The build has begun!
First up, I put together the switching power supply and immediately hit
a small problem. For some reason, the outline of the input jack is incorrect
and crowds the polyfuse and the first Schottky diode. Not enough to be
a showstopper, but if you look closely I accidentally touched the jack
with the soldering iron and melted it slightly.
Second problem was with the National Semiconductor switcher chip.
I used a pinout with a staggered pin arrangement. But I only have an
inline arrangement type in my parts box (about a hundred from an Ebay
purchase). OK, no problem, just bend the pins with a long nose pliers.
I also didn't use the exact values of the capacitors I specified on the
schematic. I had a bunch of 100uf caps, so I just used three of those.
I figure the exact values aren't too critical, and going bigger shouldn't
hurt anything.
The LM2575 is only a 52Khz switcher, and the datasheet seems to
say you can use regular electrolytics and not need to get into the
low ESR tantalum or ceramic types. Yeah, probably at that switching
speed nothing's too critical.
So I built the power supply first because when I have a new board,
I like to verify that it's going to output the proper voltage *before*
I put a bunch of rare or expensive ICs on the board. It's probably not really
necessary for this board because I have high confidence in this
Simple Switcher design and there's very little to go wrong. But
I tend to be paranoid about letting the magic smoke out.
Connected up a 12v unregulated wall wart, fired up the DMM,
and immediately remembered why I shouldn't work on this stuff
late at night. I was using my UNI-T meter instead of a Fluke, and I
had the leads plugged in to measure current, although the meter
was set to measure voltage. I measured the 5 volt output
and got nothing. Hmm, puzzled I put the probe on the polyfuse
and got a small spark.
Luckily with a switcher that is short circuit protected and a polyfuse
to handle the raw input voltage, nothing bad happened. Fixed my
test leads and sure enough, 5 volts out!
Now in looking at the PCB more closely, I spotted another mistake.
The CompactFlash card socket has two tabs near where the card is
inserted. The tabs are supposed to be soldered to the PCB.
But when I made the part in Eagle, I forgot to block out the solder
mask over the pad! Oh well, I can scrape the solder mask off with
an X-Acto knife I guess.
So where am I? I've installed the CPU, the 4Mhz master clock
oscillator, and some sockets. I've put my Rigol on the clock and
verified we're getting a good rail to rail 5V CMOS swing on the
waveform, and the Rigol says it's 4Mhz to boot.
So far so good, and I haven't yet had to cut any traces.
Scott