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2014-10-28 Joule Thief In Wikipedia

Books, documents, literature Joule Thief SMPS DC-DC
on October 28th, 2014 by - 7 Comments

I was looking at the references in the Joule Thief Wiki at Wikipedia, especially the first reference, [1] after “Armstrong” in the first line of the text. This refers to an article which describes this as an Armstrong or Meissner oscillator. This article then points back to Wikipedia as a reference. What we have is a case of one article supporting the other, with no factual support from any ‘foundation’ article, hence no actual factual support. I don’t know what Wikipedia’s policy is on this, but to me it’s unethical and a worthless reference.

Also, I don’t believe that this is an Armstrong oscillator. In the Wikipedia article for Armstrong Oscillator, it says “inductance and capacitance” (my emphasis). The Joule thief does not have any capacitance, therefore it is not an Armstrong oscillator.

One might claim that it has stray capacitance, but at the very low switching rate – less than 100 kHz – at which the Joule Thief switches, the few pF of stray capacitance is insignificant; it plays no part in the circuit’s operation.

I’m going to write this up in the discussion section of this wiki.

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2014-10-23 Nichrome Wire For DIY Resistors

Joule Thief SMPS DC-DC Meters and Test Equipment Resistors
on October 23rd, 2014 by - No Comments

I ordered some 22 AWG nichrome wire from goldmine-elec.com today.  It’s about an ohm per foot, or 3 and a fraction ohms per meter.  My plan is to make my own low value resistors for current monitoring.

I often want to monitor the current in a higher power LED or input current to a Joule Thief.  This requires a series resistance of a fraction of an ohm, typically 1/10 ohm to make it easy to calculate the current. 

The 1/10 ohm resistors are hard to find. I have plenty of 1 ohm resistors which are very handy because 1 volt across it equals 1 amp.  But the loss is excessive – 1 watt – and the drop is excessive – 1 volt.  By reducing the resistance to 1/10 ohm, the losses at 1 amp are 1/10 volt, and 1/10 watt.  At less than 1 amp, the losses are much less and tolerable because they don’t disturb the circuit much (see note at end).

The 1/10 ohm resistor I will make will consist of about 1/10 of a foot (plus a short amount for the leads) of the nichrome wire.  That’s a bit over an inch, about 1-3/8 inches or 35 mm.

It’s difficult to solder to nichrome, so I will insert the ends into a terminal strip and clamp the leads with the screws.  I’ll use 2 terminals of a Euro terminal block.  I saw off two of the terminals and file off any plastic rough edges. One side will clamp the ends of the nichrome wire, the other side, the leads of the current wire and leads to the meter. I can adjust the resistance a bit by sliding the wire in and out of the terminal.

The meter will be a cheap DMM. When I say cheap, I mean free. Harbor Freight had a coupon for a free DMM with any purchase. This meter reads 200 DC millivolts on the lowest scale, so 10 millivolts will equal 100 milliamps current. The resolution will be good enough for making comparative measurements when prototyping a circuit. And the important thing is that the voltage drop will be less than .1 volt for less than 1 amp. Another advantage is that a length of 30 AWG copper wire 0.1 ohm is about a foot long; the same nichrome wire is ten times shorter, and takes up ten times less space.

I’m patiently waiting until the package arrives early next week…..

Update Nov 1 – I received the package and I made up one of the 1/10 ohm resistors as I described. I measured it with my very accurate HP (now Agilent) multimeter and it was 0.1082 ohms. I just have to cut the nichrome wire a bit shorter and it should be exactly 0.1 ohm.

Note: I could put 2 or more 1 ohm resistors in parallel. Two 1 ohm resistors in parallel would be equal to 1/2 ohm, and 0.2 amp would give a reading of 0.1 volt on the DMM. So if the DMM reads 100 millivolts, the actual current would be 200 milliamps. More resistors in parallel would give less voltage drop but a lot of resistors gets bulky and expensive.

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2014-10-14 Multi LED Lamp

LED Lighting
on October 14th, 2014 by - No Comments

I have made an assortment of LED task lights that I’ve clipped onto places where I need a small, low power light, such as my keyboard, workbench and kitchen counter.  They typically have 3 or 4 LEDs and run off an AC adapter of about 5 volts.

I want to make a lamp with multiple LED banks, each mounted on its own steel or copper wire ‘gooseneck’ so it can be aimed at the point where light is needed.  The power will be low, 4 LEDs at 64 milliwatts each is about a quarter watt.  The 150 ohm current limiting resistors are about the same, for a total of a half watt per bank of 4 LEDs.  The 6 volt adapter puts out 1/2 amp max, so it can handle 5 of these LED banks maximum.  But then I was hoping for more, ten or fifteen seems more realistic.  I may need 1.5 to 2 amps at 5 or 6 volts. 

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2014-10-13 CRI Rating on New Lights

LED LEDs Lighting
on October 14th, 2014 by - No Comments

LED bulb prices are still coming down, but the general public is still not aware of the advantages and still think the prices are too high. And the public is still unaware of the long term savings in electricity from which they’ll benefit. I still see 60 and 75 watt incandescents in dollar stores, which I thought were banned. And as I point out below, they have big benefits in color rendering.

I have noticed that the newer LED light bulbs have a CRI or Color Rendering Index rating on the bulb and/or the packaging.  I read somewhere that to qualify for federal rebates, a light bulb must have a CRI of 90 or more.  That gives the manufacturer an incentive to make a higher quality lamp.

I and some friends have been playing Rummikub at a house that has a dining room table under a ceiling fan-light ‘chandelier’. The “Large Numbers” Rummikub tiles are made of cream colored plastic with the numbers embossed on them. We have been having problems telling the difference between the blue numbers and the green numbers. They both looked very dull, like the colors were faded and turned muddy. One time, I pulled out my LED flashlight and illuminated some tiles in question and the colors were much brighter and easy to tell apart.

The three lamps in the fan light were 13 watt ‘corkscrew’ CFL lamps, of some brand I had never heard of. Since there were no packages, I couldn’t determine if these were 60 watt equivalents. The homeowner said that the electric company had replaced the old lights with new ones, and these CFLs were probably replacements.

I read online that the original incandescent lights emit a broad, even spectrum of light, even though the light is a low color temperature. This gives good color rendering across the visible spectrum, even though the red end gets more than the blue end of the spectrum. With the switch to CFL lights, the output is different. The incandescents use a white hot filament; the CFLs, as do any fluorescent light, use phosphors on the walls of the tube, excited to produce visible light by the ultraviolet light given off from the ionized mercury vapor in the tube. These phosphors give off a band of light, typically red, green and blue, with gaps of no light between the three colors. Any surface the light hits reflects back the colors, but if the surface is a color between the red, green or blue, then it reflects back less than it would if it were red, green or blue. Apparently the ‘blue’ Rummikub tiles are actually more of an aqua or cyan color, which is between green and blue. The CFL light makes it look more like a muddy blue-green, not much different than the green tiles.

I bought a 3 pack of 60 watt LED bulbs at Costco for $19.89, or $6.63 apiece (Costco brand?). I also bought a Cree 60 watt equivalent at Home Depot, on sale for $8.00, usually about ten dollars US. I intended to use the three pack in the fan-light, and I bought the Cree just because it was a good deal. Both packages said that the lamps had better color rendering. All of the lamps, including the old ones, were soft white.

I went over to the home and put two of the three LED lamps in the fan-light, and then we went to a neighbor’s and installed the other two in the swag lamp over her dining room table. They asked how much the LED lamps cost, and when I said $8.00, they remarked that that was too expensive. I thought that replacing two out of three lamps with LEDs would be enough to improve the light quality. In the end, I was right.

Last night we played a few games of Rummikub and every time we drew a blue or green tile, we both remarked how much brighter the colors were and how much easier it was to tell the difference. It was “like night and day,” to use an old cliché. Really, the difference was dramatic, even with just two of the three lamps replaced. Her daughter came home later with a bouquet of flowers, and the colors, especially the blues, were so much brighter, almost brilliant. Subtleties between the colors were so much easier to see. Now they are believers!
They think the LED lamp’s added expense is more than worth it. We haven’t yet played under the neighbor’s swag lamp yet, but I’m fairly confident that the results will be similar. I think they are going to want LED lights over their bathroom sinks, too, so that they will get better color rendering when they put their makeup on.

This real world example of poor and good color rendering has taught me how important it is. I hope to demonstrate its importance to others, especially to those who are responsible for displays to the public. The difference really is amazing.

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2014-10-08 Nobel Prize To Blue LED Inventors

Books, documents, literature LED Other Off Topic
on October 8th, 2014 by - No Comments

My (ex) co-worker, Howard, emailed me a link to an article that said that the Nobel Prize in physics has been given to three inventors of the blue LED.  As everyone knows, the LED is a major topic of my blog.

This article excerpt says that 2015 is the International Year of Light.  We have a whole year to celebrate!!

Frances Saunders, president of the Institute of Physics, a worldwide scientific organization based in London, agreed with those sentiments. Noting in an email statement that 2015 is the International Year of Light, she said, “This is physics research that is having a direct impact on the grandest of scales, helping protect our environment, as well as turning up in our everyday electronic gadgets.”

The reason why blue is so important is that the phosphor that puts out the white light needs to be stimulated with a light that has more energy than white light so that the process, which absorbs light and re-emits light with less energy, can put out light in the visible band.  For example, a phosphor that was stimulated with red light would have to emit infrared light, which has less energy than red light, to account for the losses in the process.

However I have some problems with the following excerpt:

Red- and green-emitting diodes have been around for a long time, but nobody knew how to make a blue one, which was needed for blending with the others to create white light. The amount of information that can be packed into a light wave increases as its wavelength shortens, making blue the color of choice for conveying information.

The first sentence says that nobody knew how to make a blue LED.  Well, that sentence is not entirely true.  The first semiconductor “LED” that was discovered by an Englishman, H.J. Round, and later by a Russian, Oleg Losev, was made of silicon carbide, which emits blue light.  So the second half of that sentence is not true, because blue light was emitted from a semiconductor before red or green.

The second sentence isn’t true.  The color of choice for transmitting information over optical fibers is red or infrared.  Search for GBIC and see for yourself.  In the picture shown here, the label says 850 nm, meaning the LED wavelength is just slightly below red, in the infrared part of the spectrum.  But it still can be seen as deep red.  The single mode GBIC modules are even longer wavelength, typically about 1050 nm.

I had read about Dr. Nakamura back in the early 2000’s when I started building my own white LED flashlights.  But I didn’t know about the lawsuit he filed against Nichia.  I started buying Nichia white LEDs and they were very good, but for a long time they were more than $2.00 apiece in hundred quantities.  Nichia seemed to be very greedy and uncompetitive, more willing to sue than license their patents.  It’s quite a shock that he was given so little for such an important invention.

This was a very good article.  I hope my corrections were helpful, too.  As was noted, many researchers contributed to the process of perfecting the LED.  Now if we can get researchers to devote more manpower to perfecting the solar photovoltaic cell.  When the sunlight hits the cell, only about 10 to 15 percent is converted into electricity.  If we could double that, the amount of power per square meter would double.  Also, someone should integrate Peltier devices into the cells so that both the visible light and the infrared that ends up heating the cells could be converted into power.  Any more ideas?  😉

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2014-10-07 Fast Charging From Car’s 12V Socket

Battery or Cell Power supplies rechargeable
on October 7th, 2014 by - No Comments

Earlier this year I bought a cheap cell phone charger for plugging into the car’s cigarette lighter socket.  It works, but it’s s-l-o-w.  I can drive for a half hour with the cell phone plugged in and it says it’s charging but it only charges a few percent.  As a result, I don’t use the charger that much; I charge it in the house. The Samsung Galaxy S4 with the Google version of Android tells the percent of charge on the screen when it starts charging.

I decided to try to speed up the charging in the car a lot by maximizing the charging current at the micro USB plug. Two important factors that determine charging rate are the maximum current that the charger will put out, and the DC resistance of the wire between the charger and the micro USB plug. Some chargers say they put out 2 amps, but I could not get them to put out more than 1.2 Amps due to the resistance of the wire.

I decided I would try to minimize the resistance of the wire. I used a very short – only 2 inches – long cord. It sped up the charging to over 1 Amp. This showed me that the cable’s wire resistance is a major factor in how fast the device charges.

Even so, the charger itself is going to limit the maximum current. Chargers may do this more than one way. When the charger reaches maximum current it may reduce the output voltage, or it may shut the output off. The output may go off when the temperature gets excessive. Or the charger may sense the current and turn off. This may happen so rapidly that the output may consist of a series of pulses.

When the charger is running at its maximum, then the cable is doing a good job and is losing very little power. If the charging current is less than maximum, it could be limited by the resistance of the cable’s wires. Or the device itself may reduce the current.

I also wanted to be able to plug in a laptop AC adapter, which means that I need an outlet that puts out 120 VAC at up to 100 watts. I found an inverter that puts out 120 VAC at up to 150 watts and also has a USB port for charging. I put a very short micro USB cable between it and the phone and it charged really fast! Well over 1 amp.

But I really didn’t want to drive around with a box with heat sink fins sitting on the console between the front seats. So I went online to look for a reasonably priced 12 V to 5 V DC to DC converter, and found this small box that puts out 5 V at up to 3 A. It and other DC – DC converters can be found at ProDCtoDC.com. I decided to order two types: one that has red and black leads about a foot long with bare wire ends that go into a small plastic box about 1 by 2 inches. Then a short cable comes out of this box with a USB A socket on the end. Any charging cable can be plugged into this USB socket. The other cable I ordered has a ten foot cable with bare wires to connect to 12 v battery terminals, that go into one end of a small inline rectangular box. Coming out of the other end is a foot long cable with a micro USB connector that plugs directly into the device to be charged. This arrangement allows for charging a long distance (10 feet or 3 meters) from the car’s socket yet has minimal losses in the 5 volt wiring. Any loss in the 12 volt wiring is compensated by the converter. This inline box holding the circuit can be popped open with a thin screwdriver blade or knife.

I went to Orvac Electronics in Fullerton and bought several cigarette lighter plugs ($1.49 US) to put on the ends of the 10 foot cables. I cut off an odd jack and soldered the cable to the wires and covered the wires with heat shrink tubing.

These converters draw about 2/3 to 3/4 amp from a supply set to 13.8 volts. The load I used was a Samsung Galaxy S4 cell phone that was charging at about 50 percent. I could not measure the 5 volt current because the 5 volt output goes directly to the micro USB plug. It wouldn’t do much good if I could measure it because the phone’s load varies a lot as the CPU goes about its business. But I was impressed by how fast the phone charges compared to the cheap cigarette lighter charger plugs that are for sale in stores. The much higher current cuts the charging time to less than half. I was driving for less than an hour, probable about 50 minutes, and the S4 charged from 19 percent to 64 percent, about 45%, or almost 1 percent a minute. Excellent!

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2014-10-05 Cheap USB Cables And Low Charging Current

Battery or Cell Electronic Components
on October 5th, 2014 by - No Comments

I recently read about RadioShack losing $200 million last quarter.  I went to one RS store and found they wanted over $20 for a 5 foot USB male A to micro USB cable.  I told the salesdroid that I could get a similar cable for a fraction of that price at a dollar store.  He tried to dazzle me by throwing tech talk at me, telling me about impedances, when I interrupted him in mid sentence by telling him that I had already measured the difference and it was only about 15 to 20 percent.  (I should have told him that the impedance had nothing to do with the charging rate, it was the wire size, and hence the wire DC resistance that limited the charging current – but I figured, why embarrass him further).

I walked across the parking lot and bought a USB to micro USB cable for a dollar at Dollar Tree store.  The average consumer would not realize that the cheaper cable with thinner wire was reducing the charging current, but he would much rather buy the cheaper cable than pay 20 times more for the RadioShack cable.

Then I went to Fry’s Electronics And bought a dozen 3 foot long USB to micro USB cables for $1.99 each. The cable seems a lot thicker than the cheap Dollar store cable, but I haven’t cut one up to see if the wires really were heavier or if the jacket is just thicker. There is no doubt that they will charge at a higher current because the cable is half the length of the cheap dollar cable.

I wanted to increase the charging current, so I shortened one cable. But the thin wire requires a lot of soldering patience and small diameter heat shrink tubing. It’s just better to buy them already made short. I have a few 2 inch or 50mm very short A male to micro USB cables, so I used one between my Samsung Galaxy S4 and a charger rated for up to 2.1 amps. This charged at about 1 amp – it jumps up and down as the phone does different activities. I changed to a 6 foot or 1.8M cable and the charge current went down to 1/2 amp. This shows that the cable length has a significant effect on the charging current and hence the length of time it takes to charge.

Wire Size
I looked at the AWG (American Wire Gauge) wire tables to see what the copper wire resistance was for the various sizes. I’ll give a few examples.

I’ll start with the smallest, 32 AWG. This size has about 1/6 ohm per foot. Thus a 6 foot cable has about 1 ohm resistance in the positive wire, and 1 ohm in the negative wire, for a total of 2 ohms. A 1/2 amp current would drop 1 volt in the cable, so what starts out as 5 volts at the charger end is only 4 volts at the phone end. That is probably too low to charge, so the current would drop to a lower value to reduce the loss in voltage. The charging rate goes down and the time to fully charge increases.

AWG 30 is the next common wire size. This wire size seems to be a common size for wires used in many cables in inexpensive consumer electronics. It has about 1/10 ohm per foot, or 1.2 ohms for a 6 foot cable. A 1/2 amp current would drop about 0.6 volts across the cable. We now have 5 volts at the charging end and 4.4 volts at the phone end. Not as bad as 32 AWG, but still enough to cause a significant reduction in charging current.

AWG 28 is the next common wire size up the chart. I’ve found this size wire used in USB cables with full size A and B connectors, for the data line. For the power wires, typically a larger size is used. AWG 28 has .065 ohms per foot so 12 feet of wire has .78 ohm resistance and drops .39 volt at 1/2 amp. The phone would get 4.61 volts, still too much loss. The current would have to be reduced somewhat.

AWG 26 looks like it’s getting to the point where the voltage drop is not so unreasonable. It has .041 ohms per foot resistance for a total of about 1/2 ohm for 12 feet of wire. For 1/2 amp current, the voltage drop would be about 1/4 volt, just on the edge of being tolerable. The charging current will have to be reduced very little to compensate for the voltage drop. I would consider this to be a minimum size for a 6 foot or longer cable.

AWG 24 wire is a common size used for telephone house wiring, datacomm wiring, etc. It has .026 ohms per foot, or about .3 ohms total for 12 feet of wire. A half amp would have .15 volts drop. That’s not bad, and would be acceptable in my opinion. But consumer electronics makers find that the high price of copper makes good quality copper cable expensive so they cut the copper – use thinner wire – to save $$$.

So if the cable has thinner wire, then making it shorter will reduce the negative effect thin wire has on the cable’s performance.

You may see a ‘high current’ charger rated at 2 amps for the tablets such as the iPad being sold. This charger may put out 2 amps, but if you use the cable that came with it, you may find that the cable is restricting the current to much less than its full 2 amp capacity. Surprised? It’s the same cost cutting that I mentioned earlier. 🙁

And spending more money for a USB charging cable does not mean you will get a cable with heavier copper wire. Hopefully it might be thicker than the inexpensive cable, but it’s unlikely that the wires are heavy enough to not negatively affect the charging current. The best way to charge is with the shortest USB cable you can get, and add an extension cord to the power outlet to make the overall length long enough for your needs.

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2014-10-04 MS Win 8 Dying Away

Computer History computers Other Off Topic
on October 4th, 2014 by - No Comments

An article appeared in The Register about the percentages of Windows OSes out there being used.  I was surprised at how low the percentage of Win 8 & 8.1 were.  It looks as if Win 8/8.1 are going to be this decade’s Vista: treated as a hornets’ nest and completely avoided, even though the prospective buyer hasn’t ever been exposed to it.  Mike, my co-worker, had some thoughtful things to say.

Microsoft tried to force a change to how end users were used to using Windows in the past.  Change is fine but Microsoft missed the mark on keeping in intuitive enough so end users can easily figure out how to be productive again.

The metro tiles makes sense for touch screen tablets, mobile devices, and touch screen laptops but not for desktops (touchscreen would lead to “gorilla arms”) which are still used by most businesses.  So ditching the useful Start Menu was not a good idea.  For home use, I also think it was lame that Microsoft made Media Center a separate purchase instead of included free like it was with some Windows 7 editions.

After 8.1 came out with updates to allow booting directly to the desktop mode bypassing the metro screen and adding a third party Classic Shell start menu, then I can see myself trying to get used to using Windows 8.1 even though I might still like Windows 7 most.  Maybe given some time I can start to like Windows 8.1.
But the point for Microsoft is, why should end users have to add third party software and mess with buried configuration settings to get productive again?  Microsoft should have listened more to the end users.

What may have also kept the adoption number low for Win8/8.1 is that other software vendors have been somewhat slow on updating their software to be compatible.  Windows 7 has a nice feature called Windows XP Mode that allows a free license to run a virtual instance of Windows XP to run older software that might not be compatible with Win7.  Win8/8.1 does not offer any such feature but if they did offer a way to run a free virtual instance of Win7 and WinXP then that might have helped with the adoption numbers.  Businesses then, such as our district, just stayed on Win7 to avoid software incompatibility issues.
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Hopefully this time Microsoft does more end user surveys on their software to blend change with user expectations.

I also had another thought: MS may have feared that if they didn’t rush to get Windows to look more like tablets and mobile devices, they would miss the boat on the trend toward a universal OS that shares the same user interface on all devices. But I’ve never seen an Android device that had all the capabilities of a full keyboard and mouse interface.

I had been using Win 8 for almost a year, and I still didn’t like it. I bought a Win 8 for Dummies book when I first got it and that helped a little bit. But I’m not using it on a regular basis any more, since I sold the laptop. I was satisfied with Win 7. I will not, as long as I can avoid it, get a PC with Win 8 or 8.1.

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2014-09-27 Solarforce Z2 Flashlight

Flashlight
on September 27th, 2014 by - No Comments

I ordered and received a Solarforce Z2 flashlight a few weeks ago, and I’ve been using it for a few weeks.  It uses a AA cell, and has a push on, push off button switch on the rear end.  It also has tabs that stick out past the switch so you can set it on its rear end, shining upwards.

It has four light levels: high, medium, low and moonlight.  The high is advertised at 120 lumens, and the other three in my judgment look like the high, medium and low of the AAA cell flashlights.  In other words, the moonlight level seems to be the same as the low level in the AAA cell lights.  I like the medium, low and moonlight levels, which seem adequate for most of what I need.

For more info, go to www.Solarforce.HK.

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2014-09-23 Germanium Transistor Technology

Battery or Cell Digital Joule Thief SMPS DC-DC
on September 24th, 2014 by - 1 Comment

Paul is experimenting with a Joule Thief using some germanium transistors I sent him.  I thought about a few quirks of germanium transistors that I may not have (or may have) blogged earlier.

The vintage germanium transistors were made at a time when the solid state manufacturing technology was much less advanced than it is today.    Back then, in the late1950s through late 1970s, the processes were much less automated; they were more like baking a cake, with more dependence on humans with their frailties when it came to carrying out precisely timed procedures and precisely measured ingredients.  The chances of completing a procedure exactly as specified were much lower, and consequently greater variation in the transistor specifications had to be accommodated. 

One specification is the current gain of the transistor. This may be greatly affected by the time and temperature of the process. Those in turn may have been controlled by analog controls that were typically seen on many appliances in the home. As a result the time and/or temperature may have varied a few percent either way.

The transistors of today benefit from the technology that is used in integrated circuits. Early germanium transistors didn’t have this benefit; the transistor was a small bar of N type germanium with two dots of P type germanium baked into each side. Much of the transistor’s performance was determined by the process controls.

Another factor may have been the precision of the process. The emitter is much more like the collector, so the transistor is more symmetrical; the two may be swapped and the transistor may still have enough current gain to amplify the signal. I have seen others accidently swap the emitter and collector leads of a silicon transistor and it still worked in a Joule Thief circuit, but not as good as the correct way.

Another parameter that germanium transistors have is the emitter to base breakdown voltage. Silicon transistors may be rated at 5 or 6 volts, and germanium transistors may be rated at double that amount or more. I have seen germanium transistors rated at the same max voltage for both emitter to collector and emitter to base, 25 volts, for example.

Early germanium transistors were packaged in a package that was not hermetically sealed. The package may have been plastic or epoxy, but the humidity from the air often got between the plastic and wire leads, and caused the performance to degrade. Since the government was still a major purchaser, the transistors had to be tested for reliability for use in computers, missiles and space vehicles. Germanium transistors cannot be passivated with an oxide coating as can silicon. The manufacturers changed the package to a hermetically sealed metal package, and the reliability was greatly improved. Today you will seldom, if ever, see a germanium transistor in a plastic package.

I connected a Joule Thief to a AAA cell that I found in the street, dented and smashed by cars running over it. It keeps on going and going, like the Energizer Bunny. One part – probably the largest part – is that the germanium transistors, as I said above, have much lower gain. Thus the typical 1k resistor used for a silicon transistor, is not supplying as much current, especially at low battery voltage, to give the Joule Thief as much base current, and therefore the collector current is much lower, giving a much longer runtime. The brightness is lower, but that is not noticed; the JT just keeps on running. At low battery voltages below 1/2 volt, the resistor may be zero ohms, but that will reduce the runtime a lot.

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