Re: ICE TUBE Second digits are DIM

by jarchie on Fri Feb 15, 2013 6:38 pm

With R3 jumpered can you see the filaments glowing in a dark room?


With no segments lit, I can see the filament glowing even with the resistor installed--but only after my eyes have fully adjusted to the dark. But the filament definitely glows brighter with R3 jumpered.


I would expect some drop across the junction of Q3 but you're measuring around 1.5 V, no?


I didn't test that, but should have! In any case, your estimate based on the numbers I reported is correct. The voltage going into Q3 is 4.7v and the voltage coming out of Q3 is 3.3v (with the tube plugged in). So the voltage drop across Q3 is 1.4v. On a second clock, the voltage coming out of Q3 is 3.4v, so these numbers are probably typical.

According to one source, the current draw is highest with no segments lit, but I can't confirm that.


I can confirm that, but the difference was only ~1mA on the tube I tested. Nothing to worry about.

It would seem that the lower filament voltage is the real cause of the dim digits people are seeing.


That seems to be the case. At the other extreme, driving these tubes at higher currents tends to produce a brightness gradient across the entire display, so I'm guessing that R3 was selected to minimize that effect for most tubes. But R3 seems to be too large in some cases.

The easiest hardware solution seems to be replacing R3 with a jumper--something I've tried on two different clocks. In both cases, current running through the filament was well under 70 mA--safely below the 95 mA maximum. On one of the tubes, jumpering R3 introduced a very sight brightness gradient across the entire display.

To maximize display life, I wanted to run the tubes at the minimum current to extend display life, so I replaced the jumpers with 50 ohm potentiometers (Digi-Key, part number CT6EX500.) On one clock, the potentiometer is underneath the circuit board; the other, above the alarm switch.

ImageImageImage

At high resistance, the rightmost and leftmost digits are extremely dim, replicating the issue I observed. At medium-high resistance, only the rightmost digit is noticeably dim, replicating what others have observed. At medium-low to low resistance, the display is almost perfectly even. On only one of the clocks was there a slight gradient across the entire display at zero resistance.
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Re: ICE TUBE Second digits are DIM

by wbp on Sat Feb 16, 2013 5:36 pm

Nice work! I'm thinking I might replace Q3 with a mosfet to get the voltage up to about where it belongs, but this is not a high priority on my list right now...
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Re: ICE TUBE Second digits are DIM

by grumpygasbag on Sat Mar 02, 2013 9:57 pm

Q3 is already a MOSFET, isn't it?
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Re: ICE TUBE Second digits are DIM

by neutron spin on Sun Mar 03, 2013 2:06 pm

I have been watching these posts with extreme interest. Although the fixes for the gradient issue offer some help the main problem as mentioned before is that you are driving the filament with a D.C. voltage. In order to drive the filament properly A.C. is the design requirement for these tubes. Until the design is changed (highly unlikely) this will always be an issue. Although some tubes are less sensitive to this, the problem will always be there. The Russian criteria for these tubes was based on A.C. voltage supplied to the filaments...read the data sheet.... :D
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Re: ICE TUBE Second digits are DIM

by wbp on Sun Mar 03, 2013 3:15 pm

Much has been made about driving the filaments with AC vs DC. I think this is almost an urban myth now. Several people have gone to some length to test this and the results were always the same - it doesn't really make any difference at all!

Increasing the filament voltage to a value that is within specs (which it really isn't on the Ice Tube) does help though, so that seems worth doing.

William
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Re: ICE TUBE Second digits are DIM

by jarchie on Sun Mar 03, 2013 6:47 pm

Although the fixes for the gradient issue offer some help the main problem as mentioned before is that you are driving the filament with a D.C. voltage.


I remain skeptical that driving the filament with direct current is the problem, because driving the filament with alternating current does not fix the problem: viewtopic.php?f=41&t=23586&start=21 But if you can point out any flaws in my methodology or reasoning, I would be grateful.


The Russian criteria for these tubes was based on A.C. voltage supplied to the filaments...read the data sheet....


True, but as William mentioned, the Adafruit design violates the data sheet specifications on another count: too little current flows through the filament. Replacing R3 with a jumper increases current through the filament, and the dim digit problem disappears.

My theory is that, at insufficient voltage, the filament attachments act like heat sinks and reduce electron emission near the ends of the tube. But at sufficient voltage, the filament produces too much heat for the attachments to dissipate, and the tube operates correctly. For whatever reason, the dim-digit-issue usually effects the rightmost digit, but I have seen it affect the leftmost digit as well.

The IV-18 was indeed designed and tested with alternating current. But that does not prove the tube will not operate well under different conditions. It only proves that the tube should operate well when the filament is driven with alternating current.
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Re: ICE TUBE Second digits are DIM

by neutron spin on Sun Mar 03, 2013 8:34 pm

I respectfully disagree with your conclusion. Here is a quote from the now defunct LM9022 data sheet by National Semi.."Application Information
LM9022 FUNCTIONALITY
Typically a VFD filament requires a low voltage AC power
source in order to create a constant brightness across its
length. Such a power source is not readily available in a
most systems. Existing implementations show different cir-
cuits for supplying an AC power for a VFD filament but they
require an AC power input, or do not have a standby control,
or generate high EMI. The LM9022 can solve all these
problems in one compact circuit"

Also Nortake Inc. has similar information on driving VFD display filaments and why A.C. is a better choice. I am not here to debate your theories but just stating manufacturer's design recommendations. I have in my lab tried both methods on various VFD tube, displays, etc. and found that in fact A.C. is the most optimal method for driving the filaments. This is not to say that all VFD devices will suffer from gradient issues. It depends on construction and type of device. I would like to see proof that using A.C. on the Russian IV-18 tubes driven to their specification sheet will result in said gradient issues...that is all I will say on this subject... :)
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Re: ICE TUBE Second digits are DIM

by wbp on Sun Mar 03, 2013 8:40 pm

Actual testing showed that using the correct voltage gets rid of the dim digits. You are free to use whatever works for you.
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Re: ICE TUBE Second digits are DIM

by jarchie on Mon Mar 04, 2013 1:12 am

I have in my lab tried both methods on various VFD tube, displays, etc. and found that in fact A.C. is the most optimal method for driving the filaments. [...] I would like to see proof that using A.C. on the Russian IV-18 tubes driven to their specification sheet will result in said gradient issues...that is all I will say on this subject...


If I understand correctly, you have provided evidence to support two claims: the first claim is that, in general, VFD displays have more consistent brightness with alternating current. The second claim that if the IV-18 tube is driven according to the specifications in the data sheet, it will have acceptably consistent brightness.

I agree with you on both counts. I am making a different claim.

My claim is that the IV-18 works well when driven with direct current at around 65 mA. (Or more precisely, when the Ice Tube Clock's R3 has been replaced by a jumper.) I have tested half a dozen tubes and they all seem to work well under these conditions.

Neither of your claims contradict mine. Just because a manufacturer provides certain design recommendations does not necessarily mean that their product will not work under different conditions.

I have done more testing than disclosed in this thread, and perhaps the results of those tests will clarify things...

Driving the IV-18 at a full 5 volts (~78 mA) with direct current results in a noticeable brightness gradient across the entire tube. But the gradient is gradual and quite different from the dim digit problem reported in this thread. Driving the tube with 5 volts of alternating current (according to specs) does help make brightness more consistent across the display, but the display is still noticeably dimmer near the center.

I believe the IV-18 is one of the rare cases where driving the tube outside of specs produces the best results. Driving the tube at ~65 mA produces the most consistent brightness gradient across the tube. But I am hard pressed to tell the difference between ~65 mA alternating current and ~65 mA direct current. I do understand why alternating current, in theory, should produce better results, but I just didn't observe that.

For the IV-18 in particular, using alternating current means that the tube will work well over a wide voltage range: 3.3v to 4.7v. With direct current, the tube works well over a fairly narrow voltage range: ~3.3v to ~3.4v. Direct current voltage greater than ~3.4v works fine, but produces a noticeable brightness gradient across the tube.

For the Ice Tube Clock, jumping R3 means that you'll hit that narrow voltage range. To date, I've seen four cases where jumping R3 fixed the dim-digit problem, and I've seen zero cases where jumping R3 failed to fix the dim digit problem.

Therefore I still believe that jumping R3 is an acceptable solution to the dim digit problem.

that is all I will say on this subject...


I will understand if you do not reply, but please know that I appreciate your expertise and the contribution you have made to this thread.
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Re: ICE TUBE Second digits are DIM

by john444 on Mon Mar 04, 2013 2:33 am

Hi JArchie,

I looked at the TI datasheet for the LM9022 that Neutron Spin referenced.
However, it looked as if the AC was more for stepping up the 5-V to get the 18-20-V of anode voltage

When you are measuring the AC current be careful.
Most meters are calibrated for a sine-wave.
Pulses or square-waves will not indicate correctly on most meters.

In order to be confident in the current measurement, you could:
1 use 60-hz AC for your testing.
2 use an Oscilloscope. Although, they are not nearly as accurate as a multi-meter
(you would have to observe the waveform and calculate the current)
3 measure the DC current from the supply and subtract the current for everything else.

Although it may be true, I cannot think of why heating the filament with the same amount of AC or DC current would make a difference in display brightness.

Good Luck with your testing.
John
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Re: ICE TUBE Second digits are DIM

by jarchie on Mon Mar 04, 2013 6:39 pm

(edited post)

When you are measuring the AC current be careful.


I don't remember how I took the AC readings; I made those observations two months ago. But I really only wish to show that the IV-18 works fine with DC, so the AC measurements are not important.

Although it may be true, I cannot think of why heating the filament with the same amount of AC or DC current would make a difference in display brightness.


With DC, there is a voltage gradient across the filament because one end of the filament is positive and the other is negative. But the grid/segments, when powered, have a similar, large positive charge.

On the negative end of the filament, the difference in charge between the filament and the powered grid/segments is relatively large: more electrons travel from the filament to phosphors on nearby segments. On the positive end of the filament, the difference in charge between the filament and powered grid/segments is smaller: fewer electrons travel from the filament to nearby segments.

To demonstrate this effect, grab an IV-18, twist all the grid/segment wires together, and connect them to +12 volts. Apply +5v to one end of the filament and ground the other. There will be a noticeable brightness gradient across the tube. The brightest end will be the negative end.

Next, reverse the direction of current across the filament. The brightness gradient will be reversed.

Increasing the voltage applied to the grid/segments will reduce this effect and make the overall display brighter. I believe this was the reasoning behind the following entry in the Adafruit FAQ.

At low brightness, the seconds are dimmer than hours. Unfortunately this is an effect of the way we drive the tube. It is not apparent at medium and high brightness


If the dim digit problem were due to driving the tube with DC, increasing brightness would make the voltage gradient across the filament smaller relative to the grid/segment voltage. Brightness would become more consistent across the tube. But in my experience, increasing brightness does not seem to fix the dim digit issue.

Quite the contrary, what eliminates the Ice Tube Clock's dim digit problem is replacing R3 with a jumper to increase current across the filament. That also increases the voltage gradient across the filament, which should (and probably does) increase the overall brightness gradient. (The brightness gradient is still insignificant.) Therefore, the dim digit problem cannot be a consequence of the filament voltage gradient. It must be the result of insufficient current across the filament.

Insufficient current across the filament would result in insufficient heating, and the filament must be heated to emit electrons. The filament is in a vacuum, so it will lose heat through the metal attachments holding it in place. The rightmost attachment is larger; the filament should be coolest near the rightmost digits. The leftmost attachment is smaller, but should still cool the filament. So if the problem is insufficient heating, the problem will affect the rightmost digit most. That's the exact problem people are reporting.

There is a reason behind the claim that VFDs must be driven by AC. Modern VFDs generally have much lower grid/segment voltages, so the voltage gradient across the filament is large relative to the grid/segment voltages. For that reason, modern displays exhibit a much more extreme brightness gradient when using DC to drive the filament. So if someone only had experience with modern VFD technology, the conclusion that VFDs must be driven with AC to produce acceptable results is probably quite reasonable.

It's worth mentioning that both sources cited by neutron spin--the LM9022 data sheet and the Noritake guide--are geared toward modern VFDs which is why they both stress the importance of using AC to drive the filament.

But for the IV-18, as driven by the Ice Tube Clock, the brightness gradient caused by DC is insignificant. Driving the filament with AC would eliminate the brightness gradient, but it's is barely noticeable as is. I have to look quite carefully to convince myself that I can see any gradient at all. From my perspective, there is no compelling reason to drive the filament with AC instead of DC.
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Re: ICE TUBE Second digits are DIM

by john444 on Mon Mar 04, 2013 7:52 pm

J Archie,

jarchie wrote:With DC, there is a voltage gradient across the filament
Thanks for the info. I hadn't considered that.

John
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Re: ICE TUBE Second digits are DIM

by jarchie on Mon Mar 04, 2013 9:20 pm

Hi John,

Thanks for the info


You're welcome. Thank you for pointing out the subtitles of measuring AC current. You've convinced me to redo the AC testing when I have time.

Cheers,
John
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Re: ICE TUBE Second digits are DIM

by jarchie on Sat Mar 09, 2013 10:01 pm

I have replicated my previous IV-18 testing. All grid/segment pins were attached to 12 volts, one end of the filament (pin 13) was grounded, and the other end (pin 1) was varied from 0 to ~5 volts for both DC and AC (sine wave, 60 Hz). After playing with several tubes, I realized that the exact numbers differ depending on the particular tube. But the general observations were correct:

(1) For both AC and DC, all tubes exhibited the dim digit problem described in this thread at low current.

(2) For both AC and DC, all tubes exhibited consistent brightness once current was raised just enough to eliminate any dim digits.

(3) For DC, tubes acquired a brightness gradient at higher voltages. The tube with the worst gradient at 5 volts is picture below.

Image

In summary, the tubes could be driven with consistent brightness under both AC and DC. Under AC, brightness was consistent over a large voltage range. Under DC, the voltage range required for consistent brightness was narrower.

Just for fun, I modified my Ice Tube Clock as William and Grumpy suggested, replacing the Q3 MOSFET with a PNP transistor (PN2907A) and 200 ohm resistor. A 100 ohm potentiometer in place of R3 regulates current from the dim-digit range to the brightness-gradient range. Although adjustable filament current is nice, replacing R3 with a jumper still seems like a more elegant way eliminate dim digits.

Image
Image

Okay... so I finally looked at some actual IV-18 documentation, but I cannot read Russian. The recommended grid/segment voltage seems to be 50 volts with 70 volts as the absolute maximum. Recommend voltage for the filament seems to be 4.3 to 5.5 volts, but I found no reference to AC or DC.

On my Ice Tube Clock, the grid/segment voltage was just under 30 volts at maximum brightness. By tweaking the boost timer settings I was able to increase this to 50 volts, although the clock probably needs a larger fuse to maintain that level for extended periods. With a 50 volt grid/segment voltage, there was no noticeable brightness gradient even when applying a full 5 volts DC across the filament. Although those conditions are recommended in the IV-18 documentation, the display is extremely bright--about twice as bright as the Ice Tube Clock on its brightest setting. These tubes must have been intended to be visible in full sunlight.

The IV-18 works fine with DC current at the recommended voltages, and I could not find a reference to AC (переменный ток) in the documentation. I am beginning to wonder if the original documentation actually recommends AC over DC for the filament. I suppose the question is academic, because DC works in practice, but even so, I am curious. Can anyone on this forum read Russian? Would would anyone be willing to check this?

http://www.vitriol.com/pdf/Sprv-IND.pdf (page 278)
http://www.radiolamp.ru/sprav/ind/iv18.html
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Re: ICE TUBE Second digits are DIM

by john444 on Sat Mar 09, 2013 10:24 pm

J Archie,

Nicely done.
It is good to see someone do some actual testing and be able to make statements about actual data.

Thanks for that, John
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