ICE TUBE Second digits are DIM

[jarchie]

After driving the VFD filament with alternating current, I do not believe that dim digits are caused by using direct current. (But I don't know much about electronics, so I hope someone will challenge me if I've made mistakes.)

The alternating current is generated from the otherwise-unused PC1 and PC2 pins on the ATmega328p. After a reset, the ATmega328p sets PC1 to ground and PC2 to 5v:

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DDRC  |=  _BV(PC2) | _BV(PC1);
PORTC |=  _BV(PC2);
PORTC &= ~_BV(PC1);


And approximately once per millisecond, the PC1 and PC2 outputs are reversed, generating 500 Hz square-wave alternating current:

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PORTC ^= _BV(PC2) | _BV(PC1);


Since PC1 and PC2 cannot output sufficient amperage to drive the VFD filament, they are amplified by two push-pulls:


For the direct current test, the setup and digit brightness looked like:




For the alternating current test, I used a multimeter and oscilloscope to verify that the output used to drive the VFD filament was indeed 5-volt square-wave alternating current. The setup and display looked like:




I realize that the VFD brightness gradient is difficult to see in photographs, but there was no noticeable difference between the setups. The last digit and overall brightness gradient appeared identical. Therefore, I believe the brightness gradient is intrinsic to individual tubes and that software modification is a perfectly reasonable way to workaround the issue.

This makes me wonder... Are there any compelling reasons to use alternating current for the filament?

[twolf]

Yesterday I put together my third IceTube, and just like the second one, it had a really dim last digit when I finished. I accidentally knocked my first clock onto the floor and broke the tube when I first got it, so I had purchased some spare tubes back then. After assembling my second clock, and being unhappy with the dim last digit, I bought a spare "side pcb" and header to see if one of my other tubes would have a better output, and luckily it did. When my third clock had the exact same issue, I decided to try the original tube that had the same issue, just to see what it might look like. Surprisingly, it is very consistently bright across all the digits, even though the same tube had the dim last digit issue on the previous clock. All my clocks have ran the same firmware, DigiSage. This leads me to believe that issue is *partially* due to the exact combination of components, and building technique.

It is great to find out that there is a software fix now though. I would have never thought of such a thing myself, so thanks!

[jarchie]

Hi twolf,

Your observation about the tubes is really interesting to me.

The tube I used for the DC versus AC test had an extremely dim final digit one month ago when I purchased it. At that time, I put it in a box and used a tube with more consistent brightness. But as shown in the photos in my previous post, the inconsistent tube still has a noticeably dimmer final digit, but the difference is much less significant. Since I used the same clock for both tests, could it be that these tubes tend to even out over time after an initial use?

Have you tried moving your now-consistently-bright tube to your first clock? I wonder if the issue is really is different components/building-technique or if the issue is that these tubes tend to improve with age after an initial use.

--John

[wbp]

I've seen a small sample of tubes, 4 so far. 2 were very uniform, 1 was slightly dimmer, 1 was significantly dimmer. In both cases it was the only the last digit that was dim. One of the ones that is very uniform is not in an Ice Tube clock, it's an Akafugu clock, which runs higher filament voltage. In a dark room you can see the filament wires in the Akafugu IV-18 glowing softly red. It might be worth investigating if lower filament voltage is related to uneven digit brightness.

I also notice that my personal Ice Tube exhibits significant temperature sensitivity. If it's cold in the room that it's in, and I have the brightness set as low as it normally is in warmer weather (it has a photocell and autodimming), some of the digits become impossible to see.

William

[jarchie]

Damn, William. Good insight!

On my Ice Tube Clock, if I replace R3 with a jumper, current through the VFD filament increases. My uneven tube evens right out!

[wbp]

While you've got it apart, can you substitute a meter for R3 and measure the current? The spec for the IV-18 is 75 to 95 mA, 85 mA nominal.

William

[jarchie]

I've already reinstalled R3 and assembled the clock; I'll test the current as you suggest on Wednesday or so.

The reason for the delay is that I'm expecting three IV-18 tubes in the mail, and I'd like to do all my testing at one time. I am hoping that all tubes will have consistent brightness at ~85 mA current, and that R3 can be selected on a per-tube basis to get the desired current. The desired resistance of R3 might even be easily calculated from the resistance of the VFD filament....

Of course, it's also possible that more than 95 mA is necessary to even out the brightness, so a software solution might still be better than overtaxing the filament...

[jarchie]

I'm a bit puzzled by one thing. The output of my voltage regulator is 4.99v. The voltage between Q3 and R3 is 4.74v. But the voltage falls to 3.26v with the tube inserted. Since my clock works perfectly, I'm going to assume that's in the normal range....

On my clock I've tried six tubes, three of which have a noticeably dimmer final digit. The current going through the filaments with R3 (22 ohm resistor) installed was 52.5±0.8 mA. With a jumper installed the current increased to 62±1.2 mA.

[wbp]

The current going through the filaments with R3 (22 ohm resistor) installed was 52.5±0.8 mA. With a jumper installed the current increased to 62±1.2 mA.

Interesting. The current you measured seems quite low. The spec for this tube is 85 mA nominal (75 to 95) and 4.2 to 5.6 V, as best I can find. According to one source, the current draw is highest with no segments lit, but I can't confirm that. If those specs are right, that 22 ohm resistor is probably not necessary. Another source did indicate it would light up at much lower voltages. It would seem that the lower filament voltage is the real cause of the dim digits people are seeing.

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

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

[jarchie]

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.



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.

[wbp]

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...

[grumpygasbag]

Q3 is already a MOSFET, isn't it?

[neutron spin]

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....

[wbp]

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

[jarchie]

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.