henry21 wrote:
Hi,

As a side project I have a little mame box I'm working on. I want to
join a coin mechanism and logic board to the PC. Each device draws a
current of 65mA. I currently have them connected to the 12v mobo
header.

Is it safe to put these directly on to the PSU? The 12v molex
connectors have 20+ Amps? I'm only drawing 130mA. The wires are also
thinner, like the wires on a small exhaust fan.

Also, what is a clean method for joining the wires together inside the
case? Would I use a terminal mount?

Thanks.


When I connect home projects, I

1) Twist the copper wire ends together, to make them mechanically secure.
2) Solder the wire joint, so they can't separate. The twisting in (1)
holds the wire secure, while the solder joint cools.
3) Slide a piece of heat shrink tubing (polyolefin) over the
now cooled solder joint. (You fish that onto the wire before
step 1, so you can slide it over the joint in step 3.)
4) Heat the tubing gently so it doesn't burn. It will shrink
to about 50% of its original diameter. Such tubing remains
secure, whereas solutions like electrical tape, tend to fall
off when the weather gets warm. If you use the tubing on top
of PVC (polyvinyl chloride) insulated wire, the PVC may melt
before the outside tubing shrinks. I try to do the
shrinking step, right on top of the bare soldered area, for
best results, rather than shrinking the entire tube.

Davy has a good suggestion, to use an in-line fuse. The power
supply has a rating printed on the side, like 12V @ 20A. You
would likely be able to draw 26 amps in the event of a short
circuit, before the power supply might switch off "on overcurrent".
Using an inline fuse, limits the max current to a lesser
value, for personal safety (mainly smoke and fire).

The power supply rating, is the maximum current it can source. Your
0.13 amp load only uses a fraction of the capacity. You could
drive somewhere around 120 of those circuits in parallel, before
the power supply would be running at 100% capacity.

Using less than the full capacity of the power supply, is just fine.
It's like driving your car - a 130 horsepower engine, only puts out
perhaps 20 horsepower to compensate for wind resistance. The
unused horsepower is not an issue. It's the same with the power
supply. If it supplies 0.13 amps, it loafs along like
"an engine idling".

The purpose of adding the in-line fuse, is to limit the maximum
current the circuit can draw *downstream* from where the fuse
is connected. If you make a mistake in your circuit, something
shorts out or the like, then the fuse prevents the situation
from turning into a fire.

Fuses come in several types. There are "fast blow" and "slow blow".
If your circuit board has a massive filter capacitor on it,
charging up that capacitor might be sufficient to blow the "fast blow"
fuse. A "slow blow" can be more tolerant of initial circuit inrush
current. If you buy a fast blow, and it blows too often, then
chances are, the transient inrush current into the board, is
doing it. It might not be an actual short circuit on your board.
The disadvantage of the "slow blow" type, is there could be
minor damage to your circuit board, if there is an accident
or circuit failure. The "fast blow" opens faster. As you can
see, there is a tradeoff there. Your circuit board should only
have as much bypass capacitance as is needed for adequate
filtering. (As a hobbyist, I used to put *huge* caps on projects,
but that isn't the right thing to do.)

In the lab, we used to have a drawer full of "pico" fuses. They're
nice fuses, but they're the "fast blow" type. We used to up-size them
by about a factor of 5x, to prevent them from blowing prematurely.
So if we thought a circuit needed 1 amp max, we'd use a 5 amp fuse.
That cut down on a lot of nuisance replacements, but still provided
a measure of safely for lab prototypes. Those fuses would blow
instantly, if you dropped a ground wire onto your circuit board
by accident. They tended to be too sensitive for our liking,
which is why we got into the practice of up-sizing them.

One way to connect your project, is to purchase a Molex "Y" cable
assembly. These cost a few bucks, but they give you both male
and female Molex connectors, and some wire to work with.
Chopping this cable in half, gives you the means to wire up
your project, yet have the ability to disconnect it at the
Molex when desired. I keep a few of these as spares, for
home projects. The yellow wire is +12V, and black is ground.
As provided, the "Y" cable is a means of increasing the fanout
of your Molex power cables, so you can get more connections to it.

http://images17.newegg.com/is/image/newegg/12-198-025-S01?$S640W$

This picture, shows some heat shrink tubing, covering an
electrical connector. When it shrinks like that, it is mechanically
more secure than electrical tape.

http://projects.dimension-x.net/pict...eap_shrink.jpg

If you use connectors for both the power and data parts of your
project, it makes it easier to remove for maintenance or feature
changes. The Molex power shown in the Newegg picture, is a good
way to solve the power disconnection part of the problem. It's
more convenient than screw terminals, and yet secure enough it
won't fall off.

Your local electronics store (the one that is "better than Radio Shack),
should carry polyolefin tubing, soldering irons, solder, solder wick for
cleanup and so on. I have only one store in town, which has all that
stuff, and visiting there is like a visit to the candy store :-)

Good luck,
Paul