Custom Nixie/LED/VFD clock - Page 1
Forum section: electronics / projects

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: October 21, 2013, 01:28:08 pm
content: hello, I have re-started a project now that i have some spare time from my college worki do plan on making it fully open source once i have completed itMy plan is to make a multi board clock which you can select what display medium according to what parts you have.I have a main controller board with a micro, time keeping chip and i am also considering adding some other features to allow more flexibility on its features.Currently i am working on one of the nixie tube display boards (specifically to hold IN-14's but will also be doing IN-12's and a few other footprints).I have finalised the schematic and board layout. The board only contains passive components as the main controller board has all of the active components. I mainly would like to share this board now so that i can get feedback on it and any problems with the schematic that i cant seeI have done several simulations using LTSpice for the High side control of the tubes (thanks c4757p for the help with that in a thread quite a while agoIm trying the project using SMT to save space considering the case i would like to put it in for myself so i have tried to reduce the amount of different parts on the board.I have attached the schematic and artwork layout of the board if you want to have a look and help me to improve itas i would greatly appreciate improvements and feedback

author: sync
author_rank: Frequent Contributor
timestamp: October 21, 2013, 01:51:56 pm
content: Don't multiplex nixies unless they are specified for this. They got damaged. Every time they ignite at bit of the electrode material will be sputtered.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: October 21, 2013, 02:19:33 pm
content: I thiugh that IN-12 and IN-14 were able to do multiplexing?

author: jolshefsky
author_rank: Regular Contributor
timestamp: October 21, 2013, 02:24:15 pm
content: Using the leftmost selection subcircuit as a template, I would omit R1 and short the emitter of T1 to ground. I'm assuming the tube selection and digit selections run from 5V logic, so then I'd put R4 at 10K: the MMBTA42 has a hfe of around 40, so Ib = (5V-0.7V)/10K = 0.43mA, then Ic would be around (40)(0.43mA) = 17mA to drive the base of T2. The MMBTA92 has a bit lower gain, so assuming hfe=25, 17mA * 25 = 425mA which should fully saturate T2 right up close to its maximum Ic of 500mA. (Although without a big heat sink pad, you can't expect the device to dissipate more than 50mW or so, and with a Vce(saturation) of 0.5V, that's only 100mA.)Now you have everything at 10K except the base resistors for the digit selection (the 4.7K resistors R33-R44). If each digit will draw less than 5mA (which I'd expect, given 155V in, even 1mA is 100mW per digit which would be quite bright) you can get away with 10K resistors instead (see calculation above). However, if you'd prefer 4.7K, consider putting 2 10K in parallel instead. If you find you need 5K, you can have both installed.My point in going with the 10K resistors is something Dave mentioned in a video about getting things made: you need to buy each reel of parts, and to buy half a reel of 4.7K resistors may be more expensive than buying a full extra roll of 10K. It also frees up one more slot in your assembler's machine for a different component in case you need it.A bypass cap for the high voltage supply would make me happier, but I don't think it's actually necessary. I'd add it to the PCB and if you get some kind of high-voltage switching interference, that might help.Now, on your board layout, I'd isolate the resistors and transistors of the low-voltage logic to one part of the board near the connectors. Put a silk line around the area to clarify where it might be safe(r) to probe.And dang: filled copper planes are your friend, man! I'd make both the top and bottom of the board a full ground-potential plane. Set the clearance wide: my ballpark numbers say 1000 volts can arc 100 mils, so say a 50 mil clearance. You can make a cutout 155V plane on the top layer around the PNP transistors.And then mounting holes. Throw some on there unless you have a case that absolutely doesn't need them (e.g. slots for the board). And if you do have such a case, mark off the mounting areas so you don't put components there by accident.Maybe a couple fiducial marks too. I'm working on a design that can be wired either through-hole or surface-mount, or a combination of both. For prototyping, I'm probably going to build a reflow oven and do some SMD myself. When I get a solder mask cut, I figure I'll want fiducial marks so I can accurately align the solder mask.What else ... test points. It might be nice to get into the base and collector of T1 (et al) to check voltage. It might also be nice to be able to get a point to read out the 155 volts and ground. Maybe the collector of T2 (et al) as well to figure out if the transistor is turning on in case your digits don't display. The pins on the tube can probably be probed fairly easily. In some cases, putting the resistor pad close to the transistor pad can make it easy to get a probe between to get a reading.Now hopefully a real engineer will call me out on anything that's a bad idea. I'm ready to learn, even if I need to be schooled!

author: sync
author_rank: Frequent Contributor
timestamp: October 21, 2013, 02:52:22 pm
content: Quote from: carbon dude oxide on October 21, 2013, 02:19:33 pmRead the data sheet. If it's specified for multiplexing then do it otherwise don't. And no PWM ether. Here is an example of a PWM damaged nixie.http://www.mikrocontroller.net/topic/308329Edit:I would use high voltage open drain shift registers. Much easier and no worries about multiplexing. Maybe TPIC6B595 is sufficient. It's only 50V but the 74141 is only 60V too.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: October 21, 2013, 03:23:45 pm
content: I have checked on the IN-14 data sheet and they do support multiplexing however i need t up the input voltage from 155 to 190simple resistor change should do the trick on the smpsI did consider adding fiducial marks but i though i would wait until i have all the boards completed before preparing them for manufacturing.thanks for the 4.7k resistor reminder, i made the schematic with the correct value resistors then converted them to all 10k's but i missed the 4.7ks as i am used to working with themagain i did consider adding mounting holes for manufacturing (the final product does not actually need them when being put in its enclosurethanks for the input on this, i shall gpo back to my schematic and update the first before messing around with the artwork again

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: October 21, 2013, 03:29:36 pm
content: Quote from: jolshefsky on October 21, 2013, 02:24:15 pmR1 like you said needs to be there otherwise the voltage goes through T1 and to ground instead of going into the tube due to it having a larger resistance :/ (according to spice anyway)

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: October 21, 2013, 05:33:36 pm
content: Quote from: sync on October 21, 2013, 02:52:22 pmThat is because the 74141 has a Zener clamp on each output, and is only intended to drive Nixies. The leakage current through the Zener clamp keeps the output voltage artificially low, as it takes almost no off-state leakage current to drop the low side Nixie voltage down to a safe level.If you use another driver not intended for Nixies, then you need to add the Zener diodes yourself. An alternative would be to use a high voltage driver, which can take the full Nixie supply voltage (170V or so). Like using a long string of MPSA42 BJTs etc.

author: sync
author_rank: Frequent Contributor
timestamp: October 21, 2013, 06:42:30 pm
content: I think the TPIC6B595 clamps too. Look at the output stage. Over 50V the zener conducts and drives the gate.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: October 21, 2013, 06:48:22 pm
content: ok i have made an update, i have added: test points, ground plains and Isolation linesI have modified: resistor values, inter board connectors, silkscreen and trace layoutsedit: dw i do plan on finishing the copper fillim not leaving it with all those gaps

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: October 21, 2013, 07:10:57 pm
content: Fail.No. Multiplexing. Of. Nixie. Tubes.Evar!Seriously. You ruin them. Simple as that. They need to be driven with a static DC voltage. Not rectified AC, not PWM, not multiplexed.DC drive. Always fully on or off. You need a fully decoded display driver for each digit.

author: Balaur
author_rank: Unknown
timestamp: October 21, 2013, 08:13:42 pm
content: Quote from: sync on October 21, 2013, 01:51:56 pmQuote from: ElectroIrradiator on October 21, 2013, 07:10:57 pmNo, this is simply not true.Cathode poisoningis an inherent problem for Nixie tubes, indeed. However, that happens whether you use multiplexing or not.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: October 21, 2013, 08:21:02 pm
content: Quote from: ElectroIrradiator on October 21, 2013, 07:10:57 pmThe data sheet says that it is able to do multiplexing, and with the space that i have available to put the board into i cant really squeeze any IC chips into it. Hence the multiplexing.Unless you know of some nice small IC's which will allow me to control 96 outputs (using 8 tubes with 12 outputs each) using minimal pins from my MCU

author: sync
author_rank: Frequent Contributor
timestamp: October 21, 2013, 08:39:35 pm
content: What's about 12 TPIC6B595 in SO-20 package? That gives you 96 outputs which you connect directly to the cathodes. And you need only 3 lines from your controller.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: October 21, 2013, 09:32:49 pm
content: Quote from: carbon dude oxide on October 21, 2013, 08:21:02 pmDid it occur to you to wonder why the tube has a socket, while most western Nixies are meant to be soldered into the circuit?The Russians basically designed their equipment to use tubes like the way we eat popcorn: By the bushel. Even their expensive transmitter tubes are frequently specified as only lasting 1000 hours at full omph per the datasheet, then it is time for a new tube.Bottom line: Just because a Russian datasheet from the cold war says the tube can be used in such-and-such way, doesn't by default make it a good idea to actually do so.Any Nixie can be multiplexed, it just won't like it very much. If you expect any kind of longevity out of the tubes, then you are setting yourself up for failure by using multiplexing. You are making a clock...Quote from: carbon dude oxide on October 21, 2013, 08:21:02 pmShift registers, were mentioned previously. Clock, data and reset.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: October 21, 2013, 09:35:58 pm
content: Quote from: sync on October 21, 2013, 08:39:35 pmYou'd also need 96 current limiting resistors...

author: sync
author_rank: Frequent Contributor
timestamp: October 21, 2013, 10:00:17 pm
content: Quote from: ElectroIrradiator on October 21, 2013, 09:35:58 pmNo. Only one per anode.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: October 21, 2013, 10:04:49 pm
content: Quote from: sync on October 21, 2013, 10:00:17 pmUnless you wish to use the decimal point, if any. The IN-14 has two of them.Edit: IN-14 is of course one of the Russian Nixies, which is intended to be soldered in. No socket pins...

author: sync
author_rank: Frequent Contributor
timestamp: October 21, 2013, 10:12:36 pm
content: If you use the decimal point and a number digit a anode resistor should be enough, right? Just don't use the point alone.

author: Mr Smiley
author_rank: Frequent Contributor
timestamp: October 21, 2013, 10:20:23 pm
content: Quote from: ElectroIrradiator on October 21, 2013, 09:32:49 pmand Latch, don't forget the latch or you'll get ripple

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: October 21, 2013, 10:31:52 pm
content: Quote from: sync on October 21, 2013, 10:12:36 pmThe various digits will have different internal resistance, so even with a permanent decimal point the illumination will vary with which digit is energized. You also run the risk of of having either the digit or the decimal point carry most of the total current, exceeding the individual limits. The decimal points usually require much less current, compared to the digits.The varying distance between anode and cathode for each digit will make sharing a current limiting resistor between digits and decimal points a very problematic idea.If you only ever want to use the digits one at a time, without the decimal points, then a single resistor is fine. Otherwise you need the whole bunch.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: October 21, 2013, 10:38:40 pm
content: Quote from: Mr Smiley on October 21, 2013, 10:20:23 pm74HC595 + [Russian equivalent of] 74141.

author: sync
author_rank: Frequent Contributor
timestamp: October 21, 2013, 11:17:14 pm
content: Quote from: ElectroIrradiator on October 21, 2013, 10:31:52 pmI have old gear which only have an anode resistor. It just works. A digit + decimal point - no problem. These machines ran for many, many years. HP did it too.

author: sync
author_rank: Frequent Contributor
timestamp: October 21, 2013, 11:25:02 pm
content: I just measured a ZM1000 and ZM1020. Both ignite at ~125V, run at ~135V@2mA and go out at <120V. I'm surprised that there are basically no hysteresis. They will work with the 50V difference a TPIC6B595 provides.Edit:Brain fart! I done the measurement wrong. They ignite between 130-135V and then drop to ~125V due the resistor. Anyway 15V is a small hysteresis.

author: jolshefsky
author_rank: Regular Contributor
timestamp: October 22, 2013, 12:25:57 am
content: Quote from: carbon dude oxide on October 21, 2013, 06:48:22 pmMoar clearance! Ground-to-TP1, for instance — 190 volts difference — looks like there's just 10 mils clearance. The ground plane should be very far away from everything. I don't know if you can make it clear with a smaller distance to known low-voltage nodes, but it's probably not necessary. The inside of the nixie-tube ring of pins will probably never fill regardless, but getting it to flow to the lower-left corner would be prettier.Ironically, the "Danger High Voltage" is written in the box with the known low-voltages, not that you'd want fingers anywhere near that board when it's on.It looks like your lower-right board edge corner is a couple snap-points higher than the lower-left corner (hence, the jaggies on the bottom boundary). That, and shifting JP1-JP4 up a bit should let the ground plane flood the bottom edge of the board.As for saving the nixies from multiplexing damage, I wondered if a bunch of high-voltage capacitors from each cathode to the anode of each tube would keep it lit during mux operations. I don't know if there are SMD's that are small enoughandhave that high a voltage rating. Regardless, it would double the component count, and if all these dire warnings of multiplexing nixies are true, going with a latching shift register would be the best solution after all.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: October 22, 2013, 04:31:09 am
content: Ive just noticed the title of this post i meant to say vfd not vhd XDSo if i were to use the TPIC6B595 i would not need external transistors as it is able to drain it directly through it? Or will i need them?

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: October 22, 2013, 07:34:59 am
content: Quote from: sync on October 21, 2013, 11:17:14 pmOK, I am guessing they tested this before putting the equipment into production then. It is not a given thing it will work for all types of Nixies.Quote from: carbon dude oxide on October 22, 2013, 04:31:09 amTest it.The chip looks suited for the job at first blush, but you still need at least one current limiting resistor per Nixie tube. I would suggest also testing this detail before committing to a design, if you intend to use any of the decimal points with just a single resistor per tube.You may also want to test your power supply on a full string of tubes. Some of the Nixie types have a noticeable difference in current load between the different digits, and this may result in an annoying flicker of the individual digits as the display changes.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: October 22, 2013, 07:37:37 am
content: Quote from: ElectroIrradiator on October 22, 2013, 07:34:59 ami ahve a nixie set up with 8 tubes running all at once with the power supply i will be using. It can supply a maximum of 21ma at 155V its just enough to run all of them but occasionally 1 tube may miss a small bit of the cathode (like the no 2 it misses the top left corner of the cathode)

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: October 22, 2013, 07:52:15 am
content: Quote from: carbon dude oxide on October 22, 2013, 07:37:37 amOne thing to be aware of is, that the striking voltage will slowly increase over time as the tubes age. So you may wish to consider this, if you are having problems already.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: October 22, 2013, 09:37:42 am
content: Quote from: ElectroIrradiator on October 22, 2013, 07:52:15 ami think its because i didn't re condition the tubes before i used them (pump a higher current into them for a minute or so on each cathode t re burn them, i had to do it with my IN-13 bar tubes in order for them to light correctly), I do plan on doing with my next set of tubes

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: October 22, 2013, 10:37:07 am
content: quest question, does anyone know the holding voltage of the IN-14 nixie tubes?if i can use the shift registers on there own depends on the tubes holding voltage

author: sync
author_rank: Frequent Contributor
timestamp: October 22, 2013, 02:57:21 pm
content: Quote from: ElectroIrradiator on October 22, 2013, 07:34:59 amHave you an example for this? I have only seen circuits with an anode resistor.

author: sync
author_rank: Frequent Contributor
timestamp: October 22, 2013, 03:06:43 pm
content: Quote from: carbon dude oxide on October 22, 2013, 10:37:07 amAccordinghttp://www.tube-tester.com/sites/nixie/data/in-14/in-14.htmit's ~145V. Just test it with the TPIC6B595. If it doesn't work there are maybe shift registers with higher voltage.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: October 22, 2013, 08:28:24 pm
content: Quote from: sync on October 22, 2013, 02:57:21 pmNot so far. Any search involving Nixie tubes ends up drowning in links to clock projects. Had a particular article in mind, but am unable to find it again.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: October 23, 2013, 05:47:30 am
content: Quote from: sync on October 22, 2013, 03:06:43 pmI did a small test when i got home from college yesterday night and the holding voltage was aproxomatly 125-130v ish meaning that there is only ly about 25v on the low side of the nixiethats within range of use with the TPIC6B595they should arrive on the Thursday so i can test them then

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: October 24, 2013, 05:02:39 pm
content: Ok i did the test a few minutes ago and it worked for a full 10 minsthe TPIC6B595 works with it all and no magic smokei did howeer make a nice discoveryi only need a 10k anode resistor due to there only being an 18V drop over itwhich is probably the reason that some of te cathodes on my existing clock dont fully light up

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: October 24, 2013, 09:20:08 pm
content: I have a question about altering the brightnessThere are two solutions that i can think off one is probably a big NO in the terms of how it does it but i know it works well with LED'sMethod 1: turn the tubes off for X time within a period so that the tubes are not always emmitting light, if done fast enough like multiplexing POV would kick in and it would be dimmerMethod 2: have two sets of anode resistors that can be switched using a few transistors so that 1 path has 10k resistors and the other path can have 30k reducing the total brightness.Are there any other ways of achieving this?

author: sync
author_rank: Frequent Contributor
timestamp: October 25, 2013, 01:08:38 am
content: Quote from: carbon dude oxide on October 24, 2013, 09:20:08 pmHas the same problem like multiplexing.QuoteThat maybe work for a limited range of dimming. With low currents you get uneven glow on the cathodes. And I read that nixies need a certain minimum current for self cleaning the cathodes.QuoteNixies are not well suited for dimming. An optical solution maybe the best. Variable filter or a shutter.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: October 25, 2013, 04:04:43 am
content: Unfortunatly i cant have a physical solution on the outsise of the case

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: October 25, 2013, 05:55:06 am
content: Also would it be worth to add a few zener diodes to te serial inputs and te 5V rain with a breakdown voltge of 6v ish so that if the TPIC6B595 do fail they dont route the 155v to any of my logic?