Last update - Mar 12, 2009
DB15F CONNECTORThe DB15 connector needs to be a FEMALE
connector (sockets as opposed to pins!) for the pin numbering to be
correct. I got excited and hasty, and stupidly installed an incorrect
male connector on my first prototype board and blew the 12V trace clean
off the board in one section, just like a fuse! That was because the
pin numbers end up being inverted if you use a male connector, so don't
be stupid like me. Make sure the board-mounted connector is in fact a DB15F FEMALE type!
Also, DB connectors are available with different amounts of setback
from the mounting flange to the first row of pins. For this board to
work, you need a 0.318" short offset type connector. Here's a list of
parts from DigiKey that should work well, but you can probably find the
stuff at a local electronics supplier quite easily as well, and for
cheaper than DigiKey.
Board mounted connector: D-Sub, 15-Pos, 2-Row, FEMALE, 0.318" mounting, right angle thru hole, board locks, 4-40 nuts, DigiKey P/N A32120-ND
Jack screws: DigiKey 7230K-ND (you need two, or steal/borrow them from an old computer you've got lying around)
Mating connector: D-Sub 15-Pos, 2-Row, Male, solder cups, hollow unthreaded flange, DigiKey P/N A32503-ND or L717SDA15P-ND
Backshell
Assembly: D-Sub, 15-Pos, 2-Row, Backshell, Metalized Plastic, Assembly
Hardware, Strain Relief, Mating Screws 4-40, DigiKey P/N 915GME-ND
OPTIONAL FLYBACK DIODESThe
diodes are optional, and only intended as a low average power flyback
connection for
switched inductive loads if someone is using a few of the board's
transistor spots as general I/O
with TIP120-122 transistors. For me, the diodes were intended as a
handy way of
tying all possible flyback lines to one single pad, labeled '+12V" (and
now that I think about it, I
should change that label designator to something like FB to imply
flyback. Hmmm...) Obviously the flyback needs to go somewhere, so you
need to wire the pad labelled '+12V' to some 12V source, either in the
MS box, or outside the MS box. If you try to use the flyback diode on
a PWM controlled output line, like a PWM idle solenoid or a boost
solenoid, the flyback diode on that line may eventually succumb to
being overpowered - I don't know, because I am still in my own
prelimilary testing too. If it does, we'll look at that when it
happens.
OPTIONAL CURRENT LIMITING
A
quick note about current limiting. I don't really like it, but I
included it on the board for some strange reason. I don't like it
because it forces the high current output devices to operate in linear
mode, and that will create a LOT of heat that must be dissipated.
Whatever device they are controlling should be properly fused and the
current limiting should be thought of as a last resort. I only ever
intended it for low current limiting, and not for ignition current
limiting, although it might be able to handle it. I will state for the
record that I can't support that function, but if you try it on your
own, keep an eye on component temperatures! You don't want to set your
car's carpet or dashboard on fire! Always fuse things properly! I am
a pragmatic type, so I shy away from things like trying to get a
transistor or fet to current limit 7 amps by running it in linear mode
- it WILL create bags of heat, which is always the enemy. It doesn't
matter if it's a mosfet or a darlington bjt, they will both get smoking
hot!
Q1
through Q6 are the active component of the
current limiting system for each respective channel. If you do use
current limiting, the schematic says 2n3904, but you can also use a
2n2222
transistor or equivalent. If you are using IGBTs for ignition, I'd
leave the current limiting off, and set the dwell as per the standard
methods listed in the MS documentation. To bypass the current
limiting,
leave Q1-Q6 out, and bypass the current sense resistors (more info
below) with a small piece of jumper wire (a clipped off lead from a
1n400_ diode would do fine).
R2,
R4, R6, R8, R10, R12 are the
current sense resistors. To determine an appropriate resistor value,
0.7 divided by
your desired current will give you the nominal value for the current
sense resistor. The resistor's power dissipation is calculated as 0.49
divided by the resistor's nominal value.
Let's
say you wanted to limit current to 1.5 amps. 0.7V / 1.5A =
0.466666666Ω. You can't get such a resistor, so look at what's
available in the desired range. Here's a link to some nice resistors
from Digi-Key --> http://dkc3.digikey.com/PDF/C091/P1860.pdf
The
2W Panasonic resistors should fit the board just fine, and DigiKey
P0.47W-2BK-ND would get you a 0.47Ω resistor, that would give you
0.7 / 0.47 = 1.49A limit, and 1.04W power dissipation at a sustained 1.5A limit.
If you don't plan to use the current limiting feature, you need to
install jumpers in place of the current sense resistors.
I added extra pads at the one end of each
resistor's placement to allow the jumpers to be a bit shorter - you can
use long jumpers or short jumpers. I used some 20awg tinned buss wire
as jumper wire on my first test board. As I said before, the current
limiting was
never intended for ignition current limiting, as that would require
some higher power resistors, and honestly, the traces might not like
that too much, assuming that ignition
current limiting would happen around 7 amps or so. Aside from that,
for a 7 amp limit, you'd need a 0.1Ω resistor, which would need to
dissipate 4.9 watts. a resistor that big won't fit the board. Because
the average power in an electronically routed ignition system is low
(25% duty cycle for a 4 cylinder, 17% duty cycle for a 6 cylinder), you
might be able to fit a 3W resistor onto the board. I would try a
Digikey P/N 13FR100E-ND, but I make no guarantees about the board
surviving a 7 amp current limit thrashing without some external fuse.
The resistor will have to dissipate 4.9 watts, and the transistor, or
IGBT will be PUMPING out the heat at that point, and that resistor is
only rated for a sustained 3 watts without even considering it's
thermal derating factor.
http://www.ohmite.com/catalog/pdf/10_series.pdf
LINK WIRES FROM TRANSISTOR OUTPUTS TO DB CONNECTOR
In
order to get any function from the board, you'll need to install 18awg magnet wire
(or some equvalent copper wire with heat-shrink insulation over it) from:
OUT1 pad --> 1/2 connector pad
OUT2 pad --> 3/4 connector pad
OUT3 pad --> 5/6 connector pad
OUT4 pad --> 7/8 connector pad
OUT5 pad --> 11/12 connector pad
OUT6 pad --> 13/14 connector pad
The wires can be mounted to the back of the boards to keep
things neat. I know firsthand how this part kind of sucks, but no suitable
alternative was possible at the time I designed the board. Since then,
I revised the circuit board as a more compact four layer board that
does not require these jumpers; a panel of 4-layer prototypes will cost me upwards of
$500, so I'm not too keen to order anything until I know there is a justifiable demand for
those boards.
PIN 15 ASSIGNMENT
The pad labeled "15" connects to the DB connector's pin 15, and is unassigned. You can either use
"15" as a third ground pin by jumpering it to "9/10" (which is the
power ground that should go back to the engine block), or you can use
"15" as a place to bring the +12V pin out to the DB15 connector.