Custom nixie clock [FINISHED]

[ITman496]

[EDIT] Finished it!






Thanks to everyone who helped me!

-----------------

Sure, everyone makes these, but..  So am I.  As a gift for my girlfriend, birthday and christmas.

So I'm doing it too.  I'm using 595's to control the nixies in direct drive mode, a DS3231 spi module thing for timekeeping, MAX771 to step the voltage up, arduino because thats what I'm comfortable with, and PWM on the anode HV supply for dimming.  And a photoresistor for auto control.  I only blew it up once so far!

This will serve as my project log.

I was already here earlier when this very helpful forum helped me fix my MAX771 issues, so thank you to everyone who helped in that thread!

And now I have blown up the resistors for the pwm drive, because I am using a PNP transistor to turn on and off the HV supply to the tubes, and I had a 10k 0805 resistor, R11 and R13, to NPN's, to the micro.

Yeah, I know.  0805's don't like it when you try to dissipate 3.24 watts over them.  I found that out with a lot of smoke.

They are now 220K's and are operating fine, it seems!

So now I have pwm fading working, I realized that I was a little too conservative with the anode resistors (even at 100% duty cycle its meh) so I'm gonna swap the 15k resistors to 10k.  Should brighten things up a bit, so I can then PWM them back down.

[JoeB83]

Very cool, and very professional-looking PCB!

[ITman496]

Thank you!!  I worked very hard on it..

I've now left it running for a while, as I was afraid of how how the anode resistors might get.  Good news, they don't get that hot!  It's noticeable but certainly not bad, at around 121F max after five minutes.  That's acceptable, right?

[Cyberdragon]

Oh noes! Not again! I've already saved a Fairchild 7000 from one of you lot. Savages! Always afta me tubes!

Nixie tubes should not draw very much current at all. The anode resistors shouldn't get that hot unless they're too small. Make sure you use the recommended voltage and current for the tubes. If you run them too bright you can shorten their life, they should run around a quarter watt each.

[ITman496]

The anode resistor is 0805 1/8w and on a section of pcb with tiny tracks and very little cooling, and it only heats up to 121 F.  Does that sound bad?

And also, its a IN-14 tube, 180V and a 10K anode resistor.  Not sure how much current but I don't think its a lot.  I'll measure the resistor later.

EDIT:  I measured the anode sustain voltage on my nixie's right now are 145.9V.  So 34.1V across the 10K resistor, which means 3.4mA on the tubes.  Sounds about right.

The tiny 0805's are dissipating  0.116w, so.. within their power rating of 0.125w.  But probably at the warmer side of it.

After 20 mins the inductor L1 got to 131F.  I'm worried about that because I only had one tube in.  But maybe it's not a linear scale and just running makes it warm..  I'll have to fire the other tubes up and run it for a while and see.  Maybe I can add a heat spreader if I absolutely must, but.. probably don't need to.

[Cyberdragon]

The power draw is fine, the resistor is just a bit too small. Odd that the inductor is getting hot. Could be an efficiency problem. It probably won't overload but it's wasting power.

[ITman496]

Would going up or down in inductance fix that?

[ITman496]

Well..  the project may be over before it really started.

I transferred everything over to where my computer is so I can work on the programming, and stupidly like an idiot put the black cliplead in the red, and vise versa..  pop.  I've replaced my LDO and my microcontroller heartbeat led is flashing again, however, I cant get any high voltage.  I've replaced the max771 and the mosfet, which are the only two really sensitive components.  Still no high voltage.  I'm at a loss and I'm low on spare components, and spirits are low.  I was doing so well, too.

I cant seem to get more then 26 volts out of the hv converter..  Its a new converter ic, and a new fet.  what else could be broken???  I told the micro to not turn on its hv output so nothing should be drawing..

[ITman496]

It just seems like its not even trying to boost the voltage and is staying at minimum power.  Also, the frequency.  When it was working it was under 100khz I think?  Now its like.. 450khz.  ?

Whats so confusing is that THEY ARE NEW PARTS.  The only worry of mine is I replaced the max771 and tried it, and then the fet and tried it.  I hope the fet wasn't blown out and then blew out my only spare 771..  I just dont know what else it could be. its getting feedback.

I'm so frustrated by this.  I have no idea what to do or what could be wrong, I feel like I've hit a brick wall.  Everything was working with this setup, I've replaced all of the parts that could be malfunctioning..  It's driving the gate properly and boosting voltage and getting a proper feedback voltage (that is low so it should be upping its duty cycle)  and the scope shows everything and everything checks out but..  What is happening?  What do I do to move forward?  I don't have anymore max771's..  I'll have to get more.

[Ian.M]

As it had blown the FET, could it have smoked the inductor?  It would only take one shorted turn to explain your problem.

[ITman496]

I'll replace it now and see.  I don't know if it blew the fet, but it was hooked to an unregulated supply at the time.  I moved it from my current limiting supply at the bench to my beefy atx bench supply at my computer.  So when it went, it really went..

[ITman496]

Nope.. new inductor did not make a difference.  Fet drive is still going very low duty cycle and making a whopping 18 volts from a 12v supply..

[c4757p]

Start taking parts off the board and testing them individually. Don't skip any.

[ITman496]

It appears that the entire digital side is okay (and it wouldn't affect HV generation anyway so) 

I'm a bit new at a lot of this, how can I test parts like the fet and the like?

Also, how do I test the MAX771?  I'm actually reading its feedback voltage right off its input pin, thus testing the pcb traces and stuff.  It just doesn't want to up its duty cycle.  Could I have blown the new one out too?

In the meantime I'll just shave off that entire corner of the board with the reflow gun and test the passives out of circuit.

[ajb]

Have you replaced that 100uF cap on the 12V rail?  I'd be surprised if that didn't blow up from the reverse polarity, or at least let the magic smoke out.  Inadequate input capacitance will definitely cause problems with any switching converter.

[ITman496]

I did, but it didn't help.

The components I replaced were:

First thing, 7805 regulator.  The microcontroller came back to life, presumably the shift registers too.  Completely independant of the HV supply.

Noticed it still wasn't working.  Replaced MAX771 regulator IC.  Nothing worked, still.

Replaced Mosfet.  Still not working.  Concerned at this point, measure voltage, see around 20 volts??

Replace input cap on the 12v rail and inductor.  Still not working???  What on earth is left?

At the state it was before I desoldered the entire board, it appears the max771 was working, but at minimum power.  It was only sending the bare minimum pwm, as if it thought it was already at high voltage.  I could understand this behavior from the chip that saw -12v, but this was a new chip from the spares pile!  What gives??

[Ian.M]

Maybe it thinks its over-current.  Check the resistance of the 3x1R || shunt.

[ITman496]

......THAT MAKES SO MUCH SENSE.  That entire thing was made up of 0805 resistors, had a total resistance of probably 0.25 ohms, and got exposed, minus a fet body diode drop, to 12v..

they must have blown out.  I wish I could have measured them directly before I took it all apart. Time to put it back together. I'll keep you posted!

[ITman496]

sigh. nope.

it appears in desoldering the chip to get it out of circuit to check other things, putting it back..  its dead. And the other chip, dead too.  Both are dead, don't have any more.. have to wait for digikey.  UGH.  I wish I had just tried the resistors first..  I just want to start programming this thing and look at nixie tubes glowing and stuff.. now I have to wait a few days and do nothing.

[AustinEngy]

Keep your spirits up!  You're gonna end up with a totally awesome project when you're finished and I look forward to seeing you solve these problems.  I'm too new to this type of thing to be of much help, but admire that you're making this as a gift.  I wish you the best    Your girlfriend will love it, I'm sure!

[ITman496]

Thank you..  I've replaced every component and now I dont have a gate drive signal anymore, so.. i assume that's whats happening.  On the bright side I've learned a lot about how to use my scope.  Tomorrow I'll start attempting to machine the wood case for it.  This is my idea of what it'll look like.

[Ian.M]

Its a pity you jumped straight to replacing parts.
Swapping the regulator was a good decision, but as soon as you found the boost controller was blown, you should have removed it then cold checked the boost circuit discretes in-situ. 

Also, when designing a one-off like this, its worth providing solderpad jumpers so that various modules can be isolated for testing without having to hack up the board or desolder delicate parts.

Good luck, and have fun making gerbil bedding!

[ITman496]

Yeah..  Lessons learned for the next projects, right?  I certainly won't be making any of these mistakes again.  Using a weird not made anymore boost controller with not vetted by forum design for boost converter, not making isolating pads for stuff.. plugging things in backwards.  debugging technique. 

Alas I think I replaced everything necessarily, as in everything was blown, but my second pull of the parts is what killed the driver ic.

I am planning on it.  Trying to think of what to use.  Can I make a 2x4 look good, or do I have to use a weird wood..

[Ian.M]

How good are your carpentry skills?

At the very least, if you are going for the correct period 1950's domestic style, you'll need it to be nicely veneered.  It would probably be a lot simpler to style it as period test equipment.  Sides done in 1/2" hardwood, with lightly rounded finger-jointed corners with triangular reinforcing pieces on the inside and a rebate in the top edge for a Tufnol top panel.   Chunky rubber feet on the bottom to raise it up for ventilation and an expanded metal mesh bottom cover to keep out curious fingers.  Shellac and wax polish all the woodwork.

[krivx]

Do you have design files for the nixie sockets? Do they stay cool enough not to melt?