Dan's Data letters #199
Publication date: 9-Jun-2008.Last modified 03-Dec-2011.
Particularly effective on jet cars
Is this, "Up in Smoke: Germans Develop Exhaust Pipe Power", feasible at all? Isn't storage then the problem?
Nikolas
Answer:
Yes, it's feasible. Anything that actually impedes the exit of the exhaust gases will
reduce engine performance - so you can't just hang a big low-pressure turbine off the
exhaust pipe. But harvesting waste heat is a growth industry these days.
If something's trying to radiate heat - the condenser coils on the back of your refrigerator, say - and you cover it with Peltier tiles or any other kind of heat-harvester, you'll make power but will also impede the radiation and hurt the performance of the fridge. But when you've got hot exhaust gases that're just being vented straight to the air, there are several possible ways to make that heat do some more work.
Storage of that power can indeed then be a problem, for automotive applications in particular. If the car has no purpose for the extra power then it can only be dumped into the battery, and adding more batteries to a non-electric non-hybrid car is not a great idea, because of their weight.
But if the car's consuming quite a lot of electrical watts - headlights, in-car electronics, et cetera - an exhaust-heat-driven generator could easily take the load off the vehicle's alternator. Making such a system work is not a very difficult electronic problem.
Note that the single highest-powered "extra" system in most modern cars is the air conditioner, but that's not a candidate for improvement here, because automotive air conditioning is almost always mechanically powered, using a belt driven by the engine. This has not stopped some rip-off artists from claiming that their magical talisman for your car's electrical system will eliminate the small power loss when you turn the air-con on.
Button-battery-to-D adapters not recommended
I have some Sanyo Eneloop rechargeable batteries which get used for various things around the house. Recently the other half has decided that she wants to get some C and D cell adaptors for the Eneloops, which enable you to fit an Eneloop (or really any NiMH) AA battery where a C or D cell would normally go. I assume the adaptors are around because Sanyo don't make C or D cell Eneloop batteries and other C and D cell NiMH rechargeable batteries are fairly expensive (approx $30 for two NiMH C cells).
My question is, how long would a AA battery last if used this way? Instead of buying more AA Eneloops and some adaptors, would it be better (or more economical) to just buy some C and D cell NiMH batteries (even though they are expensive)? The batteries will mainly be used to power some C-cell Maglite torches and kids' toys.
I suppose it may just be a case of having to replace AA batteries more often than "proper" C and D cells, but do you know how often "more often" is?
Tony
Answer:
The adapters have been around for decades, but they only seem to have become popular
lately - I presume because it's now possible to buy rechargeable AAs with capacity of
more than two amp-hours. There wasn't a lot of point to filling a D-cell flashlight
or boom-box with old 500mAh NiCd AAs; you just wouldn't get enough run time for it to
be worthwhile.
How long an adapted AA will last depends on the load. NiMH cells of all sizes aren't very bothered by large discharge rates, so you pretty much get the 20 hours at 100 milliamps or 2 hours at 1 amp you'd expect from a 2Ah cell.
(Low self-discharge NiMH cells like the Eneloops have lower capacity than the old high self-discharge type, but they also don't go flat on the shelf nearly as fast - which is why they're often sold as "ready to use". The low self-discharge means that for applications where the device isn't used constantly, these lower-capacity cells will give you more run time.)
The standard bulb for a three-cell C or D Maglite will consume something approaching three watts from fresh alkalines, or a little less from rechargeables (because 1.5-volt-nominal alkalines droop under load, but larger cells won't droop all the way to the 1.2 volts of rechargeables, which themselves hardly droop at all). Let's say 2.5 watts as a ballpark figure.
2.5 watts at 3.6 volts (three 1.2-volt rechargeables) means 0.7 amps (because amps times volts equals watts, in a simple DC circuit like this), and modern low-self-discharge NiMH cells like the Eneloops have about 2.1 amp-hour capacity. So you're looking at something like three hours of run time in a three-cell Mag-Lite using the standard bulb. C alkalines will give you something like nine hours with that bulb, but three hours is perfectly respectable.
It's also now possible to buy low self-discharge C and D cells, though you'll probably have to get them from a specialist seller - low self-discharge AAs are supermarket items these days, but the bigger cells may never be. This is at least partly because they're even more expensive than regular C and D NiMHs, and so a rather attractive target for shoplifters.
Another chance to link to this comic
I am a musician and have a Monster Cable Pro 2500 power filter for my gear. I just bought a gas-powered generator as a backup for my house in case power fails. It made me think about protecting my home office gear from the generator as well as the grid.
I have the usual stuff including desktop computer, fax, two printers, copy machine, modem and wireless Internet. I like the Monster. It sells for about $300. I am thinking of getting a similar but smaller model to protect the computer gear. What do you recommend?
Steve
Answer:
I've just spent five minutes trying to find specifications for the
Pro
2500, and have failed. Monster don't seem to have them on their site, or even in
the downloadable manual for the thing. The closest they come to actual electrical specs
is a mention of "2775-Joule Surge Protection", which is unremarkable among $30 powerboards.
They go on about how it has Three Separate Isolation Filter Sections and Dual Mode
Plus™ Surge Protection for Musicians, Portable Racks, and Small Studio Applications,
but without any actual numbers I can find to tell you what these buzzwords mean.
So I can't tell you whether the Pro 2500 is actually any good, because Monster Cable will not tell me what it is. I'm sure there's some sort of filtering and surge protection in there, but I have good reason, based on pretty much everything Monster Cable have ever sold before, to think that the Pro 2500 doesn't give you anything like as much power conditioning per dollar spent than more informatively described products from less... marketing-forward manufacturers.
Monster Cable are renowned for, shall we say, "audacious" business practices (a reputation matched by the people who sell Monster cables...). So I find I cannot escape the suspicion that the Pro 2500, and its similarly specification-free relatives in the Monster Power range, may actually do a whole lot of nothing.
Monster deserve no benefit of the doubt, here; they and their "audiophile cable" competitors have all been in the business of spouting pseudoscientific nonsense since about ten minutes after they sold their first cable, so there's no reason to suppose they've stopped now.
I mean, look at what they say about their mainstay speaker cables. Apparently they have "Monster's Time Correct® windings for more natural music reproduction" (all frequencies of audio signals propagate down any kind of wire at the same speed, which is not far short of the speed of light, so all speaker cables are as "Time Correct" as each other...), and "Magnetic Flux Tube® construction for greater dynamic range and improved bass response" (dynamic range and bass response are both objectively measurable; Monster speaker cables do not do anything in these departments that similarly thick no-name wire does, and they're actually almost certain to be audibly indistinguishable from skinny bell wire, or indeed coat hangers).
It's possible that Monster's power conditioners are actually perfectly good, but who can tell? Not making specs available is just the icing on the cake, a one-finger salute to anybody who knows the first thing about actual power conditioning.
I tell you all this not to make you feel bad about buying the Pro 2500, but to make sure you realise that this thing is probably not providing very much protection, if any at all. Given the fact that it only weighs about five kilos, I would not be surprised to discover that it's just got the guts of a midrange surge protector powerboard in there, plus extras for its display, 12V XLR socket, etc. You should not count upon it to do anything more for you than that powerboard would.
And no, you absolutely should not buy another one, if they won't even tell you what it is that you're paying for.
Generator output these days can be very good. Honda's current small models, for instance, have I think at least as good an output waveform as a good-quality computer UPS, maybe better.
Cheap Chinese generators, though, may have truly horrible output, and pose an active danger to all kinds of loads.
The only way to tell whether a random generator has nice or horrible output is by testing it. The elegant way is by loading it up a bit with something it can't hurt (like a bar heater) and plugging an oscilloscope into another output. Nice sine wave, no problem; fairly nice pseudo-sine wave, still OK (and perfectly fine for computers and other hardware with switchmode power supplies); incredible spiky mountain range, problem.
The quick and dirty version of the above test is to try running something with a motor in it, like a desk fan, from the generator for a little while. If the fan motor makes a bit more noise from the generator than from wall power, you're probably OK. If it makes a lot more noise, growling and buzzing like crazy, then that generator output's going to need a great deal of filtering, probably more than you'll get from any lightweight power conditioner. Better to just get a better generator.
Proper power conditioners that're cheap and lightweight do exist these days; I wrote a bit about them, and linked to info about the older heavyweight options, here.
It's actually pretty easy to find old heavyweight ferroresonant power conditioners on eBay; the trick is finding one that's close enough to you that shipping fees won't murder you.
If that Monster Pro 2500 is supposed to have a 2500 volt-amp rating (who knows, since they don't even seem to quote VA or watts anywhere... are there at least some specs on the label of the one you've got?!), I think it's worth mentioning that a ferroresonant power conditioner with the same rating would probably weigh, oh, about TEN TIMES AS MUCH.
My personal solution to your problem would be UPSes running from a battery bank, made out of cheap car batteries, like the couple in this piece.
If you need to run your gear for days on end then this is impractical, but if all you need is a home music and computer studio that can operate for several hours without grid power, it's quite doable with a battery bank that'll fit under a coffee table. For most elegant operation you'd want a single high-rated UPS, which might cost you quite a bit. But older lower-rated UPSes with dead batteries are cheap enough, so you could split the battery bank between, say, three UPSes and get a less elegant but more affordable solution.
Weiguo wall wart
I was rather impressed with your review of the Weiguo Solutions Spotlight, so I asked around at work and we ended up buying a bulk order of them to reduce the total shipping. Everyone has been very happy with this purchase, so thank you for bringing it to our attention via your review.
I was going to use a surplus plug pack style AC-DC converter (12V 1A) from an old Linksys router as a charger for mine. However, I noted that the output voltage from the plug pack is 15.1V, with an illuminated cigarette lighter socket - I was quite surprised by how much heat the light was putting out, given that it is quite dull - and the Spotlight connected as the load. I assume this is because the plug pack is unregulated, and this is a very light load? Does this effect scale with the rated current capacity of the adapter? So with a small load will the voltage on a 12v 200mA adapter, say, be lower (closer to rated 12V) than that of a 12V 1A adapter?
I'd like to recycle the Linksys adapter if I can, rather than buy a regulated plug pack. Do you think this higher voltage (15.1v) will cause short or long term damage to the battery of the torch?
Jeffrey
Answer:
Yes, the higher voltage is because the plugpack is an unregulated
linear power
supply. I'd actually expect it to be even higher - a completely unloaded linear
"12V" PSU should give you root-2 times its rated voltage, which is very nearly seventeen
volts. Perhaps that illuminated socket's using more power than you'd expect.
Given the tiny Spotlight's very low charge rate from 12 or 13.8V, I suspect the higher voltage won't be a big problem. It'll definitely be charging harder, so I wouldn't leave the thing plugged in all day, but I also wouldn't be bothered about plugging it in for an hour or so after I'd actually used it.
Remember that the flashlight isn't meant to be charged 24/7, even at its standard voltage; it expects to be sitting in a normal car cigarette lighter socket, that's only energised when the ignition's turned on. Even from 12V, non-stop charging will probably cook the poor little battery after a few months. And if you're only using the light occasionally, it only ought to need an hour or three of charging per week anyway.
And yes, the output voltage of an unregulated linear power supply does scale with the load. These old-fashioned plugpacks therefore need to be capacity-matched to the devices they're driving; if the 200mA "6V" plugpack for some device dies and you replace it with a 1A "6V" plugpack, you may well blow up the device.
This leads to misconceptions about how the larger current capacity of the bigger plugpack somehow "drove" too much current through the device.
Modern lightweight switchmode plugpacks should all be quite well regulated, so all you need is something of the right nominal voltage that can deliver at least the necessary current. Old heavyweight plugpacks can be regulated, but if it doesn't say "regulated" on the sticker, you should assume you're looking at a simple linear power supply, and its open-circuit voltage will be root-two times its fully-loaded voltage.
If the bulb in your lighter-socket doodad runs hot, then it's an incandescent lamp, not an LED like the light in the cheap lighter-socket adapter I used to test the Spotlight from my bench power supply. Incandescent lamps emit more energy as heat than they do as light, and the long-lived low-efficiency lamps used in automotive interior lighting are even worse. This is why you can replace an automotive interior bulb with a much lower-rated LED lamp and get the same light level.
Emergency power
We have a 911 dispatch center that uses a large battery backup system rather than several HUGE UPSes.
All of our computers work on 48VDC, and our LCD monitors work on 12VDC. I have been unable to find any 12-volt monitors for under $700.00 (17 inch).
If you know of any still manufactured - Samsung, NEC, Planar or others - please drop me a quick e-mail.
Randy
Answer:
If I were you, I'd just pick up some
inverters to turn the
12 or 48V into regular mains power - 110/120VAC, for the USA. 12V inverters are cheap
and common, but 48V models are pretty easy to find as well.
The down side of this is that cheap inverters may die pretty quickly if you run them 24/7; the really cheap ones are only meant for occasional use in cars. (If an inverter is sized to fit in a cup-holder, that's a good sign that it's not meant for serious industrial applications...)
Inverters are also not 100% efficient. You should expect anything running from an inverter to draw about 10% more power than it says on the sticker.
For your purposes, though, neither of these problems should be significant. Any half-decent 48V inverter, in particular, should be fine for long-term use, and it'll let you use any ordinary PC monitors you like. A 1000VA-rated inverter should be able to run more than ten decent-sized LCDs; maybe 20.
The best toys are always dangerous
Dan, I was just looking for something I could get my grandson, he is very interested in science. Rare-earth magnets look like they could be fun. My grandson is 12 years old - can these magnets be dangerous?
I was also wondering what if anything happens to these magnets' power if they are stored stuck together?
Rick
Answer:
Big neodymium-iron-boron ("NIB")
magnets definitely can be dangerous.
Anything with two dimensions of an inch or more starts to get quite hazardous. It's now easy to buy fist-sized rare earth magnets that can smash your hand like a sledgehammer if you mis-handle them. People who use them to make wind-generator dynamos and suchlike are well advised to plan their movements carefully, and maintain a zero blood-alcohol content.
Small rare earth magnets, though, present no such hazard. They're great toys, and cheap, too!
It's theoretically possible that two decent-sized magnets (for instance, cylinders an inch long by a quarter inch in diameter) could snap together hard enough that they'd chip, and then you could catch a chip in your eye, but it's really not very likely at all. Apart from that, the magnets are just a minor "pinching hazard", which'll give you nothing worse than a small bruise if you let them grab the web of your finger, or use a couple as an ear-ring, or something.
And that'll just build character!
If you want durable magnets, avoid spheres larger than about a quarter-inch. Larger spheres stick together so hard that they grind away their (usually nickel) protective coating at the contact point. Avoid thin coin-like discs or squares, too; they'll snap very easily. Magnets that're very small in all dimensions are very sturdy, though; you can get an awful lot of 5mm by 2mm disc magnets for ten bucks these days, and they're great fun.
Note also that rare-earth magnets can be a serious hazard if swallowed. If you swallow one magnet, you're fine; it'll pass like any similarly-sized inert object, possibly dawdling a bit as it passes your belt-buckle, but that's it.
If you swallow two magnets, though, they can click together with some important part of your body in between, which can be Very Bad.
(In reality, I'm sure almost every toddler who's managed to swallow two small rare-earth magnets just had them click together harmlessly in the stomach, and then pass the rest of the gut as one object. But there definitely have been cases when kids weren't that lucky, so it's important to tell your grandson to keep the magnets away from any little 'uns. He will probably remember the warning very well, since pressure-based intestinal necrosis makes for wonderful blood-curdling stories.)
And not a thing will happen to these magnets if you store them stuck together. You're thinking of the old alnico magnets, which have to be stored with an iron "keeper" on them to complete the magnetic "circuit", and whose magnetism slowly leaks away even then.
The magnetic domains in modern ceramic magnets, in contrast, are fixed unless you expose the magnet to a very strong external magnetic field, or heat the magnet to its Curie point (which, for neodymium-iron-boron rare-earth magnets, is a relatively low 310 degrees C).
Neither rare-earth nor common cheap ferrite magnets will ever lose any magnetism, just sitting there. Well, not on human timescales, anyway.
Speak of the devil
A while back I stripped a few magnets from a hard drive. They were rather well attached to the back plate and while I usually tap a razor blade under them to pop 'em off, these particular plates had pins and bumps blocking that approach. So I thought heating them with a propane torch might melt the glue. It did... but apparently said magnets don't like heat much, as they've lost their magnetism.
They're not completely dead... Just very weak.
Any idea how I might restore them to their previous strength, or are they simply dead?
Krstofer
Answer:
The only way to fix them would be by remagnetising them in a high-powered electromagnet,
the same way they were magnetised in the factory. Practically speaking, there's no way
for anybody who doesn't own a magnet factory to do this.
You could probably make a rig that'd remagnetise them to some extent, but really high-powered magnet-making requires very high currents and involves coils that really want to blow themselves apart. It's not a job for a hobbyist.
They lost their magnetism, by the way, because the "Curie point" for neodymium-iron-boron (NIB) rare earth magnets is the lowest of all of the popular magnet types, by a wide margin. Heat a magnet above its Curie point and it loses its magnetism.
The Curie temperature for NIB magnets is only about 310 degrees C, versus about 460 degrees for cheap ferrites, and 750 degrees for samarium-cobalt. And magnetism may just "leak" out of the old alnico magnets over time no matter what you do, but they've got a Curie point of 860 degrees.
