The 48 volts comes from a kilowatt MeanWell supply. The little arrow thingies are fastons on the PCB.
Enough fun for today. Now it's time to caulk the roof; rainy season is coming soon.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
U26 should be the adjustable variety, but the pinouts are the same. I assume it's just mislabeled, or a "go-to" footprint. And you could get a little tighter on the programming resistors. For example, [1.9k/16.7k] or [1.6k / 14k] would get you closer to 12.0 VDC out, without getting into exotic resistor values.
It should be obvious from the schematic that this is the ADJ variety. I even noted the feedback voltage.
It's in the PADS library. Our MAX stock number actually controls what goes on the board; that's an attribute of every part on the schematic.
One thing we like to do is minimize the BOM, the number of different parts on the board. It's silly to load up a reel of resistors on the pick-and-place to put one resistor on the board. That's why there are extra resistors on U13 and U27.
12.18 volts is pretty close to 12.00, close enough for fans and relays and opamps.-- John Larkin Highland Technology, Inc lunatic fringe electronics
Maybe I should add a fuse, or a sacrificial resistor, in series with C57. The 48 volt supply could source 30 amps maybe, which could do some damage.
Fun, I get to explode some resistors to pick a part. Win's x-pages resistor abuse graphs will help. Win, what's that link again?
The offpage 48 volt goes to some class-D amps that are themselves fused.
We would of course production test by starting with a small current-limited supply before we connect the monster.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
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Thanks,
- Win
I have the X-book on pre-order from Amazon. This sort of real-life data is hard to find.
I'm estimating that my switcher input current might be 200 mA max, from the +48, with all 15 of my relays pulled in. So what do you think, add a 1 ohm 1206 in the switcher input before the cap? Normal power dissipation would be 40 mW, and it would open in milliseconds at, say, 10 amp fault current.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
tirsdag den 5. november 2019 kl. 00.45.15 UTC+1 skrev John Larkin:
you are supposed to use fusible resistors for that kind of thing so it doesn't catch fire, but it seems like they are mostly through hole
I don't mind blasting the resistor, but I want to reduce the downstream damage that the kilowatt power supply could do.
I guess I could use a real fuse. We'll do a trial PCB layout and see if we have room.
The probability of ever needing this protection is very low, but the potential consequences are high.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
tirsdag den 5. november 2019 kl. 02.08.00 UTC+1 skrev John Larkin:
make a thin piece of trace
Yes, a 1206 thin-film resistor, experiencing 100W, will go puff, and not with excessive flames, no problem. Blow up a few to see. Should this ever happen, it's better for the user to replace a standard 1-ohm resistor, than a fuse. Fuses are for those cases where the ratio between normal use and disaster is much smaller.
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Thanks,
- Win
Who, me?
It is cool to sometimes put a 1-ohm or 0.1 ohm resistor in series with various regulator inputs, to allow for measuring currents. That helps solve the perennial mystery, where the hell is the current going?
And it's useful to measure actual uP and FPGA currents now and then.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
I got into that habit in the late 60s, using CMOS (call COS/MOS back then), designing oceanographic instruments. Every CMOS IC had its own 10-ohms on the Vdd pin. Plus one more for the entire set of logic ICs on the board. I started my oceanographic instrument company, Sea Data, in 1972, and began using 10-ohm resistors by the tens of thousands. Funny thing, the CMOS ICs were so reliable that I rarely found a board drawing excess current, and rarely had a need to probe the individual ICs.
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Thanks,
- Win
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