homemade EFIS system and EMI

All kinds unfortunately this is for aircraft use, he's starting with 12v
DC (transformers don't work with DC) and needs to get +-12v +5 and +3.3v
all DC.
John

John,

I think you'll find that the -12V is used only in "legacy" PC design for
ISA busses, or serial ports (which he probably isn't using).

Evan


Yes I did a little research on it and the only thing the negative is needed
for is the rs232 serial ports. for this you can get a $1.50 ic to convert
+12 to -12 V low current.
That said, what I'd do is get a 1.5 amp 5v TO-3 regulator and a 1 amp TO-220
regulator along with one of the TO-220 adjustable 1 amp regulators for the
3.3 v and run it all off the 12V (highly filtered) input. That said,
(again) you might have problems with th 12V regulator if the input drops
below 13.8V! So you might need to use a separate battery for it and diode
isolate it from the aircrafts electrical systm and have it charged when it
exceeds the 13.8 volt figure.
John

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John,

I wonder if we are trying too hard on the +12V issue. Couldn't you
simply let it float free with something like a MOV to keep it from
getting too high? If the voltage goes too low, a simple current limiting
sacrificial resistor (AKA fuse) would protect the equipment. You could
even use one of those self healing plastic fuses.

Looking at the design again, there won't be any hard drives or fans to
suck juice from this voltage, so the only thing it will be powering is
the PCI bus.

John wrote:



Yes I did a little research on it and the only thing the negative is needed
for is the rs232 serial ports. for this you can get a $1.50 ic to convert
+12 to -12 V low current.
That said, what I'd do is get a 1.5 amp 5v TO-3 regulator and a 1 amp TO-220
regulator along with one of the TO-220 adjustable 1 amp regulators for the
3.3 v and run it all off the 12V (highly filtered) input. That said,
(again) you might have problems with th 12V regulator if the input drops
below 13.8V! So you might need to use a separate battery for it and diode
isolate it from the aircrafts electrical systm and have it charged when it
exceeds the 13.8 volt figure.
John


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Evan Carew wrote:

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John,

I wonder if we are trying too hard on the +12V issue. Couldn't you
simply let it float free with something like a MOV to keep it from
getting too high? If the voltage goes too low, a simple current limiting
sacrificial resistor (AKA fuse) would protect the equipment. You could
even use one of those self healing plastic fuses.

Looking at the design again, there won't be any hard drives or fans to
suck juice from this voltage, so the only thing it will be powering is
the PCI bus.



Yes it probably wouldn't matter, an MOV wouldn't do much good but you could
still feed a 12V regulator I guess and when the voltage started to go above
that it would regulate and when below it would just slump. I don't think it
would hurt the regulator to take the input too low.
That being said you could build a low differential voltage regulator for the
12V line it's possible to do it with less than 1 volt drop you just can't
do it with the simpler 3 terminal regulators.
The main point is you get rid of the switching regulators and their inherent
noise. It also makes it easier to filter the PC's noise out of the supply.
John
PS I checked out his site and found it quite interesting. I use to program
with hex and then onto assembler when I started out with computers in 1976.
I had a SWTPC 6800 with a whopping ~800KHZ clock! You had to build the whole
thing from a kit soldering on the buss connectors (SS50 bus) ic's
powersupplies etc. It did not have video you had to use an external
terminal although I later build a video board for it and modified the boot
rom's to use it. I started with a Mega memory of 4Kilobytes and eventually
got up to 48Kilobytes. Those were the days!