Dan's Data letters #207

Publication date: 28-Jun-2011.
Last modified 03-Dec-2011.

 

Smoke-Emitting Diodes = Education

I'm trying to build an LED circuit and lack the knowledge of some things so extracting it from you would be great.

I have 5x 5mm White LEDs that I'd like to put all in one parallel circuit. The details I know for them are; the forward current max is 100mA, the forward voltage max is 4v and the power dissipation is 100mW (don't have a clue what that is). Can you please tell me what resistor(s) I will need for my circuit and where they must go?

Oliver

Answer:
The best way to learn this stuff is by burning up a few LEDs and resistors. Fortunately, LEDs and resistors cost close to nothing, especially if the LEDs are the old-style low-intensity type; this pack from a UK electronics shop (Oliver is in the UK) is actually somewhat overpriced!

I think a couple of these would probably suit you quite nicely. Any bulk-pack of resistors is likely to have lots of high-value resistors that're useless for low-voltage LED circuits, but you probably won't begrudge a waste of 2.

Some batteries in a cheap plastic holder, or a 6V lantern battery, will serve as a testbed power source that can't set wires on fire if you mess up. All you need on top of that is a pack of alligator-clip test leads and an entry-level multimeter, both of which are items that anybody with any interest at all in electronics (or related topics, like "is my car's battery knackered, or is the alternator just not charging it?") should have anyway.

White LEDs are all still actually blue LEDs with a phosphor coating over them (one day quantum wizardry will create "true" white LEDs...), so they have the same specs as blue LEDs. That means a nominal voltage of about 3.6V. Specifications for LEDs aren't hard and fast, though, because they're dynamic devices that pass more current as they get hotter, which makes them hotter again, and without suitable current limitation they burn up.

In this case, your LEDs sound like standard 5mm high-intensity whites, so to keep them happy you should probably run them at 3.6 volts and about 20mA.

You can plug those numbers into the excellent array calculator here and probably end up with something that'll work. (Note that you need to know what your supply voltage is, too!)

But I must once again discourage this sort of "cargo cult electronics" where you don't really understand what you're doing. Blowing up LEDs and resistors is an inexpensive and entertaining activity, and will give you an actual grasp of what's going on.

As is usually the case, one piece of knowledge can lead to all sorts of other interesting things, too. You could, for instance, take your new resistor-smoking knowledge, tape a couple of match-heads to a soon-to-be-dead resistor, and now you've got a dandy "electric match" for igniting model rockets, fireworks, car bombs and other fascinating things.

 

Turn thrice widdershins, throw electrolyte over shoulder...

I remember being told by my parents when I was a young child that if a battery-powered device starts to behave as if the cells are running down then a quick and easy fix is to simply change their positions in the holder. I've often done this, and it's true that a quick shuffle around of batteries will make it appear as though they have fresh life in them.

Obviously, they aren't like new any more, but I'm wondering how this effect works. Can you explain it?

Michael

Answer:
I can only think of two things this'd do:

1: Reset any "smart" battery-monitoring hardware, possibly causing it to report more remaining run-time, without actually making a real difference to anything. You won't often find battery-monitoring hardware in anything that doesn't use some sort of one-piece rechargeable battery, though.

2: Clean the battery contacts a bit. Just spinning the cells without rearranging them would probably do this, too.

If you hold the batteries for a while they may deliver more power when replaced, because they're chemical devices, chemical reactions proceed faster at higher temperature, and your hand is probably warmer than ambient. Again, though, this effect would be the same even if you replaced the cells in the same locations.

 

Well there's yer problem

I was reading your Sennheiser HD 555 review, and noticed that you'd updated it to reflect recent news concerning the product. I was wondering if you were familiar with the mod that's become popular among HD555 owners, (I think) due to this YouTube video:

I really only ever listened to the headphones after having done the mod, so I can't tell if there is a significant improvement in sound quality. Perhaps you could update the review with your thoughts. I'm interested to see what your impressions are after pulling those little bastards out.

George

Answer:
Without a proper blinded test, you'll never know whether removing those two parts makes a difference. Neither would I. This mod is simple and "invisible" enough that you could probably do a decent blinded test at home by yourself with one modded HD 555 and one unmodded, but "just listening" after doing the mod on your only pair of 555s will only work if you happen to be Commander Data.

(You could also do objective testing with a fake-head frequency analyser, but not much of the audiophile community ever seems to be convinced by anything so tediously scientific.)

Artificial market segmentation by hobbling cheaper products is a real thing, and this mod seems plausible enough to me - but psychoacoustic effects are more than strong enough to account for "effects" much more dramatic than anybody seems to be claiming for this mod.

 

Autotransformers, roll out!

I've just returned from a couple of years living in the US, and brought our plasma TV home, where I'm hoping to use it.

In short:
1. The TV's internal power supply is specifically a North American AC 120V / 60Hz jobbie, the TV is specced at ~606W.
2. My basic assumption is that I need to feed the TV what it says on the tin - roughly 120V at 60Hz, from something with enough wattage headroom to deal with it (~1000W).
3. I called Jaycar and asked about their products. They were very helpful, but, I think, not particularly accurate, as they said that the frequency of the stepdown transformer output wouldn't matter and to go right ahead and plug it in...
4. In the interest of not letting the magic smoke out, I've purchased a 250W Jaycar stepdown transformer (multiple brands on the box / manual / product, Steed & PowerTech) for a different use, partly to see what it is specced as (this one) and it specifically says that the transformer does NOT convert frequency.

My Google searches for step-down transformers that convert both voltage and frequency are failing me - although I'd be pretty sure I'm not the only person in Australia to ever want to do something like this, I feel I may be barking up the wrong tree. I have photos of the manuals, boxes, products etc if that would help.

Brad

Answer:
Frequency-and-voltage converters do exist, but I don't think they're consumer products. In the olden days doing this would involve a motor/generator combination, but today it'd just be solid-state X-Hertz-mains-to-DC-to-Y-Hertz-mains.

Fortunately, you probably don't need to worry about any of this, because modern electronics almost always runs from switchmode power supplies, and switchmode is insensitive to mains frequency. Old heavyweight linear power supplies don't like frequency changes, but switchmode doesn't care.

Look, for instance, at the zillions of small-to-medium power supplies (plugpacks, phone chargers, network appliances, PC PSUs, etc) that now have "100-240V 50-60Hz" on the label. I wouldn't be surprised if those power supplies worked fine from anything from 40 to 80Hz.

This does not constitute a guarantee on my part that your US TV will be happy from about 115V and exactly 50Hz. But if it were mine I, like the Jaycar dudes, would have no trepidation whatsoever plugging it into a simple step-down transformer.

Well, actually, that's not quite true. I would have a certain amount of trepidation, but only because the quality of step-down transformers is somewhat variable. Cheap low-capacity step-down (and step-up, for that matter) transformers have a tendency to drop dead without warning. I think the "catastrophic failure" symptoms are roughly evenly divided between "attempts to kill user" and "attempts to kill connected equipment".

(Note also that all standard step-down transformers are autotransformers, which come with an impressive list of warnings and disclaimers having to do with grounding, and aquariums, and the possible insulation-failure situation in which the full input voltage cheerfully appears at the output.

You're going to have to get a big high-capacity step-down transformer, of course, which'll cost you a few hundred bucks at least. This $AU369 Jaycar unit will probably do the job just fine, provided it turns out to be reliable.

(Personally, I'd just put the $369 toward the purchase of a new TV here, but you've already gone to the trouble of bringing your huge TV to Australia, so you might as well throw some more money at it. You shouldn't, at least, be wasting much extra electricity on top of the plasma's own hefty appetite in heating up your TV room; I think the autotransformer should be at least 90% efficient and maybe better than 95%, unless there's something nasty about running it at 65% of full capacity that I've missed. You should still expect the transformer to have some tens of watts of wasted energy heating it up, though, so don't tuck it away in some ventilation-free cubby-hole.)

 

Smoothing spikes

Just got through reading a couple of your excellent articles regarding line condtioning etc. I am suddenly in need of something like that, as I seem to have a very dirty power in this house I'm renting. Mainly I'm looking to try to solve my low ADSL2 sync problem which is caused by my parents' TV set that is putting off a heap of noise. Also there seems to be a lot of voltage spikes and fluctuations in this house.

I was just hoping you could suggest a good line conditioner for me that would do both the voltage fluctuations and also the EMF/RF etc noise filtering. I'm looking to plug my PC and modem in to it. I looked at one like the APC Line-R, but I'm not sure if it does the noise filtering??

Gary

Answer:
No modern TV should emit much noise over the air or down its power cable. If it does, there may be a serious fault.

(When all TVs were CRT TVs, there were a lot of components that could emit interference, and the assumption if there was a lot of interference was usually that there was something arcing inside the set. That generally has obvious picture-quality effects, though.)

Presuming the TV's not broken (or that it works well enough so nobody wants to spend money unless it actually dies), the solution to noise getting out of it and into the mains may actually be to use a power conditioner to run the television, not something else. A proper power conditioner works "both ways", as it were.

Another thing you might like to try is getting a small UPS, if you don't already have one, and running the gear that's glitching from that. Small UPSes, even from good brands like APC, are not much more expensive than Line-R-type power conditioners, provided you don't need to run a lot of gear. Most homes can find a good use for a small UPS, too - not just the computer networking hardware, but also stuff like cordless phone base-stations.

A decent small UPS should be as good a power conditioner as an APC Line-R unit; it just won't have as high a wattage rating. I don't know of any Australian consumer-market power conditioners other than the Line-Rs, though; somebody probably makes one that isn't a $10,000 money-sink for gullible audiophiles, but I haven't found 'em. So if the UPS doesn't appeal, get a Line-R.

 

Entirely unremarkable home decor

I read your piece on rare earth magnets with interest, as I am looking into the design of a free standing portable pole for fitness/dance. Existing poles have various drawbacks in terms of their weight, bulkiness, requirement of sturdy ceiling and so on, and use mechanical means such as pressure or weighted bases to hold themselves up. I wondered whether it might be feasible to employ some kind of high powered magnetic base with a high holding value, on to which to mount a steel 8-9ft high 2in diameter pole. Would it be strong enough to hold a person of up to 18 stone at the top of the pole?!

Pole dance fitness is a tiny but phenomenally expanding niche, people do it at home and at classes instead of going to the gym. Bear with me on this one! I'm a full time instructor in UK, ambitiously trying to find a way to make running my classes easier with better kit.

Elaine
www.poleminx.co.uk [possibly NSFW, if you work at a nunnery or something]

Answer:
I don't think this would be workable.

The pole has to be attached to something. That something, if the pole is sitting out in a conventional room with enough space around it for someone to dance and/or exercise, must be the floor and/or the ceiling, unless you build a separate frame inside the room.

In reality, presumably the pole will really be anchored not to the floorboards or the ceiling panels, but to the bones of the building, most often wooden beams, that do the real structural work behind the floorboards and ceiling panels.

(This situation can obviously vary considerably if you're not looking at a conventional "stick-built" wood-framed house. If you've got a concrete-slab floor and steel ceiling beams, you can install all kinds of improbable heavyweight equipment.)

It doesn't matter whether a pole is attached to the floor and/or ceiling with bolts, or magnets, or Star-Trek force fields; however you do it, the forces exerted on the pole by the dancer will be transferred to the mounting points. Magnets can be helpful in some applications because, like Velcro, they can hold strongly without making a really solid mechanical connection that may transfer undesirable shocks, have mechanical parts that snap off, et cetera. In practice though, like Velcro, magnets are seldom actually useful for structural purposes.

I know nothing about this particular area of endeavour, but I could see it maybe being practical to have some sort of top-hat-shaped socket in the floor that you drop the pole into, with a beefy magnet retaining it against occasional upward pressure from the dancer, and just a light bump-panel of some sort attached to the top of the pole, resting quite gently against the ceiling. The magnet's not doing anything that a simple mechanical retaining system wouldn't, though, and if it's strong enough to be useful, it may make it a big pain to get the pole back out of the socket.

Or maybe you could have a large, sticky-rubber-backed base-plate with a solid metal pole coming out of it that goes up the middle of the hollow dancing pole. I think you'd have to make the base plate unmanageably large and heavy for it to not need separate anchoring to the floor, though - more of an outdoor solution, I think. That could be a workable way to semi-permanently mount a pole in some place with a flimsy ceiling, though - a big heavy steel base-plate anchored all around its edge with lightweight screws that go into the floorboards, and thereby spread the load among very many anchoring points and prevent any one enormous stress concentration ripping a chunk out of the floor and tossing the dancer across the room.

Doing it this way would in most cases be more troublesome that doing it with relatively small top-and-bottom mounts connected to the building's structural beams, though. The conventional both-top-and-bottom mounting system's great advantage, I think, would be that it to prevents the huge leverage of a human hurling themselves around in the middle of the pole from all being applied to one end or the other. Anchor firmly at both ends and, provided the mounting points are firm, the stress will be evenly spread at all times.

(A quick Web search turns up plenty of people selling poles that really do seem to attach to the ceiling by just... pushing against it. They do at least have a load-spreading plate around the top of the pole, but I am nonetheless entertained by the thought of the many and varied ways in which physics could have its vengeance upon someone who attempted to use such a pole, if the person who installed it hadn't had a damn good look at the ceiling from the other side before picking the pole's location.)

In any case, I'm afraid I doubt magnets will be much help.

 

Perpetual inverse tachyon neutron flow

I stumbled across this video...

...showing how to "hack" the old electric meters using a cup of distilled water and two different size magnets.

I'm truly puzzled at how they think it will work, or if they are laughing at all the cups of water now sitting on top of electric meters.

Any insight to their supposed operation would be appreciated.

Andrew

Answer:
This is, basically, bullshit. The water and wires cannot possibly do anything. It's another fake how-to video, of the type perfected by Household Hacker.

The tiny kernel of truth in this, though, is that you can slow, or even stop, a spinning-disc electricity meter by strapping magnets onto it from the outside. I think you want to "augment" the magnetic field of the standard brake magnets, and/or swamp out the eddy currents that make the meter spin in the first place.

The magnets-on-the-outside power-meter scam is preferable to the version that involves an earth stake and new and exciting wiring connections, because it's pretty much impossible to kill yourself while setting it up. But it's still a dumb thing to do, because an electricity-company person may come around at any time, and if they see weird stuff taped to your meter, even if it's the thing in that fake video that doesn't even work, you're still probably going to find yourself paying a lot more money than you could possibly have saved, even if the gadget had stopped the meter altogether.

(And if some gadget does greatly slow a meter, and you manage to remove and hide it before the meter reader comes around, you're still likely to find yourself put on the audit list. Someone whose house drew an average 1.5 kilowatts for the first quarter of the year, then an average 25 watts for the next, is someone who is now probably going to get checked on quite frequently.)

 

Everything fun is illegal, immoral or causes cancer in rats

A couple of years ago I was at the end of a very long night with a mate and made this fairly tame potassium permanganate + glycerol reaction video [not viewable in Australia, at least, for soundtrack-copyright reasons; herewith, one from my blog post on the subject].

A couple of the commenters warned that the smoke was actually very dangerous. When glycerol burns, it forms acrolein gas, used as a chemical weapon in WWI and toxic above 2ppm. Subsequent measurement with an IR thermometer revealed this reaction burns at around 350C.

With this in mind, I just noticed your blog post. I too have smelt the pleasant firework-y smell, but apparently it is quite nasty.

Jonathan

Answer: I strongly doubt the acrolein production from an experiment-sized version of this reaction poses a real hazard. There's acrolein in pretty much all normal combustion smoke, and it is indeed unhealthy to breathe large amounts of just about any kind of smoke in a short period of time (acrolein is apparently a major factor in the unpleasant effects of acute smoke inhalation), or to breathe small amounts of just about any kind of smoke habitually (acrolein may play a part in causing smoking-related lung cancer).

But that doesn't mean any whiff of the stuff poses a risk worth worrying about. Which is just as well, or the world would be littered with circles of dead Boy Scouts around piles of campfire ashes.

If you want to ask me a question, feel free - but please read this and this first.

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