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Author Topic: [SOLVED] Leakage current in a driver IC; how to compensate?  (Read 1915 times)
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Hi all,

I'm building a nixie tube display using an UNO R3 and the ArduiNix shield (arduinix.com). Things are great with the shield and the nixie tubes I'm trying to drive. The shield is essentially a 170V power supply, which is switchable via the Arduino. On the 'nix shield are two driver ICs that convert BCD signal from the Arduino to one of ten pins (one for each digit in the nixie tube). The drivers control the cathodes of the tubes, so you supply 170V to the anode and then ground one of the cathodes to turn it on. That's all background, really---just how I got where I am now.

Those driver ICs have some leakage when they are supposed to shut the cathodes off. Instead of getting 0V to a pin in the 'off' state, it reads about 70V. 70V is well under the amount needed to even partially light the tubes, but I'm also trying to run some neon bulbs off these. The leakage is more than enough to light a bulb partially. Just putting a bigger resistor in there isn't an option; it works but the 'on' state then becomes very dim.

So, my question is: is there something I can do to prevent the bulbs from getting partially lit at 70V? I'd like to control the on/off state of these bulbs with the same driver IC if possible.

I thought about maybe using a transistor that only switches current on once the base goes above 100V or so, but I'm pretty new to electronics and have no idea how to go about finding one with that exact capability (or, really, if they even exist...)

Thanks!
« Last Edit: April 29, 2012, 04:54:36 pm by Risen » Logged

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Quote
and the ArduiNix shield (arduinix.com)
When talking about a product please provide a link so we know what it is.

Your problem seems to be with the hardware of a commercial product so it is your supplier who should answer the question.

Quote
Those driver ICs
What driver IC, link again please.

Quote
Just putting a bigger resistor in there isn't an option;
In where and why is it not an option?
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When talking about a product please provide a link so we know what it is.
arduinix.com

Your problem seems to be with the hardware of a commercial product so it is your supplier who should answer the question.

The chip is working as intended, it's just not designed to drive these small bulbs. My question isn't about the operation of the chip at all. This chip I have is the russian equivalent of the 74141 BCD-to-decimal nixie driver. But I think that's irrelevant... the question is really about how and if I can to control a device with a smaller draw when I only have the capability to switch my input voltage from 170V to 70V. I'm only used to working with a "low" voltage of 0V.

In where and why is it not an option?
Assume we're talking about a single path here, where I can only control the voltage to be either 170V or 70V. I want the bulb to be off when the line is at 70V and on at 170V.

I've already got 200KOhm resistance on it so it lights properly at 170V. If I make that any bigger, (I've tried 1M) the voltage does drop enough to blank it at 70V, but then the bulb is too dim at the "on" voltage of 170V.
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Where on that site is a schematic so we can tell what we are dealing with, as I said a link would be good.

Where are you getting this information about 70V being the minimum you can get from the driver, it is not what the 74141 gives.

Do you understand the fundamentals of neon lamps?
They have a strike voltage that turns on the discharge. Once on there is very little voltage dropped across it way smaller than 70V. The only way to turn it off is to reduce the current to a point lower than a critical value. This is normally done by reducing the voltage to zero, or by having a seriese resistor in the order of several meg ohms.

Small neon lamps can make very good audio oscillators using a 2M resistor feeding the lamp with a 0.1uF capacitor across the lamp.
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Where on that site is a schematic so we can tell what we are dealing with, as I said a link would be good.
http://arduinix.com/images/Main/News/2009/3.0Schematic.jpg

Where are you getting this information about 70V being the minimum you can get from the driver, it is not what the 74141 gives.

It's a direct multimeter measurement from my 170V source to one the driver pins in its 'off' state. This is without the bulb or the resistors connected.

Do you understand the fundamentals of neon lamps?
Not in depth, but I know that the 'off' state from the driver IC isn't dropping current enough to shut all the bulbs back off, which is what I want it to do. (Some do go off, but some don't.)

Since I don't have control of that leakage current, I'm trying to figure out what else could be done about it.

Thanks for the continued help.
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The 74141 is an open collector output with  Zener protection diodes.  When the outputs are OFF... no current flows through the TTL IC gate output transistor though the zener protection diode will happily work independently of gate status.   When the TTL Gate is ON, it supplies a ground path to the  NIXIE segment, closing the circuit and allowing it to light.  A standard neon lamp operates at 60-80V, a Nixie operates at 170V.  using the same ttl gate IC for the two devices is impractical.
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Thank makes sense, but can it be done? The shield is good for everything else I'm doing (driving 6 tubes).

I had assumed this would be easy since the ArduiNix folks supply code on their site for using the shield directly with bulbs as clock separators, and state elsewhere on the forum that it works great. There's only a simple note about using a bigger resistor for the separators (which so far has only given me a very dim 'on' when it's large enough to blank it when 'off'). I do have a thread on the ArduiNix forum about this, but there's been no activity on the forum at all since I posted it two weeks ago.

Is this not possible? Should I try something completely different? I did assemble my shield with headers that allow me access to the Arduino pins at 0/5V, and I do have one anode from the ArduiNix at 0/170V not being used for tubes. I suppose I could route those to something different and bypass the driver ICs? Something with transistors there? Would that be better?
« Last Edit: April 26, 2012, 10:19:22 am by Risen » Logged

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A resistor across the Neon bulb would probably do the job. A neon bulb does not require much current so you might try a 1K resistor to start.
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You mean in parallel with instead of in series? And do I need one for each bulb?
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Yeah, I didn't think parallel made sense. I'd blow the bulb out that way for sure.
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Odd I thought you said a resistor was out of the question.
A resistor in Parallel will not blow the lamp. In fact it will act as a potential divider and reduce your minimum an maximum voltage, it would be worth trying.
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I was working with series resistance and found that to be unworkable. I hadn't considered one in parallel. (If it works I will need to seek further explanation.)

How would I determine its value? I tried a 1M in parallel and the neon went so bright I was certain it would burn out if left for long. I found with the series resistor that I couldn't just select a value in the normal way -- I suppose I could go for trial and error again.
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The 74141 is an open collector output with  Zener protection diodes.  When the outputs are OFF... no current flows through the TTL IC gate output transistor though the zener protection diode will happily work independently of gate status.   When the TTL Gate is ON, it supplies a ground path to the  NIXIE segment, closing the circuit and allowing it to light.  A standard neon lamp operates at 60-80V, a Nixie operates at 170V.  using the same ttl gate IC for the two devices is impractical.

If it is true that a standard neon lamp operates at 60-80v (and I have no reason to doubt it), and the zener diodes in the TTL IC are conducting at 70v, then it follows that 170v is far too much voltage to be supplying. So I think the solution is to run the nixie tubes from 170v and the neon lamps from something much lower, in the region of 100 to 120v.

If you only have 170v available, then one way of doing this would be to connect the neon lamps like this:

  TTL driver output ----- one side of neon lamp
  other side of neon lamp ----- resistor to +170v AND second resistor to ground

If 200K in series with the neon lamp is about right when you run the combo from 170v, then try 180K to +170v and 330k or 390K to ground.
« Last Edit: April 27, 2012, 09:29:00 am by dc42 » Logged

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dc42, I agree about the voltage not being ideal, but I don't believe I have the expertise to build my own power supply right now.

Your divider solution that sounds like it has some promise. Would you mind explaining it a bit further with how you calculated it?

Thanks!
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I tried a 1M in parallel and the neon went so bright I
No that is impossible. You are missunderstanding something.
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