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IceTube Clock display goes out after a minute
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IceTube Clock display goes out after a minute

by erikd on Tue Jul 06, 2010 8:27 pm

I had a great time building this clock but discovered a problem shortly after testing. The clock works great for few minutes after plug in although the display is somewhat dim in my opinion. High voltage is only around 27vdc at max brightness (with tube installed, about 42vdc without the tube) but after a few minutes the display goes out completely. Using a scope I can see the PWM signal from the processor that drives the boost converter goes flat to zero volts. The processor has stop running. Unplugging the clock and then plugging back in restores operation but a minute later the display goes out again. I notice when this happens the processor is extremely hot, as is the fuse. I have about 8 volts on the input and a solid 5 volts out of the regulator. I double checked all parts are installed and correctly. I believe my processor is defective, or part of it anyway. Is there anything else I should check? If not, how can I get a replacement processor? I do not have the means to program one myself.
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Re: IceTube Clock display goes out after a minute

by adafruit_support_bill on Tue Jul 06, 2010 9:11 pm

It could be due to a short on the board or some other assembly issue. Post some good clear photos of the fronts & backs of the boards.

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Re: IceTube Clock display goes out after a minute

by erikd on Wed Jul 07, 2010 7:13 pm

Okay, here are some pic of the board.

icetubetop.JPG
top
icetubetop.JPG (660.37 KiB) Viewed 3106 times

icetubebot.JPG
icetubebot.JPG (854.04 KiB) Viewed 3106 times
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Re: IceTube Clock display goes out after a minute

by stinkbutt on Wed Jul 07, 2010 11:51 pm

Wow. Not only are those some of the bigger images I've seen on these boards, but that's some of the cleanest soldering I've seen in a while.

However, it appears that several of your resistors, (R1, R3, and R5) are in backwards.
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Re: IceTube Clock display goes out after a minute

by stinkbutt on Wed Jul 07, 2010 11:51 pm

Yes, I was just joking.

Can you perhaps post images of the standoff PCB and the tube? Because the board itself looks decent to me (I could be wrong, perhaps someone else can detect a problem that I'm missing...)
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Re: IceTube Clock display goes out after a minute

by adafruit_support_bill on Thu Jul 08, 2010 6:29 am

I haven't built one of these myself, so I don't know what else to look for.
But excellent work on the soldering and board-pics!
As stinkbutt says, post pics of the tube subassembly and the Icetube regulars will look it over.

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Re: IceTube Clock display goes out after a minute

by erikd on Thu Jul 08, 2010 6:45 am

Thanks for the kind words on the solder job :) For me, appearance is as important as function. I could post pics of the tube board but I don't think the problem is there. The processor still gets hot even without the tube installed. Has anyone else noticed their processor getting hot?
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Re: IceTube Clock display goes out after a minute

by stinkbutt on Thu Jul 08, 2010 10:18 am

That's odd. The processor should be neither sourcing nor sinking much power at all. It's just sending serial to the driver chip. When you say the processor are you talking about the ATMega168 or the MAX driver chip?

Also check the temperature of the fuse. If there's too much power being dissipated then the fuse will eventually blow, and that might explain display going out, though it still won't tell us where the short is.

[EDIT]Oh yeah, you did check the fuse, and it's getting hot. And it's the 168 getting hot, not the driver. Perhaps I should read the whole thread, hmm?[/EDIT]

It might not necessarily be the ATMega, it might still be the driver chip that's defective, and it might be somehow sending current the wrong way up to the ATMega. Try pulling the driver chip and see if the PWM craps out again. The PWM should still be coming out of the ATMega, no?
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Re: IceTube Clock display goes out after a minute

by erikd on Thu Jul 08, 2010 10:37 am

Processor = ATMega168. The fuse gets hot too. Something is consuming power and since the ATMega168 is the only other device getting hot I surmise it is the device consuming excessive power which is why I think it is defective. I can not explain why it still seems to work correctly for a while though. Perhaps an unused I/O port pin is shorted internally which is causing the part to heat up and eventually lock-up. How do I get a replacement?
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Re: IceTube Clock display goes out after a minute

by adafruit on Thu Jul 08, 2010 11:38 am

hi erik,
does the display ever light anymore or is it broken now.
please remove the IC and check the 5V supply. does it go out after a minute?

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Re: IceTube Clock display goes out after a minute

by erikd on Thu Jul 08, 2010 12:24 pm

The clock always functions normally when first plugged in. Then after the ATMega heats for a while it stops working which causes the display to go out. When this happens I still have 5 volts available from the regulator. I believe it's the ATMega itself shutting down. One thing I've noticed is that there is no series current limiting resister on the base of the VFDPWR transistor. Since this pin must be low to turn on the transistor (and display), doesn't this put 5volts -0.7(Vbe) to ground through the ATMega? This could cause some heating or destruction of the I/O buffer of the part if the current through the base is sufficient. This current is only limited by the base-emitter junction, which is basically a forward biased diode, of the transistor and the I/O buffer of the ATMega which can only source/sink 40mA.
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Re: IceTube Clock display goes out after a minute

by stinkbutt on Thu Jul 08, 2010 2:49 pm

I seriously doubt it's the 168 that's overheating. If it was shorted to something it'd probably have just died on you. That's why I think it might be the MAX Driver chip, or perhaps there's just a short that we're having difficulty identifying.

Perhaps it might be helpful to confirm the display's going out because of the fuse. Try measuring the resistance across the fuse before you plug it in and immediately after the display goes out (you may need to unplug it and then hurry to measure the fuse - I dunno what'd happen if your multimeter tries to measure a live circuit.)
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Re: IceTube Clock display goes out after a minute

by stinkbutt on Thu Jul 08, 2010 4:07 pm

erikd wrote:One thing I've noticed is that there is no series current limiting resister on the base of the VFDPWR transistor. Since this pin must be low to turn on the transistor (and display), doesn't this put 5volts -0.7(Vbe) to ground through the ATMega? This could cause some heating or destruction of the I/O buffer of the part if the current through the base is sufficient. This current is only limited by the base-emitter junction, which is basically a forward biased diode, of the transistor and the I/O buffer of the ATMega which can only source/sink 40mA.


I don't precisely know how to characterize the base of the boost transistor. However, I don't think it matters:

1. There have been a gagillion copies of this kit shipped. What you're describing is a design flaw in the system itself (assuming it is, in fact, the issue.) If that were the case, you wouldn't be the first to encounter it.

2. The boost circuit's designed to source less than 200 mA. In fact, the design document suggests it's going to draw only ~20 mA. I don't precisely know what the hfe (gain) of the Hexfet is, but I'm fairly sure it's a hell of a lot more than 0.5. Like closer to 50 than 0.5.

3. The datasheet from Vishay list two parameters:

"Gate-Source Leakage" ~100nA
"Zero Gate Voltage Drain Current" 25uA or 250uA.

While I don't precisely understand the conditions these parameters are measured, I think it's likely that it gives us an order of magnitude estimate of the leakage current out of the gate, and it's not nearly enough to make the microcontroller cry.

Why don't you try pulling the MAX 6921 chip out of it's socket? Does that bypass the overheating issue? What about if the tube's not plugged it?
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Re: IceTube Clock display goes out after a minute

by erikd on Thu Jul 08, 2010 6:50 pm

No, not the boost MOSFET I'm talking about the VFDPWR transistor Q3. I am suggesting it is a design flaw. I just measured 70mA from the base of that transistor to pin 5 of ATMega168 chip. The datasheet for the ATMega168 list a max current for the I/O pins at 40mA.
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Re: IceTube Clock display goes out after a minute

by stinkbutt on Thu Jul 08, 2010 7:10 pm

erikd wrote:No, not the boost MOSFET I'm talking about the VFDPWR transistor Q3. I am suggesting it is a design flaw. I just measured 70mA from the base of that transistor to pin 5 of ATMega168 chip. The datasheet for the ATMega168 list a max current for the I/O pins at 40mA.


Oh. Well, then perhaps you've accidentally bridged the pins on the Q3 (VFDPWR) transistor. Can you check those three pins from above? They look fine from below, but they might be bridged on the top side of the board. This behavior is consistent with all three pins being shorted, I would imagine...
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Please be positive and constructive with your questions and comments.