Deuterium Arc Lamp

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On Saturday I found a deuterium arc lamp at a local surplus store. It was used, and most likely pulled from an ultraviolet spectroscopy machine. I could not find data on the specific lamp model, but I found a similar lamp. On the chance any of you might know what it is, the lamp is marked

D 805 K

56066349

West Germany

H9

Before running any tests with the lamp, I wiped it down with isopropyl alchohol to remove any fingerprint oils. When heated, they can cause the glass envelope to bubble and even melt, destroying it.

To run this lamp, which is a gas-discharge type, you first have to heat up the cathode. There is a very thick double-spiral tungsten filament inside that uses 2V at 4.5A (or 9 watts!). Once it’s warmed up for a minute or two, you apply the high voltage to the anode. I connected it to a current-limited electrophoresis power supply set to 50mA. The lamp started at 350V and settled to an operating voltage of about 84V. Incidentally, the heat generated by this helps keep the cathode hot, and the filament current can be reduced to improve its lifetime.

Here’s a quick video showing what it looks like.

Deuterium is an isotope of hydrogen: hydrogen has one electron and one proton, and deuterium takes that and adds a neutron. It is not a radioactive isotope, unlike tritium, which has two additional neutrons. According to Wikipedia, Deuterium is used in these lamps because it emits more UV with a wavelength less than 400nm.

If you’ve got one of these lamps and you plan to light it up, you’ll need eye protection. I ran it at a very low beam current (most likely it was designed for 300mA!) and the light was not so intense, but you might want more than just a pair of sunglasses if you’re going to full power…

Miniature Nixie Power Supply – How it Works

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For more details and the video about the miniature Nixie power supply, see my original post.

First off, this circuit is not a Royer oscillator. As summarized by Jim Williams in his famous app note AN65, Royer developed a power converter using a transformer that saturates every cycle.

A coil saturates when the magnetic field (the B field) has reached the maximum that the magnetic core material can support: if the current (which is what creates the magnetizing H field) increases more, the magnetic field increases very little. The inductance, which is proportional to (B ÷ H), rapidly drops off, causing the current in the coil to increase at a much faster rate. Royer’s design detects this current spike and uses it to switch the transistors (2N74s, in his original paper) into their opposite state.


My circuit is a more common LC resonant converter. There are two transistors, Q1A and Q1B. Resistor R1 provides the bias current for the transistors and gets things started. Current (represented by the large red arrow) flows through the center tap of the coil T1 out to Q1A through its collector. The current in the upper half of coil creates a magnetic field, and the magnetic field induces a voltage in the feedback coil. This voltage reduces the base drive for Q1A and increases the base drive for Q1B (represented by the small blue arrow). When that happens, Q1A shuts off and Q1B turns on. The current in Q1B’s collector (represented by the large blue arrow) creates a magnetic field of the opposite polarity in the coil, and therefore causes the feedback winding voltage to reverse polarity (see the small red arrow), turning off Q1B and turning on Q1A. The cycle repeats as long as there is power.


Above are some approximate waveforms. You can see that the transistors go back and forth, driving the coil first one way and then another. Capacitor C1 and inductor L1 help determine the resonant frequency of the circuit. If you measure the voltage across the entire coil, you’ll see a sine wave.

The output winding of the coil has a lot more turns than the input winding, and it increases the voltage (at the expense of the current) dramatically. This high voltage AC goes through the half wave rectifier formed by D1 and gets filtered to DC by C2. R3 limits the current into the Nixie tube.

You might be wondering why T1 is actually an inductor–an 8.2 millihenry one. It just makes the project easier to build. You only have to wind 12 turns on an off-the-shelf part instead of buying a hard-to-find transformer core and adding all the windings yourself.

If you feel so inclined, try adjusting the component values. Start with C1 and then maybe R1 or even L1. Try changing the number of turns on the coil.

Miniature Coin Cell Nixie Tube Power Supply

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Update 11/3/2014: Fixed the coil connections on the schematic, along with the inductance. Also, check out this post to see how the circuit works.

This project has been a long time in progress. It started years ago at a Maker Faire where I built a Nixie tube pendant powered by a lithium coin cell battery. Since then, I’ve decided to make a PC board and put together some instructions on how to build such a power supply yourself. These little supplies are great for steampunk jewelry or possibly single-digit Nixie tube clocks (they’re not quite strong enough to drive multiple tubes). The battery life should be around 3 hours or so for a CR2032 lithium coin cell.

The schematic is below–click for a larger view. The bill of materials is located here, including Mouser Electronics part numbers. If you decide to order, get at least 5 of each part just in case you lose or burn up some of them.  Note: Mouser seems to be out of stock for the T1 inductor, but Digikey has it here.

Q1 is a single device; it actually contains two transistors which is why it looks like that on the schematic.

To make it easier to build, I’ve put up a convenient OSH Park project page link so you can order boards. When I ordered from them, it cost $2.80 for a set of three boards (with free USPS shipping). Not a bad deal at all!

When you have boards and components, there’s a specific order of assembly that makes things a bit easier. See this YouTube video:

Basically you need to assemble the components in the following order:

  1. Solder C1.
  2. Solder Q1. Be sure you line up the beveled edge with the extra-wide silkscreen. If you put it in backwards the power supply will not work.
  3. Flip the board over.
  4. Solder D1, then R2, and then C2. C2 is 0.01uF, similar to C1, but it has a 250V rating. It is very important not to mix these up.
  5. Solder R1, and then T1 (the big coil, not marked on the silkscreen).
  6. Take a piece of 32 gauge magnet wire that is 13 inches long and tin about 1/8 of an inch at one end. I use a soldering iron to burn off the varnish. Solder it into the upper left through hole that is below the coil.
  7. Wrap 5 turns clockwise around the coil T1. Thread the end of the wire into the middle through hole below the coil.
  8. Turn the wire around and thread it back through the same hole, pulling it tight to form a tiny loop. Solder the loop to the through hole.
  9. Take the wire and wrap another 5 turns clockwise around the coil. Thread the end through the top right through hole below the coil, and solder it in place. Trim off any excess.
  10. Get another piece of 32 gauge magnet wire that is 5 inches long, and tin about 1/8 of an inch at one end. Solder it into the lower left hole that is below the coil.
  11. Wrap 2 turns clockwise around the coil T1. Thread the end of the wire through the lower right through hole that is below the coil. Solder it, and then trim off any excess
  12. Solder the inductor L1. The reason it needs to be soldered last is that it makes it hard to wind wire around the coil T1.
  13. Solder connecting leads to the +, -, and OUT terminals.

To use it, connect a coin cell’s negative terminal to “-,” the coin cell positive to “+,” and “OUT” to the anode of a Nixie tube. The Nixie tube cathode goes to the coin cell negative terminal. Don’t touch the “OUT” terminal–you could get a shock. In fact, if you build the power supply into jewelry or something people will be touching, insulate all the connections.

Have fun!

 

A Miniaturized Discrete MC1466

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Update: the bill of materials is now available. You can order boards from OSH Park using this direct link. Last time I ordered, it cost me a grand total of $3.55 for three boards (free shipping), and it took about two weeks for the boards to arrive.

I screwed up. My bench power supply is a Lambda LPT-7202-FM triple output (0-7V @ 5A, 0-20V @ 1.5A, 0-20V @ 1.5A), and I blew it up by trying to desulfate a lead-acid battery. The idea is to take a dead lead-acid battery and recondition it by charging it with a current-limited 15V source while feeding it high voltage pulses. I had a diode connected in between the battery and the bench supply to protect the power supply from the high voltage pulses. Well, the diode failed. It was a sad day.

Fortunately the service manual for the supply is available online. I traced around the circuit and found that two of the power control chips were fried, but everything else seemed OK–I could move the one remaining functional chip from channel to channel to confirm that. The control chips were marked with the Motorola logo and a Lambda house part number: FBT-031. A forum thread indicated that the part was actually the MC1466. Sadly this chip is long out of production and a bit hard to find, although a popular auction site had several listed from a seller in China (but who knows if the parts were counterfeit or not).

The datasheet has the full schematic including resistor values, but how do I know that it actually matches the chip? Since the IC is packaged in a ceramic DIP, I followed reader Uwe’s suggestion and took a chisel to one of the dead parts.

It worked and nothing was damaged! The die looks like this:

I went over the layout and it matches up with the datasheet schematic. Those funny round elements are actually zener diodes. You can see the long skinny resistors and the lateral PNPs as well as the NPN transistors and diodes. Below is the schematic (click to enlarge):

The IC design is pretty archaic. I’d say it dates to the late 1960s. There are fairly ordinary differential amplifiers, but the current mirrors are really strange, and the voltage reference circuit uses Zener diodes and series-connected diodes instead of a temperature compensated bandgap reference. The two Zener diodes (the only round features on the die) are probably just reverse biased NPN transistors, using the ~7.5V avalanche breakdown of the base-emitter junction. The lateral PNPs have a much higher breakdown voltage so they can’t be used this way.

Here’s a labeled die photo (click for a larger image) so you can see where each of the components are. The component designators match up with my schematic, not the IC datasheet schematic.

The device is simple enough that I decided to build a really small PC board with discrete components. I found that the BC847BVN (NPN/PNP dual transistor), BC847BV (dual NPN), and BAS16VV (triple diode) came in a really tiny SOT-563 package. Believe it or not, this is not the tightest or smallest layout I’ve done. This is a 2-layer board with 6 mil traces and 6 mil spaces.

To give you an idea just how small the SOT-563 is, take a look at the first BC847BVN I soldered:

The part is 1.05mm x 1.05mm! I had to use a very fine soldering iron tip and a microscope. Another trick is to use really thin solder (I used 0.38mm). As you can see, the resulting board is just slightly larger than the original DIP IC:

It really is pin compatible. I plugged two of them into my Lambda supply and now it works perfectly!

Inside a TTL Logic IC

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Since 2014 is generally considered the 50th anniversary of TTL logic, I thought I’d take a TTL logic chip apart and do a little analysis.

So I started with a DM7438N, lot code M:P9006Y. Looking at National Semiconductor’s device marking convention document, I take this to mean that it was manufactured in week 6 of 1990 at a subcontractor’s fab in the United States and assembled in Malaysia.

The 7438 is a quad 2-input NAND buffer with open-collector outputs. That means the die should look symmetrical to a degree.

To take it apart, I used a rotary tool to carve out the encapsulation material on the top and the bottom, and then picked at it with side cutters until the chip fell out. Sadly I cracked off a corner of the die including one bond pad, but it’s still possible to figure out how it works.

What does all this do? See the image below. I’ve cropped all but one gate and highlighted the various semiconducting regions in different colors. I’ve also given designators to all the components.

Red represents the N-type collector epitaxial diffusion. Cyan represents the P-type base diffusion, and purple represents the N+ emitter region.

The schematic looks like this:

That dual-emitter transistor (Q1) sure looks strange!

How does it work? Well, if both A and B inputs are a logic high, then Q1 is off, but some current flows from R1 (4K ohm) through the base collector junction (since it is, after all, a PN junction) and feeds the base of Q2. Q2 turns on, and its emitter current feeds R3 (1K ohm) and Q3. Q3 turns on as well, and the output Y gets driven low. The non-inverted version of the output signal is available at the collector of Q3 (biased through R2, a 1.6K ohm resistor), but this particular chip doesn’t use it.

If either A or B goes low, then Q1 gets turned on. Current flows through the base emitter junction and the base gets pulled to about 0.6V above ground. No current flows through the base of Q2 because the voltage on the collector of Q1 is just too low for any current to flow. Q3 therefore stays off, and the output Y goes high impedance. By the way, this is what open collector means–the collector of the output stage transistor is left “open” with no corresponding transistor above it to pull it high.

Diodes D1 and D2 are just for input protection.

There are a couple of unused components. There is a resistor right below R1, and another resistor below R2. There are two extra transistors with a shared collector to the left of Q3. A different top metal mask could connect these extra components into the circuit and change the function of the device.

Can you think of some other gates that could be built by changing the top metal mask? Remember that there is only one metal layer which limits where you can route the traces.

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