Multiplexing IV-9 Numitrons - Page 1
Forum section: electronics / projects

author: DomesticHacks
author_rank: Regular Contributor
timestamp: February 10, 2014, 09:28:19 am
content: I ordered some Numitron tubes (IV-9) and since they draw not much current I can cook dem up directly to a µC pin.For a single tube this is just fine but for two i need some kind of multiplexing, since every tube has 8 segments, so I need 16 outputs.The obvious thing would be to use shift registers for multiplexing but I the whole thing should be battery powered, so I want to minimize the number of devices which draw power even when they are switched off.Multiplexing by time dooesn't seem to be a good idea since in my opinion this should lower the lifespan of the tubes. Or not?So has anybody an idea how I can implement a multiplexing for those tubes?

author: Paul Price
author_rank: Super Contributor
timestamp: February 10, 2014, 02:53:52 pm
content: HCT or HC type shift registers do not draw any significant power when not sourcing current.You could use 74C164, for example.Mplx by time won't work since the filaments of these numatrons take a long time to heat up with perhaps, facing even a shorter time to cool. Not good idea, not practical an idea, multiplexing, since the required ON sustain time to light each display is so long for each digit displayed.The current may also be quite higher with a cold numatron digit warming up, wasting battery power and may require the use of high current drivers between SR's and each segment lit up on these power-hungry tubes.You could use a MCU with many outputs to feed various chains of SR's.

author: sync
author_rank: Frequent Contributor
timestamp: February 10, 2014, 03:27:06 pm
content: These are 3.15+1.35V devices. Don't use a static 5V on them. They also draw about 20mA per segment. That's too much for a micro controller or normal logic ICs. There are power shift registers like TPIC6B595, TPIC6C595 which can handle the current. Use one per IV-9 with a 3.15V supply and you don't need multiplexing.Multiplexing doesn't reduce power here because they are incandescent lamps. It's exactly power that controls the brightness of a segment.http://www.jogis-roehrenbude.de/Roehren-Geschichtliches/Nixie/IV-9.htm

author: DomesticHacks
author_rank: Regular Contributor
timestamp: February 10, 2014, 04:39:45 pm
content: I plan to use 3 or 3.3V, I read that they should light up on a lower voltage, too.I can't test this at the moment because I don't have the tubes yet, but I want to order some components, so I can start testing when I get them.For an ATmega 20mA isn't a problem and shift registers should be able to handle 20mA (maybe realy close but should work). If not I will add an ULN2008 or so.

author: sync
author_rank: Frequent Contributor
timestamp: February 10, 2014, 05:37:41 pm
content: Out of curiosity I measured the inrush current of a3015Fnumitron. It draws about 8mA. The inrush current is about 5 times higher.An Atmega328 has an absolute current rating of 200mA for GND or Vcc. With 20mA per segment it could drive 9 segments at the same time which is somewhat limiting. But the inrush current will overload the ATmega anyway. A 74HC595 shift register won't work too.The ULNs are bipolar darlington transistor array. You will loose about 1V across them. Better use something with MOSFET outputs.

author: DomesticHacks
author_rank: Regular Contributor
timestamp: February 10, 2014, 09:30:05 pm
content: What would you suggest as driver?

author: dannyf
author_rank: Super Contributor
timestamp: February 10, 2014, 09:57:59 pm
content: "They should light up on a lower voltage, too.""For an ATmega 20mA isn't a problem and shift registers should be able to handle 20mA "If I were you, I would read the datasheets hard to make sure that's indeed the case, or experiment first if you intend to push the chips out of their specs.And multiplexing a tube is going to be tricky given their long time constants.

author: sync
author_rank: Frequent Contributor
timestamp: February 10, 2014, 10:23:42 pm
content: Quote from: DomesticHacks on February 10, 2014, 09:30:05 pmTPIC6B595 or TPIC6C595. These are basically a 74HC595 shift register with beefy open drain MOSFET outputs. They are cascadable and only need three signal line from the micro controller. Look at their datasheets.

author: dannyf
author_rank: Super Contributor
timestamp: February 10, 2014, 10:32:27 pm
content: I would use a bunch of transistors rather than a specialty chip.

author: sync
author_rank: Frequent Contributor
timestamp: February 10, 2014, 10:39:30 pm
content: Quote from: dannyf on February 10, 2014, 10:32:27 pmWhy?

author: JoeyP
author_rank: Frequent Contributor
timestamp: February 10, 2014, 11:13:31 pm
content: The current for filaments in general is not linearly proportional to applied voltage. They act like self-heating thermistors. For example, a 5v filament which draws 20ma would be expected to draw much more than 10ma at 2.5v. It is this "negative resistance" characteristic which made them popular for use in vintage AGC circuits. It's also why incandescent lamps become significantly more efficient as their operating temperature increases (while lifespan is inversely proportional to operating temperature).

author: DomesticHacks
author_rank: Regular Contributor
timestamp: February 10, 2014, 11:33:37 pm
content: Quote from: sync on February 10, 2014, 10:23:42 pmThey look nice thanks, but they only work with 5V, I need someting for 3V.

author: sync
author_rank: Frequent Contributor
timestamp: February 11, 2014, 12:41:40 am
content: Sorry, I didn't considered your 3V supply voltage. I didn't know any 3.3V parts and a quick search gave no results. Maybe transistors then.

author: Paul Price
author_rank: Super Contributor
timestamp: February 11, 2014, 07:17:18 am
content: You can add a single-IC step-up converter to develop 1 to 3 amps of power using a tiny inductor to convert 3.3 to 5V and get enough 5V power for many Numatron tubes with a conversion efficiency of >90%. Microchip, TI and National have a large selection of these chips, and many to choose from, some are < US$ .50 per IC if ordered 10+ quan.I assume you are not building a mission-critical device, so why not try to see what sink/source drive currents a HC or HCT SR can be pushed to before failure.I have used 74HC164's and 74HCT164's with VDD=5V for this purpose without a failure over thousands of hours of operation directly driving high efficiency 7-Seg LED displays using only one resistor per digit, a common cathode resistor of 200-ohms 1/8-W per digit to circuit ground. The 74HC164 itself provides some of the current limiting along with the 200-ohm resistor. Segment brightness balance is also quite acceptable, although there is a noticeable contrast between adjacent digits, especially as in the case of displaying an 8 next to a 1, but overall, this small digit display brightness variation is neither distracting nor aesthetically a concern. The simplicity of this approach is outstanding, using only two wires, clock and data to daisy-chain tens of digits . This same circuit might work with a Numatron. since 20mA or more of current is possible, enough for one numeral segment..try it!

author: DomesticHacks
author_rank: Regular Contributor
timestamp: February 11, 2014, 08:47:27 am
content: At the moment I think about using 3,7V, so I could also use a LiPo. But I have to try if this works with the tubes when I get them.@Paul PriceI have some 74HC164 on hand and I will try those. If it won't work long, I will try some of the other suggestions.

author: DomesticHacks
author_rank: Regular Contributor
timestamp: February 21, 2014, 02:49:53 pm
content: My tubes arrived and I tried them with a 74HCT595 and 3,7VWhen I switch on many segments at once the other ones are getting a litte bit darker but I have to check if i can solve this problem with a cap (I'm also out of decoupling caps ;-) ).Aside from this they are working fine since 2 days (displaying the time in 8 second intervals).

author: Kaptein QK
author_rank: Regular Contributor
timestamp: February 22, 2014, 09:45:56 pm
content: Quote from: sync on February 10, 2014, 03:27:06 pmHey, thanks for the tip!I'm sourcing parts for a Numitron clock and was going to use 74HC595 shift registers and transistors.

author: DomesticHacks
author_rank: Regular Contributor
timestamp: March 29, 2014, 12:10:56 pm
content: I found a beautiful shift register for this task: The TLC6C598Works with low voltages (>= 3V) and can handle 50mA.

author: edavid
author_rank: Super Contributor
timestamp: March 31, 2014, 05:33:20 am
content: Using shift registers to get more outputs is *not* multiplexing, it's still direct drive, so the title here is confusing.Multiplexing does work, although there's not much reason for it now. RCA had app notes showing multiplexing of real Numitrons - the main point was that you need a diode for each segment to avoid sneak paths. So, instead of wiring those diodes, you might as well use a driver chip instead.

author: calexanian
author_rank: Super Contributor
timestamp: March 31, 2014, 06:17:46 am
content: Octal latches and one address pin on your micro for each. Just make an 8 bit data buss, trip the latch (74hct574 comes to mind) and then on to the next digit. they are cheap and plentiful. fast also. They also have an enable for digit masking.