £18 tomorrow. There really is no way of preventing this.

Being as RasbPi is only £25, it kinda hurts to spend another £15-£20 on a suitable PSU.

[Tass]

WOW!!  You're all such nerds

Only 2 real questions that need to be answered here:

1) Will I be able to buy a PSU with my Pi?

2) Will it have a lovely RPi logo on it?

[Burngate]

1) Yes though it might be some while off

2) Don't know, but if it hasn't, a couple of pots of nail varnish might help.

[Prometheus]

Burngate said:

2) Don't know, but if it hasn't, a couple of pots of nail varnish might help.

Or one of the keyboard-stickers!

[cheapskate]

Tass said:


WOW!!  You"re all such nerds

Only 2 real questions that need to be answered here:

1) Will I be able to buy a PSU with my Pi?

2) Will it have a lovely RPi logo on it?



cheapskate said:


As with other things, price is no indication of quality. The fact that you can buy lousy open-loop 50p psus from China on eBay for £3 today, doesn't stop you buying the same thing in a nice blister pack in a shop for £18 tomorrow. There really is no way of preventing this.

Being as RasbPi is only £25, it kinda hurts to spend another £15-£20 on a suitable PSU.



Follow up. I've just tried one of these: http://www.amazon.co.uk/gp/pro.....B003E3YQNM

It's 6 quid, and my example has a no-load output voltage of 5.08V, dropping to 5.04V at 990mA. That'll do!

I also have one of those £2 cheap ones : http://www.amazon.co.uk/gp/pro.....B002TQUDBO

That unfortunately outputs 5.6V, and is still pushing out 5.5V at 1A! Fine for charging, bad for USB peripherals.

[drgeoff]

Maplin don't seem to stock micro-USB plugs so I crossed the road to Poundland and bought a "Retractable USB cable for Blackberry and Kindle".  The plug on one end is the same as the leftmost one in the photo.   http://en.wikipedia.org/wiki/F.....ectors.JPG

So I'm ready to go.

[grumpyoldgit]

drgeoff said:


Maplin don't seem to stock micro-USB plugs so I crossed the road to Poundland and bought a "Retractable USB cable for Blackberry and Kindle".  The plug on one end is the same as the leftmost one in the photo.   http://en.wikipedia.org/wiki/F.....ectors.JPG

So I'm ready to go.


How much did it cost?

[drgeoff]

Grumpyoldgit said:


drgeoff said:


Maplin don't seem to stock micro-USB plugs so I crossed the road to Poundland and bought a "Retractable USB cable for Blackberry and Kindle".  The plug on one end is the same as the leftmost one in the photo.   http://en.wikipedia.org/wiki/F.....ectors.JPG

So I'm ready to go.


How much did it cost?


The same as everything else in Poundland.

It is branded Signalex which I think is their own brand.

[cheapskate]

I've just ordered an assortment of these:

http://cgi.ebay.com/ws/eBayISA.....0650881112

Cheap way of powering R-Pi from an unregulated source, or even a battery of some kind. Watch this space!

[drgeoff]

cheapskate said:


I've just ordered an assortment of these:

http://cgi.ebay.com/ws/eBayISA.....0650881112

Cheap way of powering R-Pi from an unregulated source, or even a battery of some kind. Watch this space!


Good find.  I've ordered a couple of these.  Thanks.

[Flyballthecat]

The Kaito PST-15U is a rock solid 5v 2A usb supply for those in the US in my experience.

I have used dozens of them in various ways.

All have been spot on at 5v

Thanks for this information, I ordered one off Amazon...Before seeing this discussion, I had picked up a $5 wall wart but it didn't have any specs.