Driving Nixie tubes - Page 1
Forum section: electronics / projects

author: aheid
author_rank: Regular Contributor
timestamp: March 12, 2021, 02:02:36 pm
content: So of course I wanted to make my own nixie tube clock...So while I wait for the arrival of the tubes, I'm thinking about how to drive them.I've seen Daves videos on his Nixie project and many other similar ones which use high-voltage shift-registers, but I was thinking about taking a slightly different course and would like to know if it's a decent approach or stupid. The motivation for doing it this way was that I could then have a smaller and relatively cheap "driver board" per tube, so that I wouldn't have to waste an entire "motherboard" in case I screwed things up during production or similar.The core idea was to use a single 74HC595 shift register per tube, and drive the 12 tube elements with a transistor each through a kind of 3x4 multiplexing scheme. To light up a given digit the corresponding row (base) would get pulled high, the other rows low, and the corresponding column (emitter) low, the other columns high.For current limiting I added a fairly high emitter resistor (~1.1k) in addition to the typical anode resistor (~15k), as this seems to provide a lot better current control in face of varying supply voltage. It also seems to make it relatively easy to have individual current control for the dots which only require ~0.5mA rather than ~2mA. Simulations look good, but is this a viable strategy or are there some gotchas? FWIW I'll be using 1% resistors, and will be driving the tubes at 170V.I've not added any protection to the schematic yet, as I'm not sure what is needed. At the very least I'll be putting some protection diodes etc on the "motherboard" to protect the microcontroller.Any feedback much appreciated.

author: pqass
author_rank: Super Contributor
timestamp: March 12, 2021, 04:01:24 pm
content: To further limit the current for RDP and LDP, I would put R11 and R12 on the collector side. It seems more natural to me and won't cause issues with turning on/off the transistor (having the emitter not directly grounded albeit through the '595).Also, you don't need R6, R8, and R4.Normally, in a switching application the emitters are grounded but you're using '595 outputs to switch whole columns out (QH, QE, QA) by raising the emitters to H (3.3V = whole column off) or L (0V = column on, row follow corresponding base).I just looked at the TI 74HC595 datasheet and you have more issues... "±6-mA Output Drive at 5 V"I'm not sure if the max current spec changes at 3.3V operation.Also, using a 15K anode resistor makes the switched current needed 11mA (170V/15K; adding 1.1K doesn't really change it much).

author: aheid
author_rank: Regular Contributor
timestamp: March 12, 2021, 07:23:38 pm
content: Thank you for the input.Quote from: pqass on March 12, 2021, 04:01:24 pmAs I mentioned these were added as they seemed to do a much better job at limiting the current than having just an anode (or cathode) resistor and the emitter being directly driven to ground by the shift register. With the emitter resistors at 1.1k, it limits the current to roughly (Vb - Vbe)/Re = (3.3V - 0.65V)/1.1kOhm = 2.4mA, which simulations agree with. At those current levels I was thinking the shift register should handle it fine, given that only a single digit would be on at any time (I can always ensure that through programming).Are you saying this is a poor way of doing it?I've attached the simulation. I've been using the transistor model is fromNexperia(under documentation), but I replaced it in the attached file just for convenience as it doesn't seem to matter much.

author: pqass
author_rank: Super Contributor
timestamp: March 12, 2021, 08:51:25 pm
content: I cannot comment whether your approach is acceptable by using emitter resistor(s) in that way to limit current.To me, the straight forward approach is to ground the emitter and use the usual anode resistor as well as applying an additional cathod-side resistor to limit it further for LHD,RHD segments.See attached circuit.The CLKs are: a row (B1), a column (E1), and another column (E2).The transistor-switched verticals are: using 15K gives 11mA, a LHD,RHD 1mA segment, a normal 2mA segment, and a normal 2mA segment on another column clock.NOTE: The 11mA vertical doesn't share the same base resistor with the rest because of some odd behaviour. It was being limited to 2mA. I'm not sure why. However, the right three verticals were okay with sharing just one base resistor.Click here to edit the circuit:http://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWEAWaAmBayQBxvpAGxoDsAnCApJZdQgKYC0YYAUAEoiPI4gaFcwaXv3B9wccNEJIw0pNRgJWAJ3ADRLNHwQDqyEnFYAXcEJ0CwZbaOoQlJZAGYShODkgkwbyE6jSfYTg0J28nQhIJagATegAzAEMAVwAbY1UUSSFeZElCZCgQPHgOTLsBXOpGJ21FTPBDQsVoZRd9LIq8gvsoVm9tSvBhMoaNEFjE1ONGFPposTqYSDY2rh4LdQ2eyFLvEBqq7P38MW0qKRkLhX9lAHdNg-BrY8UMwbBXEbRcwuKje6s2keg0eO36T20R0Bo3EE2SaRmcwWTVgK0a0L20PyyJ2nAxlme1VqhR+H0WNwyGOGWIKdioRlMj1Eg1sCxaJBwhjIVgQYFC+EgFBghDAzi83KoOCcgqcFGwMXi8PSAOe3zszw+elYq2hpEhz0e221jRZJAGkkNTVYAPMwl4TN0vTUWj4OF4eztJMMOwBGjdm1sxrobj4ZsoIctr1MPGool0scddn8BhqTl0ODAOGcsrw-hFYqsYEl0rIsqi40VU2tI0e8ZevXBdaOTc+2jhU0R822-mWQfD5X7IGxRs4TYEdaJTXqZKnSgyTeGdex1D+PpQHg2Dq19zrasHmt69zNsZ+x5hOzUZ6OZ+XRQFuJAV4EZ8ndVJjUWLT7T8fJ2HVvBK9hivVsK0mBFZi7WdURMIcflEM9WQgZhCDIdAnCzRwyFyaUcEwLg0GkPDiA8LNuVQohy3bNJvxOPVf2oSNDzghMKg3QMjxOWsI3wh89jTbRmGGATTh0K5xOaO4HnwusRJ2AB7R8UBJQVeAUWA4A5IRrm0SI-HYAB5AB1AAdABnABRAA5ABBAAhAAZCyAG5zPoABzcy7LYRSchUsg1J7HSin2cAzOMcyAGEDIcgBVABZKzzOs+ynNcsyPOSnyUAgWlMgCmgYB6AYID8NAADtoiimKEqSyzbMcly3M8yy0FYIA

author: pqass
author_rank: Super Contributor
timestamp: March 12, 2021, 09:04:28 pm
content: Duh! I think I see now. You are using the emitter resistor like in a constant current load circuit.If it matters to you,you can design for 4 columns and 3 rows, then you would only need 3 base resistors, 1 anode resistor, and two additional cathod-side LHD, RHD resistors; for 6 in total for my approach.

author: retiredfeline
author_rank: Frequent Contributor
timestamp: March 12, 2021, 09:55:14 pm
content: The 74HC595 is cheap. Why not just use 2 and keep it simple? You can wire the transistors in common base configuration and drive the emitters, effectively using the transistors as a level shifters.

author: aheid
author_rank: Regular Contributor
timestamp: March 12, 2021, 10:15:40 pm
content: Quote from: pqass on March 12, 2021, 09:04:28 pmYeah I guess that's what it is, in effect. I was thinking of emitter degeneration in amplifier circuits. Thank you for your input, much appreciated.Quote from: retiredfeline on March 12, 2021, 09:55:14 pmI must admit it didn't even cross my mind... An advantage that I can see with my single 74HC595 variant is board real estate, but it shouldn't be a huge difference I imagine. The tubes I'm waiting for aren't huge so would be nice with a not-overly-large PCB.Anyway I will give that a whirl, thanks!

author: oPossum
author_rank: Super Contributor
timestamp: March 12, 2021, 11:10:45 pm
content: Quote from: aheid on March 12, 2021, 02:02:36 pmWorked great for me. I used the 4094 to eliminate the need for base or emitter resistors. Most 4000 series chips have rather week MOSFETs that are limited to about 4 ma when supplied with 5 volts. This is within the max allowed current so it is possible to directly drive LED and bipolar transistors. The same can be done with 74HC series but the current will be much higher and over the max allowed. You could get by with only 2 emitter resistors by using a 2x5 matrix with 5 base circuits and 2 emitter circuits.

author: pqass
author_rank: Super Contributor
timestamp: March 12, 2021, 11:13:46 pm
content: Quote from: aheid on March 12, 2021, 10:15:40 pmI very much like the matrix idea. At the expense of software complexity, with just 8 bits, you can encode up to 16 cathods by enabling one row and one column bit (4x4 arrangement).Not to dissuade you from your approach, but you can really cut-down the BOM, yet still get side-by-side modular plug-ability by multiplexing digits:a. using, say, a 2x10 connector on each side of a digit (one male, one female),b. tying all similar cathods to the same connecter pin (using up to 12 pins),c. shifting the left side anode connector pin by one to the right side (using up to 8 anode pins) with the tube anode connected to the first anode pin on the right-side connector,d. using a single driver board with 12 MMBTA42 cathod drivers (with additionally cathod resistor if necessary), and 8 MMBTA92+MMBTA42 anode drivers (with anode resistor before the anode connecter pin), two '595 shift registers (one for the matrix, one for anode drivers),e. microcontroller just needs to source an SPI stream such that each of the 8 digits gets 50Hz refresh.

author: aheid
author_rank: Regular Contributor
timestamp: March 12, 2021, 11:20:38 pm
content: Quote from: oPossum on March 12, 2021, 11:10:45 pmExcellentLooks great as well.Quote from: pqass on March 12, 2021, 11:13:46 pmGood plan. The only reason I figured I shouldn't multiplex digits was the need for higher drive currents. The datasheet for the tubes I got says it supports multiplexing, but I was worried it might reduce lifetime due to higher instantaneous currents. But I may be mistaken, in which case multiplexing digits is indeed an interesting option.Thanks again guys, great feedback!

author: james_s
author_rank: Super Contributor
timestamp: March 12, 2021, 11:23:47 pm
content: Multiplexing nixie tubes is generally not recommended. Some later types were specifically designed to tolerate it but many are not. Even with those that are, you will get shorter lifespan for a given brightness and some tubes can audibly "sing" depending on your multiplexing frequency. The lifespan of a cold cathode display drops sharply as drive current rises, so the best way to optimize life is to direct drive at the lowest current resulting in acceptable brightness. Since most nixie tubes are not made anymore maximizing life seems like a good idea.I have built quite a few nixie clocks and usually used a 74HC595 shift registers driving either MPSA42 transistors or HV transistor arrays. Supertex also make a line of HV shift registers that are essentially this arrangement all integrated into a single package.

author: retiredfeline
author_rank: Frequent Contributor
timestamp: March 12, 2021, 11:52:14 pm
content: Quote from: aheid on March 12, 2021, 10:15:40 pmBut if you still want to use a matrix to select the cathode, which isn't a terrible idea, you could use 4 common base configuration transistors, driving the emitter, at the bottom, driving the columns, then 4 common emitter configuration transistors per column, driving the base, driving the rows, to get 16 lines. For 12 omit one row or column.On further thought you might even be able to dispense with the column driver transistors.

author: xavier60
author_rank: Super Contributor
timestamp: March 13, 2021, 07:54:59 am
content: Common Base drive with 74HC595 works well.https://www.eevblog.com/forum/projects/nixie-clock-247134/msg3219154/#msg3219154I current drive the anode BJTs.Also make provision for anti-ghosting diodes.

author: aheid
author_rank: Regular Contributor
timestamp: March 13, 2021, 09:40:19 pm
content: Thanks a lot guys, great stuff!Quote from: xavier60 on March 13, 2021, 07:54:59 amYeah planned to do something like that but didn't add anything like that yet. Diodes vs just high-value series resistor to 100V, does it matter?

author: xavier60
author_rank: Super Contributor
timestamp: March 13, 2021, 09:55:13 pm
content: Quote from: aheid on March 13, 2021, 09:40:19 pmHave you seen resistors used? I suspect that it should work.I tested the reverse breakdown voltage of many of my 1N4148 and 1N914 diodes and found it to be always close to 200V from memory.

author: aheid
author_rank: Regular Contributor
timestamp: March 13, 2021, 10:12:37 pm
content: Quote from: xavier60 on March 13, 2021, 09:55:13 pmYes, one example herehttps://github.com/panzer412/Nixies_Board(see schematic pdf). Also pretty sure I saw one or two other cases but can't find reference right now.Anyway thanks again

author: SYJON
author_rank: Newbie
timestamp: March 16, 2021, 09:29:56 am
content: Maybe try using BCD to decimal IC? With this:https://web.jfet.org/nixie-1/NixieTransistors.pdfit could work as popular 74141 chip

author: aheid
author_rank: Regular Contributor
timestamp: March 16, 2021, 09:44:29 am
content: Quote from: SYJON on March 16, 2021, 09:29:56 amI initially thought about this, but it would mean an extra chip per tube, and since I got a beefy microcontroller driving this, doing weird encodings isn't an issue. So that's why I figured it wasn't really needed. Maybe I missed something though.