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henry21 November 10th 10 06:44 AM

Building mame box
 

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.



Paul November 10th 10 04:53 PM

Building mame box
 
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

henry21[_2_] November 10th 10 10:27 PM

Building mame box
 

Could I just continue to use the 12v from the motherboard system fan
source? This is only 1-2 amp I think? This is working now, I am just
ignoring the white sense connector.

Thanks.



Paul November 11th 10 01:54 PM

Building mame box
 
henry21 wrote:
Could I just continue to use the 12v from the motherboard system fan
source? This is only 1-2 amp I think? This is working now, I am just
ignoring the white sense connector.

Thanks.


Fan headers have a current limit. Some retail motherboards, in the
user manual, will tell you how much current you can safely draw.
Exceeding the number by a large amount, will burn out a copper
track in the motherboard. The fan headers are *not* protected
by a fuse. The motherboard has fuses on USB ports and on PS/2,
but there are never fuses on the fan headers. They're an accident
waiting to happen.

That's why, getting your power from a Molex 1x4 disk connector, is
a lot safer. The Molex pins are good to about 8 amps or so.
They'll burn eventually, if abused (I've had to replace one here),
but they'll take a lot more current than a fan header.

Paul


GMAN[_13_] November 11th 10 04:44 PM

Building mame box
 
In article , henry21 wrote:

Could I just continue to use the 12v from the motherboard system fan
source? This is only 1-2 amp I think? This is working now, I am just
ignoring the white sense connector.

Thanks.


You could but youd be safer just tapping into a Molex connector for the 12V

henry21[_3_] November 13th 10 07:48 AM

Building mame box
 

Should be OK but would be advisable to use a in-line fuse say 1 Amp max.
the min. guess 500mA being better just for peace of mind should anything
go wrong.


The coin mech unit is self-contained and comes with warranty.

Connecting it to th 4-pin 12v molex connectors sounds good. Is an
in-line fuse really need? Say, the coin unit itself can be trusted...
could the PSU itself cause damage?

Do all other PC components have fuses? ie FDDs, fans, LED lamps etc? Is
it a hazard without one? What could happen?

Thanks.



Paul November 13th 10 06:12 PM

Building mame box
 
henry21 wrote:
Should be OK but would be advisable to use a in-line fuse say 1 Amp max.
the min. guess 500mA being better just for peace of mind should anything
go wrong.


The coin mech unit is self-contained and comes with warranty.

Connecting it to th 4-pin 12v molex connectors sounds good. Is an
in-line fuse really need? Say, the coin unit itself can be trusted...
could the PSU itself cause damage?

Do all other PC components have fuses? ie FDDs, fans, LED lamps etc? Is
it a hazard without one? What could happen?

Thanks.


In the industry I used to work in, we had a "fusing hierarchy",
and the purpose was to prevent fires. The components used,
have their own fire ratings, and one of the tests we did
for our equipment, was to actually burn one at an approved
facility. Then, changes would be made, to remove the more
flammable parts.

Computers are a bit different, in that they usually only
fuse items with "external" interfaces. The USB connectors
are fused on +5V. PS/2 is fused on +5V. Firewire may be fused
on +12V. The box centric view, is assuming nothing bad can
happen inside the computer, but a shorted cable outside
could cause a problem.

Well, what kind of incidents have happened inside computers ?

1) Computer case speaker (the "beeper"). Some of those were
powered on one of the leads, by +5V directly. If you closed the side
on the case, and pinched that +5V wire (to chassis ground),
it would become red hot, the insulation would melt and smoke,
and if you were lucky, the wire would burn off.

2) Vcore. I've seen a picture of at least one motherboard, that
drew large amounts of current from the supply, because the
processor Vcore was shorted to ground. The result was the
area all around the processor socket was blackened from
overheating. I expect there would have been significant
smoke and smell to go with that.

So plenty of things have happened inside a computer. It's
rarely that an incident results in live flames shooting out
of the case. That happened in at least one incident, but
that was probably a power supply failure. Flames actually
shot out of the fan hole on the back, on that one.

You would add an inline fuse, if you didn't trust the
construction techniques or the quality of the design.
Or, if there were recorded incidents of that same box
having failed before. You can use your own best
judgment, as to how likely that is.

If the thing is sitting loose on the table, you might
think about protection for it. If it is securely
fastened inside the computer case, perhaps less can
happen to it.

There are a number of ATX power supplies, that have
the potential to cause real problems. The ones
with a single 12V rail rated at 50 amps or more.
If one of those is shorted, I'd expect some
nice fireworks. While the lower power supplies have
current limiters on each output (at perhaps the
20 amp level), it isn't stated on the high end units,
whether they have any individual limits or just one
global limit (at 50 amps). That's a lot of power in
the event of trouble.

Years ago, I was working in a lab at 8PM in the evening.
My buddy had his head inside a large VME computer case
(room for 22 9U cards). There was a flash of white light,
that lasted for 1/2 a second and lit up the lab. My buddy's
head, quickly came out of the case. He'd shorted
the +5V in the box. The power supply was rated
+5V at 100 amps. The ground clip on an instrument,
touched the +5V leg of one of the ICs he was working
on. The leg burned off completely (nothing left) in
that half second. Impressed the hell out of me :-)
I'm sure that power supply, didn't even notice the load.
It didn't shut off or anything. You can imagine,
if a monster supply like that had a problem at night,
when you were out of the building, there'd be a
sooty mess the next morning.

Some of the wires inside your computer, are thin enough
to act as fuses. The fan headers on the motherboard,
don't have a very large copper track feeding them,
and that track tends to burn out if overloaded. But
some other things have lots of copper, and so the
conductors can't act as fuses. and then there's going
to be smoldering or flames somewhere. A lot of the components
are designed to be self-extinguishing (i.e. hard to keep
them lit), but still, you're going to get a lot of smoke
damage to the room.

Your power supply probably has a fuse on the entry side,
but I hardly ever hear about one of those blowing.

Use your best judgment, as to whether a fuse is right for you.
If there are lots of exposed wires, you haven't taken care
to insulate everything carefully, a fuse might be worth a
buck or two, to you.

Paul


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