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

Projects 33 Comments

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

Cleverness, Projects 12 Comments

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.

XL741 – Discrete Op-Amp

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We’ve done it again! My friends at Evil Mad Scientist Laboratories have a new kit for sale. Following on the success of our Three Fives discrete 555 timer kit, we’ve had a lot of requests for a discrete 741 op-amp.

The XL741 is based on the datasheet schematic of the original uA741 op-amp IC from 1968. You can wire it up in a classic op-amp circuit and probe nodes inside the IC so you can see how the chip works. Play with differential pairs, modify the compensation, and change bias currents to your heart’s content!

Flea Market Find–Dual Gun CRT

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At the electronics flea market on Saturday, I found a 3ABP7 dual gun CRT. This one was built by DuMont, most likely intended for the 3″ version of their 5″ Type 279 Dual Beam Oscilloscope.

So of course I had to fire it up. There are two sets of deflection coils, so I drove them with one deflection board and cross-wired the deflection coils to flip the image around on the second gun.

The guns themselves have a common cathode connection and separate grids, which forces me to drive them both in parallel since my deflection board video amplifier keeps the grid at a constant potential and drives the video onto the cathode.
Dual gun 3ABP2 CRT

Here’s a closer look at the guns. The filaments are connected in parallel so this tube uses twice the normal current.
Dual gun 3ABP2 CRT

Compensating CRT Deflection Coils

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Things look really weird when you use an uncompensated deflection coil with a vector graphics display. This is because the coil looks like an inductive load to the driver amplifier, and the parasitic capacitance makes it ring. Ringing and overshoot create the strange-looking display, basically extending every line past its destination.

One way to compensate for that is to add a series RC snubber in parallel with each deflection coil. You can perform some calculations to figure out the values of the resistor and capacitor but they won’t get you very close to the answer. There are just too many parasitics to model. It’s much faster just to build a RC substitution box and tweak the values until you get the result you want.

CRT Board BOM Updates

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I have corrected some mistakes in the bills of materials for the CRT driver boards (the files in GitHub have already been updated):

  • ScopePower U2 part number from TS271CN (DIP package) to TS271IDT (SOIC).
  • ScopePower R3 part number from VR37000003305JR500 (33 meg) to VR37000001005JR500 (10 meg).

If you already ordered parts, I apologize. The TS271CN is a great little op amp and the DIP package version will work fine in a breadboard. That’s what I will do with the ones I accidentally ordered. The 33 meg resistor is useful as a bleeder resistor if you build the post deflection acceleration module (to be posted soon).

Mel, Blackjack, and the LGP-30

Cleverness No Comments

A friend passed on the following articles regarding a rather famous hack:

https://news.ycombinator.com/item?id=7869771
http://www.jamtronix.com/blog/2011/03/25/on-the-trail-of-a-real-programmer/

The LGP-30 was an early computer manufactured in 1956 and sold by the Royal McBee division of the Royal Typewriter Company. It had 113 vacuum tubes, 1450 diodes, a magnetic drum memory, and an oscilloscope screen with a template marking the register bits (instead of the more traditional panel with blinking lights). Mel wrote a lot of code for this machine, including a blackjack demo program, entirely in raw machine code, taking advantage of various quirks in the machine to keep the program very compact.

The Jamtronix blog has a link to a purported paper tape dump of Mel’s program.

Foolishly I decided to figure out the paper tape dump. I succeeded, and it looks like it’s in a format that a bootloader was supposed to understand, but not the bootloader documented in the LGP-30 manual. So first I ran the raw file into a popcount routine to see what the most common letters were. It was a subset of the alphabet, but had all the numbers except for the number ‘1’.

Wat?

Back then they did a few strange things. One of them was using the lowercase letter ‘L’ as the number 1. Right. Also, since the LGP-30 was hexadecimal, they used extra letters to represent numbers beyond 9, but instead of using a-f, they used fgjkqw.

Then the single quote ‘ is used as a record delimiter, and the letter ‘v’ at the beginning of the block of records identifies the track and sector where the data is to be loaded. Each block has 64 words, 8 per line (which fits perfectly in a single track on the magnetic drum memory). To save space, they omit leading zeros, so incoming characters of 4 bits each are shifted left 4 at a time. At the end of the block is a stray word whose purpose I cannot figure out. I suspect it’s a parity or checksum, but it doesn’t match a simple checksum of all the data.

This machine’s architecture is a clever nightmare. Self modifying code was encouraged and was the standard way to accomplish common tasks. For example, to branch to a subroutine, you do this:

1000: r 3050     <- Set up return address by writing PC+2 to the destination address of the instruction at 3050.
1001: u 3000     <- Unconditional branch to address 3000.
1002: ...        <- Magically we pop up back here!
...
3050  u 0000     <- Looks like a unconditional branch to location 0 right? No, it's just a placeholder address. Yay, we can do without a link register!

And Mel coded this up in machine code. Probably because the “assembly” mnemonics are not particularly helpful, he must have quickly memorized the opcodes themselves. And why did he use instructions as data? The machine doesn’t decode the upper 12 bits of a word, so if you didn’t use it, it was just wasted. Basically the architecture of the machine naturally led Mel down this slippery slope.

So you might think that Librascope/Royal McBee, based on customer feedback for this machine, must have simplified the design for a later machine, the RPC-4000. And you would be wrong–it’s even worse…

Anyway, here’s my disassembled version: blackjack.txt. The track number is listed first, once for every block. In each block, every line has the sector number first, followed by the instruction mnemonic, following by the track and sector numbers of the instruction’s target. After that are the raw bits. The first two instructions are clear instructions followed by an unconditional branch. This goes to track 49 sector 34. You can follow the instruction flow from there.

As for me, I’m going to stop now before I accidentally learn how to program the LGP-30.

 

P10 Dark Trace CRT – The Skiatron

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I have a little story to tell. Years ago, I met someone who had a very large collection of CRTs. He had everything from common 3BP1s all the way to rare little gems like the 1EP1 and some vintage prototype CRTs. He showed me an item in his collection which was a rectangular CRT with a P10 phosphor. Finding a CRT with the P10 phosphor is like finding a unicorn. P10 is not really a phosphor; it designates a screen coated with some sort of alkali-halide (potassium chloride) that darkens when hit with an electron beam–a scotophor. The darkening effect lasts until the coating is heated, and typical P10 CRTs have a built-in heater that erases whatever was recorded on the screen.

Anyway, a few years go by and I lose contact with the guy. Rumors are flying around, and it turns out that he has decided to sell his entire collection. Bits and pieces of it start showing up at auction houses and flea markets. Another friend of mine mentions that he picked up a lot of CRTs at an auction house and asked if I wanted to pick through it. While sorting through it, I recognized the rare beast and bought it on the spot.

P10 Dark Phosphor CRT

So I finally got the time to hook it up and try it out. There doesn’t seem to be any documentation. The part number is 06E024P10, made by Thomas Electronics. It works, but not particularly well. Since the pin connections are nonstandard and the electron gun has some extra elements, I’ve probably got it connected all wrong. Anyway, I was able to put some scribbles on the screen.

P10 Dark Phosphor Screen

Notice the dark purple areas. I am shining a lamp through an aperture in the top of the CRT.

P10 Dark Phosphor Screen

Looking through the aperture, you can see how the CRT has a standard green phosphor section on the top third. This might have been used to verify that the tube’s electron gun was in focus. It could also have been used for status information. Most likely this tube would have been used in an early form of storage oscilloscope for capturing single-shot high speed events, although most examples of P10 tubes were designed for radar displays.

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