Need some help wrapping my mind around driving IN-9's with A TLC-5940

[Chill4844]

Good evening folks.

I hope someone out there has some thoughts on this and is willing to help out this noob.

The senerio is this:

I am attempting to use an Arduino in conjunction with a TLC5940 to drive an array of IN-9's (And not kill myself or blow anything up) the problem being if i understand it right (which I probably don't) is that instead of a positive PWM out the arduino the TLC5940 works by sinking the current on the cathode side. I don't think i can use this with my standard mpsa42 (and I'm not going to attempt anything until i get some advice) I do have some mpsa92's available.

Anyone who can help straighten me out I would greatly appreciate it.

Thanks

Chase

[John Rehwinkel]

> I hope someone out there has some thoughts on this and is willing to help out this noob.
>
> The senerio is this:
>
> I am attempting to use an Arduino in conjunction with a TLC5940 to drive an array of IN-9's (And not kill myself or blow anything up) the problem being if i understand it right (which I probably don't) is that instead of a positive PWM out the arduino the TLC5940 works by sinking the current on the cathode side. I don't think i can use this with my standard mpsa42 (and I'm not going to attempt anything until i get some advice) I do have some mpsa92's available.

That's a somehat peculiar choice for driving an IN-9, but if that's the way you're going, you'll want a common-base setup for your transistor, to provide voltage gain but not current gain (the current through the IN-9 will still be controlled by the TLC5940). You could use your MPSA42 for this (note that I'm just scribbling here, this lashup might not work):

http://www.vitriol.com/images/tech/nixies/TLC5940.gif

Have you read Jan's writeup on driving IN-9s?

http://www.die-wuestens.de/iz/IN9-2.pdf

You might also want to look at some other designs used to drive IN-9s, such as Brian Stuckey's Nixietherm:

http://www.tubeclockdb.com/forum/Builders-Forum/1280-Nixietherm-IN-9-bargraph-based-thermomer.html

- John

[John Rehwinkel]

> Have you read Jan's writeup on driving IN-9s?
>
> http://www.die-wuestens.de/iz/IN9-2.pdf

Sorry, I misspoke - it's hosted at Jan's site, but that writeup is by Jeff Malins!

- John

[Chill4844]

Thanks for the quick reply John, the main reason for he 5940 in this case is that an arduino does not have enough pwm outs to drive the 14x in-9 array for my "spectrum analyzer"  probably actually an audio visualizer. Yes I have read that write up on driving in-9's and have done a few projects with that already.  After much research I found something very similar to the lash up you posted. I just have to figure out weather the 5940 will sink that much current.  If you have any other ideas for driving 14 of them I am all ears I've been thinking on it for months I could use a fresh mind. I am sure there is a better way to go about all of this. That's what I get for being a noob.

Thanks again John

Chase

[Jeff Malins]

It was my experience and learning from this list that originally inspired the research in that IN-9 document. Based on some private correspondence, Jan asked me to write something for his site, hence the doc.

I believe John's suggestion of an MPSA42 configured as common-base amplifier will work. I've used a similar common-base circuit with 7400 TTL outputs, which are current-sinking as well. 

I would add the following additions to John's recommendation (unverified):

1) Consider placing R1 (in John's schematic) between the MPSA42 emitter and the TLC5940 output instead of between the IN-9 and B+. This will limit base current regardless of what the TLC5940 driver tries to do (says person who has blown more than a few MPSA42s by somehow putting base @ 5V and emitter @ GND).

2) The TLC5940's PWM feature will probably not work. PWM of LEDs is "integrated" by the eye to see intensity modulation... while the IN-9 needs true current modulation. Assuming the driver circuit can switch fast enough, PWM would likely just intensity modulate an unchanging bar length on the IN-9. However, it looks like the TLC5940's "dot correction" feature is exactly current modulation of the individual outputs. The big caveat is that it is only 6-bit vs. 12-bit for the PWM (64 instead of 4096 steps). If you can live with only 64-steps in your spectrum analyzer (I probably could), that may not be a big deal.

3) A proposed design approach:

 - Choose IREF so that the max output current from the TLC5940 is 10mA, the full scale value for the IN-9. This looks like a reference resistor (R-IREF) of about ~3.9k, based Figure 3.

 - Choose R1, to be placed between MPSA42 emitter and TLC5940 output so that the TLC5940 output voltage will be about ~2.5V (arbitrarily chosen to be the center of the supposed 5V power supply) -> (5V - 0.7V - 2.5V) / 10mA -> 180ohm. This would basically be Figure C from my IN-9 doc, with R1 = 180ohms, Vin hardwired to 5V, but with the TLC5940 connected in place of a fixed GND.

- To control, set all TLC5940 channels to 100% PWM, but dynamically set the dot correction (DCx) on each channel to vary output current from 0 to 10mA in 64 steps... hopefully allowing 0 to 100% on the IN-9 in 64 steps.

Anyone see any major errors here? 

Chase, please let me know if any of this doesn't quite make sense... I was a total noob too when I first joined this list 12+ years ago (I've been reading it every day since, but actually post something about once every World Cup or so).

Jeff

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[Jon]

On Tuesday, December 3, 2013 2:24:03 AM UTC, Chill4844 wrote:

I am attempting to use an Arduino in conjunction with a TLC5940 to drive an array of IN-9's (And not kill myself or blow anything up) the problem being if i understand it right (which I probably don't) is that instead of a positive PWM out the arduino the TLC5940 works by sinking the current on the cathode side. I don't think i can use this with my standard mpsa42 (and I'm not going to attempt anything until i get some advice) I do have some mpsa92's available.

 

I don't think you want the PWM capability of this chip for IN9. For what you're trying to do, take a look at the TLC5628 which is an 8 channel DAC with a simple 4 wire interface to the microcontroller. Hook the output of each DAC channel up to the simple MPSA42 current sinks per Jeff's paper, and that's all you need. You can manage around the Vbe offset in software to keep the hardware simple. That's the technology which sits behind this: http://youtu.be/mQ1567EFCY0

 

Grahame rolled his own version and documented it here (http://www.sgitheach.org.uk/ss.html) including a full schematic.

 

BTW, IN9 are very variable and suffer a lot from cathode poisoning. To get an array of them to perform consistently well and in a uniform way is going to take quite some tube selection and conditioning. IN13 may be a better bet, though they are rather dim to my eye.

 

Cheers,

 

Jon.

[Jan Rychter]

Ok, I realize I'm really late to this thread, but I've just only found it going through the archives and I thought I'd pitch in, because it might help someone in the future: I spent quite a bit of time looking for inexpensive multi-channel DACs.

I have a "bargraph" project on the back burner and I wanted to drive 8-12 IN-13 tubes. The best solution I could find was a ROHM BU2501FV DAC. It's a 12-channel DAC with rail-to-rail output capability, driven via a serial shift-register-like interface. Seems perfect for this kind of application. I was planning to use it with transistors to drive the tubes. I got the chips from RS Components and they were not overly expensive. The BU2500FV is 5V, BU2501FV is 3V.

The nice thing is that it also has a VrefL pin that allows you to supply a lower reference voltage for the DAC. I was planning to supply it with a diode drop, to offset the B-E voltage of the driver transistors, which would give me a lot of room to work with.

You might also want to look at BH2221FV.

--J.

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[Chill4844]

Thanks to all of you for the help, sorry I haven't been really present in my own thread lately, I've been busy with some other projects for Christmas presents.  Here is how the project stands thus far. The project is working well as a visualizer using the TLC5940, the only thing that needs some work is the overall linearity of the output, I have a plan to create use a custom scale that I made dividing the tubes into 128 parts and to create an array of the representive numbers for each segment. then using that number to get accurate 128 segment representation. I'm basically using Jeff Malins purposed approach as of this moment and when i plot the points it looks somewhat like a Power Curve.  Does my plan sound reasonable? Or does anyone have any other ideas to better the linearity, perhaps different value resistors?? ? Again thank you to all of you who have helped me greatly in this and other projects, It's quite a great blessing to have access to such a pool of knowledge.

[Michael Gregg]

I have a project where I used TLC5940 and TLC5947's to drive nixie tubes a while back.

Chill4844, I am not completely sure what you are trying to accomplish, but using a few transistors, you can drive nixie tube segments with the Ti parts.

I used a pair of transistors in front of the nixie tubes to drive various nixie tube parts. You need a low voltage PNP transistor, and a high voltage NPN transistor for every segment. I have some boards that work for in-2 tubes around here. I can post schematics if people here are interested.

Chase, does that clear things up for you? I can post links to the parts on mouser that I used if you like. If you are in the US, I can mail you a few transistors to make your project work. I believe I have extras.

Michael-

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