Ice Tube Clock IV-18 To-Spec Hack

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jarchie
 
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Re: Ice Tube Clock IV-18 To-Spec Hack

Post by jarchie »

Russell 27 wrote:Input voltage is not a direct correlation to output current. The regulator regulates voltage, not current. The capacitor will not charge above output voltage, it will just stabilize at full charge capacity @ 5.7 volts (according to your drawing resistors) with no load.
The problem is that LM317 is not guaranteed to regulate voltage if the load is less than 10 mA, according to the datasheet. I've read on other forums that the LM317 tends to output too much voltage when the load is small, but suspect that the exact behavior depends on the manufacturer.
Russell 27 wrote:The TO-92 is only good for about 200 MA max, so it may get a little warm. Until the push pull circuit starts to oscillate, there would be no real connection there.
I'm using a TO-92 that is rated to 100-200 mA, so I was thinking it might be dodgy above 100 mA, but the filament will consume 95 mA max. Perhaps more importantly, the TO-92 package should only be able to dissipate ~600 mW. On my clock the TO-92 drops ~2.7 volts at ~85 mA, so the power is ~2.7 * 85 = 230 mA. The regulator gets slightly warm to the touch (maybe 40 degC?), but that seems normal for a regulator.

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phild13
 
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Re: Ice Tube Clock IV-18 To-Spec Hack

Post by phild13 »

Since the current from the adjustment terminal (IAdj) represents an error term in the equation, the LM317 is designed to control IAdj to less than 100 uA and keep it constant. To do this, all quiescent operating current is returned to the output terminal. This imposes the requirement for a minimum load current. If the load current is less than the minimum, the output voltage will rise.

I think you can get away with a load current minimum of 3.5mA, though I would shoot for 4mA-5mA. 10mA is probably excessive.

You can get LM317 in 500mA in a TO220 package which will probably fit the space and provide a reasonable margin if your concerned about the warmth of the T0-92 package. The TO-92 is supposed to be good for 200mA max so even if the filament resistance varies rather greatly (is very likely due to manufacturing) it should be ok.
While all the LM317's from different manufacturers are about the same, the ON Semiconductor has a better datasheet to use for reference.
http://www.onsemi.com/pub_link/Collateral/LM317M-D.PDF

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jarchie
 
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Re: Ice Tube Clock IV-18 To-Spec Hack

Post by jarchie »

PhilD13 wrote:Since the current from the adjustment terminal (IAdj) represents an error term in the equation, the LM317 is designed to control IAdj to less than 100 uA and keep it constant. To do this, all quiescent operating current is returned to the output terminal. This imposes the requirement for a minimum load current. If the load current is less than the minimum, the output voltage will rise.
That totally makes sense, and thank you for the explanation.

PhilD13 wrote:You can get LM317 in 500mA in a TO220 package which will probably fit the space and provide a reasonable margin if your concerned about the warmth of the T0-92 package.
The TO-220 package will fit, although it's a bit of a squeeze. I was actually planning to mention that the TO-220 package should work in the xmas-icetube Rev. D board (even though the silkscreen outlines a TO-92), but I was waiting until after I tested Rev. D.... so you must be something of a mind reader!

EDIT: I prefer a TO-92 package for aesthetic reasons, but using a TO-220 package in the upright position on the xmas Rev D board doesn't look bad. So if AC-to-DC power supply robustness is your priority, the TO-220 package is the superior choice. The only reason I suggest a TO-92 package on the xmas-icetube board is for aesthetic reasons. For hackers who would rather be extra safe, the TO-220 package is fine!

Assuming the Rev. D is my final revision for a while, my (as yet unwritten) documentation will recommend a TO-220 package for use with an unregulated wall-wort or any other AC adapter that could provide significantly more than 9 volts DC.

PhilD13 wrote:[T]he ON Semiconductor has a better datasheet to use for reference.
Thanks for the ref! Although the datasheet you linked is for the TO-220 package, the ON Semiconductor datasheet for the TO-92 package is indeed much better and makes me more comfortable with the decision to go with a TO-92 package:

The IV-18 datasheet specifies a filament current of 75-95 mA at 5v, which is consistent with the tubes I've played with. (If I had never seen the IV-18 datasheet, I would have naively guessed a narrower range centered at ~85 mA.) With a regulated 9v switching power supply (like the one that comes with the Ice Tube Clock kit), I would guess the worst-case voltage drop across the LM317 would be 3 volts, and the worst case current would be 100 mA, as the resistor divider only consumes ~1.3 mA. The ON Semiconductor datasheet gives a thermal resistance of 160 degC/W. In the absolute worst case, I can imagine the internal temperature of the clock reaching 50 degC, which would place the regulator temperature at 50degC + 3v * 0.100A * 160degC/W = 98degC. According to the ON Semiconductor datasheet, the maximum operating temperature of the regulator is 150 degC. I am comfortable with that margin of error.

EDIT: The calculations above were for the Adafruit kit. My xmas-icetube board schematic specifies D1 as a 1N4001--a cheaper part which will lower the voltage drop across the LM317 by ~0.6v and further increase the margin of error.

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russell 27
 
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Re: Ice Tube Clock IV-18 To-Spec Hack

Post by russell 27 »

I'll leave you with this one last thought and give it a rest. The data sheet is just a guide and even so only showed a millivolt scale deviation without a load. I have not used a ton of 317s, but I have used one for the last five years in a time clock circuit. Input voltage varies by 5 volts or better, I'm eeking out the absolute minimum of about 1.25 volts output. If my output deviated by a volt, my circuit probably would have been toast some time ago. I did my own test. Set up the 317 to make about 4 volts, a 1k and 2k2 resistor setup. With no load and some filter capacitance, and with 9 and 17.5 volts, respectively the output voltage was 4.18 and 4.36, a .18 volt difference, nothing of note, a battery can easily change this much. With an LED load of 18 Ma, with same 9 and 17.5 volt, respectively the output voltage was 4.17 and 4.18, load made small difference. My circuit uses less than 10 Ma much of the time. This would never be a problem for the ICE TUBE. It's always important to test the circuit, data sheet is just a guide. I would probably never use this chip without filter capacitance.

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jarchie
 
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Re: Ice Tube Clock IV-18 To-Spec Hack

Post by jarchie »

Russell 27 wrote:The data sheet is just a guide...
True, and I follow datasheets perhaps too religiously. But I do think that datasheets are written by engineers and designers who understand the chip in great detail, and that the specifications incorporate the results of significant testing. Although a part might work quite well outside the datasheet parameters, following the datasheet seems like a safe choice.
Russell 27 wrote:My circuit uses less than 10 Ma much of the time. This would never be a problem for the ICE TUBE.
In the circuit I posted, the LM317 provides voltage only to the push-pulls, so when the display is inactive, the only current--about 1 mA--will flow through the resistor divider. So the low current problem could be an issue.
Russell 27 wrote:I would probably never use this chip without filter capacitance.
Note that all the sensitive parts, including the microcontroller, VFD driver chip, and temperature sensor, are using the the 5v regulator with filter caps. Only the push-pulls and VFD filament are exposed to the LM317 output, which is why I'm willing to omit the filter cap there, as the output AC waveform seems okay for the filament.

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