Dan's Data letters #23
Publication date: 20-Jan-2003.Last modified 03-Dec-2011.
Faking it
Just a quick question regarding the Optus cable question in letters #21. If Optus uses the MAC address to verify, is it possible (theoretically of course) to spoof MAC addresses - as is a common feature in a lot of routers - and therefore use someone else's Optus account?
Leyton
Answer:
If you can change the MAC address of the cable modem, then yes. Since
you probably can't, though, then no. The cable modem's own MAC address
is what Optus looks at; the stuff it's connected to on the LAN side is irrelevant.
Some broadband ISPs do authenticate by the MAC address of the network
adapter that's connected to whatever broadband "modem" they provide, which
can be annoying for people who don't want to use the provided Ethernet adapter.
But that's not the case here.
Characterful computers
I bought a new computer from what you can consider to be a DIY shop or a VERY off brand no-name company, which is the usual way you buy computers here in Hong Kong if you don't want a prebuilt box from a major company. It's got an Asus A7V333, AMD Athlon XP 1800+, GeForce4 MX, 256MB DDR SDRAM, CDRW, DVD-ROM, single hard drive and "300W" PSU.
This computer has been flaky as hell since day one. After I installed Windows XP, it hung numerous times without apparent reason on the first day, except it seemed that it liked to hang after I start typing on the PS/2 keyboard!
Then it sort of stabilized for a bit in the next month. Then, this month, it suddenly took on this habit of rebooting itself while I'm booting it up. These days it happens 2-5 times in the first 10 minutes after I switch on the computer, so that the first time XP wouldn't have finished loading before it rebooted... but then it is relatively OK after the first 10 minutes (but still hangs in Counter-Strike...). Seeing as this problem started in winter (here) and goes away after the computer has warmed up, it almost seems like there's some part that malfunctions when "overcooled" rather than "overheated"!
Is this a power problem?
My other computer (Athlon 1133, 128MB RAM, GF2MX, Win 98) is also acting up. It used to have overheating problems, which I fixed by changing the cooler. But now it would also reboot itself for no reason (but much less often than the other computer) and now its GUI and character set is screwed up - the minimize, maximize and close buttons have been replaced by "1", "2" and "T"! Fonts on Web pages are not displayed correctly, with small fonts munged up most badly. I suppose the best thing to do is reinstall Windows? Also, how can I get a bit more stability out of this computer? PSU change again?
Joseph
Answer:
A PSU problem would be my first guess for what's wrong with your
first PC, but there are various other things that could be causing it.
Since another PSU isn't going to cost you a fortune, though, you might as well buy one and swap it in and see if it does any good. If it doesn't, then you've got yourself a spare PSU, which is a useful thing to have anyway.
If changing the PSU doesn't help, then you're down to shotgun debugging - removing/replacing components one at a time to see what, if anything, helps. Start with the RAM, and work from there. Note that it's not necessarily going to be an actual defective component; if the BIOS settings are screwed up (a correspondent has told me of an A7V333 machine that flaked out not unlike yours, because its RAM was set in the BIOS to the rather aggressive "Turbo" mode), that could be it too.
Also bear in mind that it could be something as simple as a mis-seated CPU cooler. Or as simple, and annoying, as an extra standoff under the mobo that's just barely touching a trace, pulling some line or other low as a result, and making the computer flaky. With the machine warmed up, something expands and flexes the standoff out of contact. I'm not saying that that's the problem - I'd bet that if it's not the PSU, then it's actually a bad component - but it could be.
For the other computer - yup, sounds like a software problem, and reinstalling Windows from scratch is likely to cure it (Hey! Look at me! I'm a Microsoft phone support guy!). If the computer's having hardware problems, though (bad RAM, again, could be involved), an attempt at a reinstall could leave it completely unusable.
A PSU change would be worth a try to make the second PC more stable - especially if your new PSU doesn't help the other machine!
Beaucoup batteries
I just got a query from a coworker.
We are installing a system on a mine site and they are concerned that they often have power outages, brownouts etc.
"UPS", they cried, but they want a UPS that will run a modern PC for 12 hours!
I think the PC would be pretty standard, except running two 21" monitors. Say a P4 2GHz, 1-2 HDD's, plus the monitors.
Any idea how many VA I would require to run that for 12 hours? How do you get minutes runtime from the VA rating of a UPS?
Simon
Answer:
The PC you mention shouldn't draw more than 500 watts of real power for
any length of time (that allows for surges, but not for transient
monitor degauss spikes, which could be a lot higher). Divide 500 watts by
a pessimistic power factor of 0.6 to get VA; say 850VA apparent power. The
real power is what the battery bank has to deliver; the apparent power is
what the UPS has to be rated for. This stuff, by the way, is quite well
explained in the PDF document
here.
How do you get runtime from VA? You don't. The VA rating tells you how large a load the UPS' inverter can power; the battery capacity tells you how long it can power that load for. VA is the engine, battery capacity is the fuel tank.
No consumer UPS can come anywhere near running a computer for 12 hours. They're generally made to keep the computer up through an outage of well under an hour, or (more often) to just allow the computer enough time to shut down elegantly. For your application you either need a proper separate-boxes solution - one (not-terribly-big) inverter, one (quite-hefty) charger, one (pretty-enormous) battery bank - or a mid-range UPS that can accept some big outboard batteries. The first option is the more flexible; the second is simpler.
Let's say you're being pessimistic (always a good idea, in power system design) and allowing for some expansion (there's probably something else that needs to be on when the computer is...), and figure on drawing 1000 watts out of the batteries constantly. Then you'll need 12 kilowatt-hours of battery capacity to run the UPS for 12 hours, which adds up to a thousand amp-hours (Ah) worth of 12 volt batteries (or 500Ah worth of 24 volt batteries, to power a 24V UPS; the exact same bunch of batteries can do each job, it's just a matter of connecting them up differently, provided you've got an even number of batteries).
You wouldn't want the battery bank to be completely discharged at the end of its 12 hours, though; you'll be using lead acid because nothing else is anything like as cheap, and lead acid doesn't like full discharge. Regular lead acid batteries will sulfate and become useless if they're left discharged for any length of time; even "deep cycle" lead acid batteries shouldn't be left flat. Plus, capacity will naturally decay as the batteries age. So you'd need to raise the starting nominal capacity to, say, 1500 amp-hours worth of 12 volt batteries.
And you'd need to raise it even further, because even six great big "250Ah" deep cycle 12V lead acid batteries won't actually deliver the 1500Ah you'd expect, in this situation.
A 12 volt battery will only deliver its full rated capacity at a relatively low discharge rate. Lead acid batteries are commonly specified at a "20 hour rate" - the rate of discharge that'll flatten them in 20 hours. Now, the 20 hour rate for a 250Ah battery is a solid 12.5 amps, but you're thinking about a twelve hour discharge, which is rather more ferocious.
At 25 amps (a commonly quoted "heavy load" number), a "250Ah" battery will only deliver about 200Ah. Smaller batteries will do worse; a "35Ah" battery has a 20 hour rate of only 1.75A, and will probably only deliver 20 to 25Ah at 25A.
Fortunately, 12 hour discharge isn't too high a load, so this isn't a major concern for a bank of big fat batteries. But it's yet another reason to over-specify your battery bank.
So let's go for a nice round 2000Ah. That ought to give enough headroom. You're looking at more than 450 kilograms of lead acid batteries to do it.
Thanks to a chap from Battery World who happened to ask me a question about a tiny light bulb right after you asked me your question, I happen to know that 16 decent quality 130Ah-rated Trojan deep cycle batteries will cost you about $AU4700 (about $US2775, as I write this; they'd probably be a bit cheaper in the USA). You can pay less if you use budget car batteries (don't run 'em flat...), or more if you buy really top class batteries; large single lead acid cells, for instance, are sold specifically for power applications like these. Buying cell-by-cell lets you swap out individual cells as they die.
Whichever way you cut it, it ain't cheap; you still haven't bought the inverter and charger.
It's not hard to find a consumer UPS with an inverter powerful enough to run the one PC you're talking about, but you can't just hook up a UPS that isn't made for expansion to a vast battery bank and walk away; the thing's built-in charger may not charge the big bank much at all before it gives up on the grounds that something appears to be Very Wrong, and it certainly won't charge it very fast.
For long run time applications, there's something to be said for a good old pull-start emergency generator. You have to connect cheap generators to a decent power conditioner, because their output waveform is generally very rough indeed and tests the garbage-accepting abilities of even computer PSUs (better generators are, well, better), but a generator is a solution that you can just pour more fuel into if you need more run time. And a little thousand watt generator with four hours of fuel in it is likely to weigh only about 20 kilos.
And now, the light bulb question
I am looking for a 1.5 volt light globe, 0.22 watts, that is powered by a single AA cell. It is a standard miniature Edison screw (MES) globe.
Matthew
Answer:
I scanned the usual traps here in Australia, and found that
DSE have one that almost matches
your requirements. It's catalogue number P8147.
That globe might be brighter than you want, but 1.5V MES lamps are hard enough to find that I reckon you ought to give it a go. Its rated current may not have much to do with what it really draws, anyway.
Energy for nothing, and your chicks for free
I am writing to find out your opinion of this.
In short, it's information from a magazine called Science & Mechanics, whatever the heck that is. It tells about some guy who invented a motor using only permanent magnets. I have no clue what to think but am very interested to hear what you think. The theory to me seems sound, but to me a lot of things do.
On this page is a wonderfully animated GIF of what should happen.
Jordan
Answer:
Yeah, that's great, and all, except I'll bet my liver that the thing doesn't
work.
The fact that the USPTO granted someone a patent doesn't mean anything. Yes, patent offices the world over do try to avoid granting perpetual motion/"over unity" patents (unless provided with a working model, that is...), but it's not actually up to the patent office of any nation to verify that patented things work. Patent offices have granted patents to all sorts of wiggy things over the years. A patent is just a piece of paper that protects you from other people ripping off your idea. The idea may or may not be worth ripping off; the patent office doesn't care.
There are lots of "permanent magnet motor" designs, which unlike the permanent magnet motors doing duty in zillions of electric devices all over the world, contain nothing but permanent magnets - there are no energised rotor or stator coils. All of these sorts of alleged motors have one or another pile of pseudoscientific hoo-hah (a la this lot) behind them; none of them work. Not even a little bit.
This does not greatly dampen the enthusiasm of independent thinkers for such things, of course. Dig this, and the rest of that site, for a representative example.
This particular motor, by the way, scored one whole line here.
Immortal batteries?
Since you are the foremost expert I know of on the subject of batteries, in addition to being about the only expert on batteries I'd enjoy reading a response from (I love your writing style), I am writing to ask you a silly question that's been batted back and forth in my head:
Do batteries (disposable cells) last longer sitting on the shelf in my pantry, or being used a little bit at a time, like in a TV IR remote control? Because every time I have the batteries die to such a point that they don't work anymore in a TV remote, I open the battery cover and see that the batteries in there are the "Not For Resale" set that came with the thing, sometimes up to 15 years ago. Or is it just that remotes use such little charge that the batteries last for a much longer time?
Dan
Answer:
Remote controls are very low-current devices; they don't draw much
when they're being used, and their duty cycle (ratio of on-time to off-time)
is really small, so batteries do last a very long time in them. Remote
controls also, often, still work fine when their batteries are quite a long
way below their nominal voltage; this means a remote can use more of its batteries'
total energy than, say, a flashlight can.
Different non-rechargeable battery chemistries have different shelf lives - dry cells don't last as long on the shelf as alkalines, which in turn don't last as long as lithium cells. And different makes of battery that have the same basic chemistry can have quite different shelf lives as well. The environment plays a part, too; batteries kept somewhere cool will last longer than batteries kept somewhere hot.
All of this together means that while batteries won't last quite as long in a remote control as they would on the shelf, they can indeed last for a really long time.
