Ice tube clock - display problems and Q3

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redwire
 
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Ice tube clock - display problems and Q3

Post by redwire »

I had odd troubles after assembling my Adafruit ice tube clock display.
The symptom is only one segment lights! Occasionally, the display flickers and shows the time or fades in/out. Soldering/parts placement is fine. Oddly, putting my DVM from GND to JP2-2 got the display to light up steady...
I took some measurements, and found the P-channel mosfet Q3 (ZVP3306A) is lazy, it is only partially on. Verified pinout- the rounded-side matches the pcb silkscreen. Source=4.72V Gate=0V Drain=3.06V (*too low); Vin=9V, Vcc=4.72V, Vboost=14V, Vfilament=2.0V (*low)

Fix: Swapped in a venerable 2N3906+1k base resistor and the display lights now (RBase= 1k gave 4.56V out; 10k gave only 3.76V out, B=200)

Looked at datasheet and Q3 (mosfet) is a dog, as finding TO-92 P-ch mosfets with low-threshold is very difficult.
http://www.diodes.com/datasheets/ZVP3306A.pdf
FJN4301R, PDCT123T, DTA114 digital transistors might work, but pinout is different.

Just letting people know, as the symptoms were pretty strange and that mosfet is not best suited for this.

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adafruit_support_bill
 
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Re: Ice tube clock - display problems and Q3

Post by adafruit_support_bill »

Thank you for the suggestions.

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neutron spin
 
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Re: Ice tube clock - display problems and Q3

Post by neutron spin »

The original design used a 2N2907 PNP transistor but they had problems. The change to the ZVP3306 Mosfet was to improve reliability. When the ZVP3306 was used the issues went away...Perhaps trying a different ZVP3306 Mosfet may solve the problem.....

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redwire
 
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Re: Ice tube clock - display problems and Q3

Post by redwire »

As a follow up, I bench-tested the mosfet and it's within spec.
VGS(th) is -3.10V for 1mA, at VGS=-4.76V IDSS is 48mA, which is a problem. It's "on" weakly, not enough for the tube filament load; the mosfet really would like 5-10V gate drive. I measured: -4.76V for 48mA, -5V for 60mA, -5.85V for 100mA.

This means the tube (filament) is starved, and for me it was enough to make the display wonky.
It's something to check, as the tube filament is under-volted in the design anyhow. It needs 5V at 75-95mA and we have 4.76V power and a 22R resistor giving 58mA and 3.5VDC. Then it looks like people over-volt the anode past 30VDC to make up for the brightness loss. Tubes don't last full life with filament voltage even 5-10% low.
I would change the design, maybe for the next revision.

I thought the 2N2907 didn't work because there was no base current limit resistor. I added tiny 1k 1/8W and it's fine.

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neutron spin
 
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Re: Ice tube clock - display problems and Q3

Post by neutron spin »

I believe you are correct as to the reason the mosfet was substituted. Perhaps a consortium of interested users could suggest a new improved design and present it to the Adafruit management and a new improved Ice Tube Clock could be next?....I am sure it would be a welcome project. :D

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redwire
 
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Re: Ice tube clock - display problems and Q3

Post by redwire »

I don't know how it would work for a next revision Ice Tube Clock. It looks like one person comes up with cool open-source hardware, and then everyone or any company can kit it?
So I don't think Adafruit is the "parent" of this clock, even though they provide the kit and tons of information on building it. The community should be the parent. I'm not sure, I'm still trying to figure out how to contribute to projects, like this.

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adafruit_support_mike
 
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Re: Ice tube clock - display problems and Q3

Post by adafruit_support_mike »

The Ice Tube clock is very much an Adafruit design. We owe serious props to Riad, who created the inGrid clock (http://web.jfet.org/inGrid/), but Ladyada put a ton of work into this specific design: http://learn.adafruit.com/ice-tube-clock-kit/design

It's Open Hardware, so yes, other people can copy the design and offer their own kit (if they can find a reliable source for the VFD tubes), but if you want the original accept-no-substitutes source of this product's hardware design, that's the very smart lady with the pink hair over there playing with her new pick-and-place machine.

To get involved in an Open Hardware project, build stuff.

It really is as simple as that. The whole point of Open Hardware is to remove the friction that keeps you from doing whatever you want with the product you've paid for. Our role is to curate of the core reference design, but that design does move now and then. No design is ever 'done', you just reach a point where you have to stop tinkering and ship.

Design, build, modify, measure, test, test some more, bang your head against the wall to see which one is stronger, ask questions, trade suggestions, tear it all down and do it all over again. Those are enough of a challenge without a rights-holder saying, "no, we don't feel any need to give you the documentation for those parts" or "how dare you tamper with our work!"

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