Before we look at preparing the Arduino/Freeduino microcontroller for connection to the nixie tube driver modules described in Part I and Part II, you can build this power supply to provide the high firing voltage required by the nixie tubes. This switch mode power supply easily outputs 50 mA, which is more than most, and offers a variable output from 150 to 220 VDC, when driven by a 9 to 16 VDC source.
Step 1: About the Circuit
A 12 volt source at one amp will easily drive this nixie tube supply. There is sufficient power produced by this switch-mode supply to drive at least eight of the nixie tube driver modules (I've had 12 of the nixie tube driver modules running off of one of these boards, that's 24 IN-12A nixie tubes!).
A typical nixie tube power supply offers 170 to 250 VDC at 10 to 50 mA. A switch-mode power supply is desirable because it's small and very efficient. You can fit it inside your clock and it won't heat up. The schematic for the project is taken directly from the MAX1771 datasheet, however, because of the large voltage jump from input to output, board layout and low ESR type components are critical.
Step 2: Parts List
The Following are Digi-Key Part numbers for all components:
CAP TANT 100UF 20V 10% LOESR SMD
CAP CER .1UF 25V Y5V 0805
CAP 4.7UF 450V ELECT EB SMD
CAP TANT 10UF 25V 10% LOESR SMD
CAP .1UF 250V PEN FILM 2420 5%
CONN TERM BLOCK 2POS 5MM PCB
J1, J2, J3
INDUCTOR POWER 100UH 2A SMD
RES 10.0K OHM 1/8W 1% 0805 SMD
RES 1.5M OHM 1W 5% 2512 SMD
RESISTOR .050 OHM 1W 1% 2512
TRIMPOT 5K OHM 4MM SQ CERM SMD
IC DC/DC CTRLR STEP-UP HE 8-SOIC
MOSFET N-CHAN 300V 14A TO-220F
DIODE ULTRA FAST 3A 400V SMC
Step 3: Preparing Parts for the Printed Circuit Board
These parts I Ieave to solder conventionally after I've got all the smaller surface mounts parts on the board.
Step 4: Oven Soldering
Here are the smaller parts that we'll apply to the printed circuit board with solder paste, and then toast in our oven.
Step 5: Solder Paste
Get with the gooey stuff. Pull the solder paste out of your fridge and give it a chance to warm up. Then it's not so stiff when you try to force it out of the tube. The best part is that if your board has a good solder mask, you don't have to be quite so precise. Once the paste hits the oven it will flow to just where you want it (most of the time - see step 9).
Step 6: Solder Paste Application
Settle in and hold the caffeine because you need steady hands for this work. Put your thumb over the plunger and gently squeeze the paste onto the pads. Don't worry so much if you're not always on the mark. Excess paste will clog fine pitch parts, so go easy.
Step 7: Pre-Heat Oven
Once you know where the components go, it's quick to apply this amount of paste to a small board. This is about the right amount of paste for successful toasting. Get out your pick-up tool and lay on the SMDs.
Step 8: Seat Components Into the Paste - and Toast
The solder paste used here is lead-free, and though it looks dull and murky now, just wait until it pops up in the oven. The standard issue toaster oven I'm using I got for $20. It's got 3/8" wide quartz heaters above and below the oven rack. I can toast six of these boards at a time. Here's the temperature curve you'd like to adhere to:
Preheat your oven to 200 deg F
1. insert the board into the oven and hold it at 200 deg F for 4 minutes
2. Bring temperature up to 325 deg F for 2 minutes
3. Hold at 450 deg F for about 30 seconds until solder pops up, then wait 30 more seconds
4. Tap the side of the oven, and drop temperature to 300 deg F for 1 minute
5. Allow to cool, but not too fast. You don't want to thermally shock the components.
Step 9: Post-Toast Inspection
After the board is cool, examine it for shifted parts and solder bridges. You can see some beads of solder in places where they might get into trouble. Gently tap them away and off the board.
Uh oh. It looks like we've got two solder bridges on the right side of the 8-pin IC.
Step 10: Solder Wick Is Your Friend
Here's where the truly deft work occurs. Fan open the end of the braided solder wick mesh so that it will pick up molten solder. Place it over the solder-bridged location, and press down with a hot iron. Apply heat for no more than 5 to 7 seconds. This is usually all you need to do to remove the solder bridge. If it doesn't work for you the first time, maybe try approaching the board from a different angle.
Step 11: Solder Remaining Components to the Printed Circuit Board
Ok, pull up to your solder station, and locate the components set aside in Step 3. The MOSFET is static sensitive so don't run across the carpet with this one. We're almost done.
The two solder bridges on the step-up converter have been removed with the solder wick, and the board is now complete.
Step 12: Connecting HV Power to Nixie Tube Driver Modules
If you are connecting this high voltage nixie tube power supply to a nixie tube driver module, here is a simple test set-up. Refer to the markings beside the green terminals on the printed circuit board.
For main PWR input voltages supplied to the nixie tube power supply that are lower than 15 volts DC, you can connect the PWR and Vcc terminals together.
For main PWR input voltages supplied to the nixie tube power supply that are higher than 15 volts DC, you will need to insert a regulator (7812) to provide 12 volts DC to the Vcc terminal.
If using a 12 volt AC adapter, for example, the PWR terminal and Vcc terminal should be connected with a short jumper wire. For normal operation, also connect the Shdn terminal to GND with a jumper wire. This will enable the nixie tube power supply to produce an output when input power is supplied.
Step 13: Power Input Pins
The HV+ and HV- labels on the nixie tube power supply correspond with HV and gnd on the nixie tube driver module.
The HV- lead connects to pin 1 of SV1 (gnd), and the HV lead connects to pin 4 of SV1. For SV1 and SV4, pins 1, 2, 5, and 6 are all connected to gnd.
Only pins 3 and 4 of SV1 and SV2 carry the high voltage required by the nixie tubes.
Step 14: High Voltage Threading Throughout the Modules
Now that you have power supplied to the nixie tube driver modules, you should see all the elements in both nixie tube digits illuminated.
Use caution not to touch the high voltage output to the nixie tube driver modules. There is potentially enough energy here to cause a severe shock.
When nixie tube driver modules are connected edge-to-edge, left-to-right, both high voltage power and serial data from the external microcontroller are threaded through to all the boards.
A microcontroller is required to take full advantage of the nixie tube driver module shift register chain. The nixie tube driver module permits a microcontroller ( Arduino, etc.) to address two nixie tube digits, and via this shift register chain, multiple pairs of nixie tube digits.
For an example of how the nixie tube driver modules may be supported by an external microcontroller, see the sample Arduino digits driver code. Multiple nixie tube driver modules are seen operating together in the nixie tube driver module movie.
Depending how brightly you wish your nixie tubes to be illuminated, you can adjust VR1 to generate output between 170 and 250 volts DC. Increasing the output power will also allow you to drive more nixie tubes simultaneously.
Stay tuned for Part IV, where we'll hook up an Arduino Diecimila, and make some very long numbers. Extra special thanks to Nick de Smith.
See also this nice bit of work by Marc Pelletreau.