Author Topic: Regular Nixie Tube Clock  (Read 18767 times)

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Offline cprobertson1Topic starter

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Re: Regular Nixie Tube Clock
« Reply #50 on: April 21, 2015, 12:17:48 pm »
Quote
archaic but temptingly cheap stuff.
- aye, damn my tightfistedness! - just checked out a few datasheets for these inductors - q factor isn't even listed, must be a good sign ;)

Let us assume we tried to salvage the existing design (I'll get a schematic up later - though it's pretty much the step-up application circuit with this extra part to it to bump up the duty cycle and a little bit of extra filtering on the output)

Would a 2200R series inductor or similar be of any use for the power supply?

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For starters...
MC34063...
Really?
Reeeally?

Oh aye! Meant to ask, what's wrong with the MC34063 out of curiosity :P? (might as well turn this into a learning experience!!!)

I was originally going to use a MAX1771; but once again was tempted by the lower price and the DIP package of the MC34063. I was also (perhaps naively) reassured by the number of nixie clocks using the MC34063 in it's PSU.

I don't think I cut corners anywhere else though; so with any luck nothing else will fail miserably because of a cost-saving measure!
« Last Edit: April 21, 2015, 12:22:23 pm by cprobertson1 »
 

Offline T3sl4co1l

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Re: Regular Nixie Tube Clock
« Reply #51 on: April 21, 2015, 01:20:07 pm »
Let us assume we tried to salvage the existing design (I'll get a schematic up later - though it's pretty much the step-up application circuit with this extra part to it to bump up the duty cycle and a little bit of extra filtering on the output)

I would suggest not, but I'll cover that in more detail... :)

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Would a 2200R series inductor or similar be of any use for the power supply?

Umm, probably.  Looks to be a ferrite bobbin type.  Winding losses are dominant on those, so make sure you get a low enough DCR to handle it (minus skin effect and all that).

What you really want is a step-up autoformer or transformer, like in my circuit.  Then you don't need to worry about ridiculous duty cycles, fast pulses and huge losses.  Examples are also in the '34063 datasheet.

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Oh aye! Meant to ask, what's wrong with the MC34063 out of curiosity :P? (might as well turn this into a learning experience!!!)

- It's old.  OOoooollld.  Not necessarily a bad thing, but that brings inevitable process and design limitations that modern parts don't have.
- The operating frequency is low.
- The control method sucks.
- Actually, they never even say what the control method *is*.  There's that "current sense" block, but it's not peak current mode, it doesn't turn off a latch.  It just kind of... pokes at the oscillator duty cycle.  WTF?
- Regulation is typically (always?) hysteretic, so the output ripple is incessant, low frequency, chaotic and hard to filter.
- Because the frequency is low, you spend extra on caps and inductor(s).  Doubly so if you need to filter down the ripple.

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I was originally going to use a MAX1771; but once again was tempted by the lower price and the DIP package of the MC34063. I was also (perhaps naively) reassured by the number of nixie clocks using the MC34063 in it's PSU.

This lesson repeats time and time again: ;)

Just because everyone is doing it, doesn't mean it's a smart thing to do.

Weekend projects are very frequently that sort of thing.  Slap together a nixie clock, or Arduino-whatever, or a Tesla coil, or...  Almost none of them have real consideration involved, they're just following what someone else did.  But usually making "this looks close enough" compromises that degrade performance in poorly defined ways.  It's still useful learning or "doing" experience, but don't mistake it for understanding.

As for modern/ish suggestions:
UC384x -- also bipolar, but somewhat faster (<500kHz), and designed to drive MOSFETs (high efficiency!); peak current mode control; Micrel makes a BiCMOS version MIC38C4x.
UCC3808 -- CMOS, made for push-pull applications, acts like a 3844 with both output phases brought out to driver pins.
Any of a number of integrated and external-switch controllers, from LT, ADI, TI and others.  TPS54xxx series are generally good.
I tend not to remember a wide selection of "newer" parts unfortunately, but sticking to the brand names and following appnotes will generally go well.

All of these are more expensive, but inductors and capacitors are expensive, too.  You save a lot of space by going to a higher frequency, and not having to dissipate half your converted power as heat!

Tim
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Offline cprobertson1Topic starter

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Re: Regular Nixie Tube Clock
« Reply #52 on: April 21, 2015, 05:07:57 pm »
I just shouted "SOD IT" (exceptionally loudly as well) and bought a cheap PSU (based round the MAX1771 I believe) from some nixie enthusiast.

That will tide us over and allow me to get the clock up and running - and then I can start the building of a proper power supply with better efficiency ;)

Thanks for all your input! It's been very useful - and with any luck will teach me a thing or two when I get round to building my own supply! I'll need to have a good study of power supply design at some point.

If only I went to uni to study all this... I probably would be sick of it and never do it for fun again... so actually starting as a hobbyist is probably a better idea (for me at least!)
 

Offline linux-works

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Re: Regular Nixie Tube Clock
« Reply #53 on: April 22, 2015, 03:01:29 am »
this is a good example of 'make vs. buy'.  for me, it was clear : 'buy'.

its so cheap, its already worked out, why re-invent what is commodity?

figure where your value-add is.  for me,  the psu is NOT a differentiating factor.  the software and features are.


Offline cprobertson1Topic starter

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Re: Regular Nixie Tube Clock
« Reply #54 on: April 22, 2015, 06:32:39 am »
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its so cheap, its already worked out, why re-invent what is commodity?
Ah, you see my problem is I like a challenge too much - figured I'd take something I don't know a whole lot about and make myself learn a whole lot about it ;) It's much more fun than just reading a book!

Called it wrong on this one though; should have bought a module in the first place and learned about stepping up DC afterwards. Auch well, nothing ventured; nothing gained!

While I'm waiting for the PSU to arrive I'll take another crack at a fake-nixie. I can use the spare MC34063s for the LED drivers. Soldering these 0803 leds is going to be great fun though. Once I finally get a bigger place with dedicated lab space I'll get an oven and try my hand at reflow soldering - just now everything is done by hand. Which works fine 90% of the time, but it does generally limit me to through-hole parts.

I suppose I should also start developing the driver boards for the nixie clock at some point. Might leave that to the weekend though!
 

Offline aargee

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Re: Regular Nixie Tube Clock
« Reply #55 on: April 22, 2015, 06:38:07 am »
Did you get the photos and circuit I sent you CP?
Not easy, not hard, just need to be incentivised.
 

Offline cprobertson1Topic starter

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Re: Regular Nixie Tube Clock
« Reply #56 on: April 22, 2015, 07:46:52 am »
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Did you get the photos and circuit I sent you CP?

I certainly did! :D Helped quite a bit actually! I've tried it and another couple of variants and got between 40 and 76.4V - highest voltage design I found was pretty much the application note with an additional NPN on it before the main switching transistor. As T3sl4co1l mentioned, my choice of ferrite is probably a big part of the problem (amongst other things of course)

Efficiency wasn't all that great on the circuit I had either, I reckoned it was less than 60% (based on power in vs power out (when supplying a dummy load via the step-up converter)) - tested at 40.0V, 60.0V and 74.6V (+/- error in my meter ) - efficiency varied depending on the exact test circuit - I was actually surprised I managed to get it as a high as 60% - I was expecting even less for the first couple of runs due to contact resistance in the breadboard.

I got a cheap...ish board for £10, which seemed to be the going price for many of them on ebay (bought this one - claims to be "professionally designed" - from some nixie enthusiast website that seemed about as reliable as you can get for this sort of thing. Time will tell if my investment has paid off!)
 

Offline cprobertson1Topic starter

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Re: Regular Nixie Tube Clock
« Reply #57 on: April 24, 2015, 09:19:45 am »
PURELY out of curiosity - could a Cockroft-Walton generator with a pulsed DC input be used to provide the necessary voltage for the nixies? (as I said, I've bought a module - was just curious xD)
 

Offline T3sl4co1l

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Re: Regular Nixie Tube Clock
« Reply #58 on: April 24, 2015, 01:28:19 pm »
Yes, but: not from a very low supply.  You probably need a transformer or inductor to get enough voltage worth starting with.  And if you're doing more than a few digits, the current consumption gets a bit unwieldy too.

Reason being, if you need say +170V from +5V, you can generate 5Vpp with something like a MOSFET driver chip (which pulls fully rail to rail with low resistance).  You need effectively 170Vpp at the end, so you need a 34 stage multiplier.  But each diode in that multiplier drops at least 0.3V (assuming schottky), so you need at least 10.2V to supply all those diodes alone!  Instead, it'll peter out somewhere around 50-60V DC.

If you start with 12V, the ratio is much lower, and the total forward drop smaller as well.  Tedious, but possible.  You still need a substantial current supply, which needs big bypass capacitors to handle the ripple drawn by the thing.  Which you'd need anyway for the inductive case, but the inductor will save some size and cost over the stack.

If you use both positive and negative multipliers, you get better performance: you only need +/-85V.  Still not likely to work at 5V, but pretty easy at 12V.  You do need a level shifter to communicate the digital signals to the drivers down at -85V -- or a pair of big coupling caps to bypass the drive point to the driver, grounding the negative end of the chain.  (This might not be too bad of a compromise; electrolytics of that rating aren't a terrible problem, at least if you only need a pair.)

There are other configurations that trade DC voltage across the capacitors for diode drops.  Instead of a straight ladder stack, you can use the "half wave" style configuration, where capacitors are used to couple AC voltage to higher and higher doubler stages; because the AC signal isn't being transmitted through the entire diode chain, diode drops subtract linearly (per stage) rather than incrementally.  But the capacitors get much larger.

It's noteworthy that you could use an H-bridge driver.  But now the ground reference of the multiplier chain is bouncing up and down.  A balanced, symmetrical chain might be possible, or if using level translators for the negative end, simply having devices with enough CMRR would suffice (who cares if the negative end is bouncing between -90 and -80V, as long as the signals still get there in order?).

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline cprobertson1Topic starter

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Re: Regular Nixie Tube Clock
« Reply #59 on: April 24, 2015, 07:34:21 pm »
Another very informative reply!

Thanks again - you're a goldmine of infoT3sl4co1l!
 

Offline cprobertson1Topic starter

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Re: Regular Nixie Tube Clock
« Reply #60 on: May 08, 2015, 10:49:44 pm »
Auch, you were right about those powder-core inductors - 40 ohm impedance and 1.6kohm resistance at 10khz test frequency. Youch!

Tried a different inductor with a solid core and managed to get 150V out, but the efficiency was in the region of 35%

Anyway! I'll get some pics up soon - I've decided to see if I can mill some PCBs on one of the CNC centres (overkill much?), so I'll get pics of that once they're on the go.

I've also had a test run of the nixie tubes and they're looking good!

Anyway, pics to follow!
 


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