Tube Time

My IEE clock runs on about 6.5VDC. Why the odd voltage? There are sundry voltage drops throughout the circuit that require the power supply voltage to be higher than the 6V rating for the light bulbs. The problem is that you just can’t find 6.5V AC adapters.

Fortunately it’s quite simple to modify an existing AC adapter to run at the new voltage. I’m sure there are other people who might want to get their own custom output voltages, so here’s a short tutorial.

Disclaimer: if you go ahead with this project, you’re doing it at your own risk. AC line voltage is quite dangerous and you could be injured or even killed. If you get shocked, get medical attention right away: there have been people (often with a previously undiagnosed heart condition) who have received a “small shock” only to drop dead a couple of hours later.

Here is what you need: A switching AC adapter, a flat screwdriver, a soldering iron, an assortment of surface mount resistors, and some time. It is important that the AC adapter is of the new switching style rather than the old transformer style–usually you can tell if the AC adapter feels light and is small enough not to block adjacent outlets, yet has a relatively high current rating for its size. I am starting off with a fairly generic 5V 2 amp AC adapter. Yours will likely use a similar circuit inside.

Quick side note: Why are the new AC adapters so much smaller? First, to provide a given amount of power, a transformer (copper windings around some type of core material) has a physical size that decreases as the frequency goes up. A transformer designed to operate at 60Hz or 50Hz  is going to be much larger than a transformer that is designed to operate at 15KHz. Since the frequency of the AC line is fixed, these new AC adapters work by converting the input AC to a much higher frequency. There is also a little circuit that looks at the output voltage and tweaks the converter circuit to maintain a constant output voltage. Thus, the new AC adapters have much better regulation than the old styles. As an added bonus, this type of design can operate at 50Hz, 60Hz, 120V, or 240V! To get into more detail, there is a feedback circuit that compares the output voltage to a voltage reference, then sends an error signal to the converter circuit at the primary side of the transformer. The error signal is isolated (for your safety) using an optocoupler.

To take apart the AC adapter, crowbar the plastic halves of the case where they meet in the middle. Try to find the little plastic catches that keep the halves together. You might need two screwdrivers to get them to come apart. Inside you can see a little circuit board with components and some wires.

To modify the output circuit, the feedback circuit must be modified. I sketched out the circuit and learned that this particular AC adapter uses a clone of the TL431 reference+error amplifier circuit (which is the weird zener-diode looking thing in the photo above). This particular device, the AM431, has an internal 2.495V reference. The output voltage is divided down so it can be compared with 2.495V, and this is done with a resistive divider formed by R1 (4.87K) and R10 (5.23K). R10 goes to the output voltage, and R1 goes to ground. In this design, I can only modify R1 since R10 (acting in combination with the feedback capacitor) also determines the loop compensation pole (no need to mess with this). Based on my reverse-engineered schematic of the secondary side, the formula for the output voltage is Vout = (1 + R10/R1) * 2.495. Replacing R1 with a 3.3K resistor should bump up the output voltage.

It’s easier to remove a surface mount resistor using two soldering irons, but if you only have one, you can blob a bunch of solder on top of the resistor to wet both sides, and then flick it off with the iron. Be careful to get rid of any splattered solder since it could cause some dangerous short circuits.

You can test it before putting the case back on by plugging it into a power strip that has a circuit breaker and a switch–turn it off first! Connect a multimeter to the outputs with alligator clips, and use one hand to switch on the power and check the voltage. If you want to be a little extra safe you could plug the power strip into a GFI outlet. The safest approach (especially if you want to take measurements on the AC line side of the isolation barrier) is to connect the whole rig to an isolation transformer. Once you’ve confirmed that it works, put the case back together and mark the label with the new output voltage. If the output voltage is higher, you will need to decrease the current rating proportionally. If you’re decreasing the output voltage, you shouldn’t increase the current rating since internal components may have a pretty low maximum current capability.

If you want more information about the TL431 (which is a really neat little device that could be useful for lots of other circuits), check out this application note: Designing with the TL431.

Recently I purchased a number of nixie tubes from the local electronics flea market:

A few of them caught my eye. This Burroughs B-5448 one was most likely used in a calculator or possibly a meter to indicate plus and minus as well as the overload condition.

And this National NL-989 was used for indicating the unit or mode of a multimeter. There are two “partitions” in this nixie tube: one contains the symbols “A”, “M”, and “K”, while the other contains “C”, “V”, and “Ω”. The multimeter would have used this to indicate “AC”, “MV”, “KV”, “MΩ”, “KΩ”, and so on.

Notice how the glow is a different color in each tube? My camera did an excellent job of reproducing the correct color, so what you see is very close to what the ionized gas actually looks like. The National tube appears to contain mostly neon, while the Burroughs shows light blue “fringing”, indicating the presence of mercury, which was used to increase the lifespan of the tube.

At the electronics flea market I found a rather interesting-looking vacuum tube. It appears to be a CRT but with a metal cap at the end.

As it turns out, this device is a memory which could have been used in some old computers, storing around 16 kilobits. It could also have been used in radar systems for converting polar coordinate-based sweeps to raster sweeps (like a TV). The socket end contains an ordinary electron gun like the one found in a CRT, but the front end does not contain the usual phosphor screen. Instead, there is a 1mil thick sheet of mica with a fine grid of wires laid on top. On the other side of the mica there is a metal plate.

Here’s a closeup showing the metallic screen on the front. The mica underneath capacitively stores electrons that are laid down by the electron beam. This memory can store analog waveforms since higher voltages are represented by a higher density of electrons at a particular spot, and lower voltages correspond with a lower electron density.

Data for the tube is available from David Forbes.

Here are a couple of other sites with information:

Åke’s Tubedata

World Power Systems

Virtual Valve Museum

Cold War Infrastructure (A full-page RCA ad for the device)

In this previous post I disassembled the Coby DP-151SX digital picture frame. This device is very hackable, and includes a lot of goodies such as a Li-Ion battery and battery charger circuit as well as a neat little color LCD display with a white LED backlight. The pinout for the LCD is in the previous post.

The MAXQ2000 microcontroller development board I have uses a 0.1″ spacing header to connect to the I/O pins, so I made a little adapter and wired it up to the LCD connector using wire-wrap wire. It uses 13 I/O lines, but that could be reduced 11 if CS# is wired to ground and RST# tied to a separate reset IC (such as a MAX811). It’s actually a good idea to use CS#, because you can then multiplex the functionality of all the other pins and recover that I/O.

Here is a picture showing the LCD up and running with a simple test pattern:

It’s not 128×128, but actually 132×132 pixels. The color depth is 16-bit using a fairly standard 5-6-5 bit encoding. See the PCF8833 datasheet for more details.

Spark Fun has a similar LCD display which uses the same controller, only it costs $20. Amazon.com sells the Coby-151SX in black for $10. Not a bad deal: for $10 less you get a Li-Ion battery, mini-USB cable, and a driver CD, which you could use as a coaster for your Mountain Dew to help with the LCD programming. Spark Fun has some sample code which you should easily be able to adapt for parallel mode (since the Coby LCD connector brings out the parallel data lines, unlike the Spark Fun LCD).

The source code for my test program will get posted once I clean it up and possibly add functionality (Character fonts? Bit blitters?)

CNET’s Crave has a really fascinating post about a gentleman in Vietnam who has figured out how to unlock 3G iPhones–as of this writing, there is no software way to unlock this phone.

Unlocking iPhone 3Gs–the Vietnamese way

There are some interesting pictures showing some methods used by the Vietnamese technicians to remove and replace the flash memory device. They use a cheap hardware store heat gun, which is normally used to strip paint, to heat the solder until it melts, and then they remove it with tweezers. One of the problems with this sort of rework is that the adjacent components often loosen and shift or even fly off completely.

Look at the last picture in the article. The technician is using some small metal plates to protect the other components from the blast of hot air. Genius!