Mount external modem inside PC? Power from PSU?

If I have an external phone modem that uses a 1amp 9vDC tranformer, could I
power this from the 12v line of my PC PSU? (assuming that the modem will run
on 12v)

Noozer wrote:

If I have an external phone modem that uses a 1amp 9vDC tranformer,
could I power this from the 12v line of my PC PSU? (assuming that
the modem will run on 12v)

As usual, that depends. If you publish the schematic of the modem,
we can probably better evaluate it. The most likely thing is that
the 9 VAC gets full wave rectified resulting in about 12 VDC inside
the unit, in which case it should work (but the turn on load will
be high, to charge the input filter capacitor).

Oops, you said it was 9 VDC. Now we need the rest of the
schematic, including chip data sheets. However a set of 4 silicon
power diodes (say 1N4001) in series will drop the 12V DC to about 9
VDC, which should also work. They will be dissipating about 3
watts.

--
http://www.cs.auckland.ac.nz/~pgut001/pubs/vista_cost.txt

"A man who is right every time is not likely to do very much."
-- Francis Crick, co-discover of DNA
"There is nothing more amazing than stupidity in action."
-- Thomas Matthews

On Jan 27, 5:38 am, "Noozer" wrote:
If I have an external phone modem that uses a 1amp 9vDC tranformer, could I
power this from the 12v line of my PC PSU? (assuming that the modem will run
on 12v)

Mismatching power supplies is a craps shoot.

Noozer wrote:

If I have an external phone modem that uses a 1amp 9vDC tranformer,

That would be unusual.

could I power this from the 12v line of my PC PSU? (assuming that the modem will run on 12v)

It should be ok voltage wise, it should be regulated in the modem.

BUT there may be only enough heatsink on the regulator
to be able to handle 9V and not 12V without overheating
on the hottest days of summer if you arent airconditioned.

And it isnt a great idea to be taking the 12V rail outside
the case either, its a very high current rail in modern ATX
supplys, so the effect of a short can be rather spectacular
and electronically fatal if its a cheap power supply.

If I have an external phone modem that uses a 1amp 9vDC tranformer, could I
power this from the 12v line of my PC PSU? (assuming that the modem will
run
on 12v)

Might I ask what the purpose of this is ? (vs a PCI modem instead)

Chris

On Sun, 28 Jan 2007 06:16:16 +1100, "Rod Speed"
wrote:

Noozer wrote:

If I have an external phone modem that uses a 1amp 9vDC tranformer,

That would be unusual.

True, but if it is really 9VDC, next is to determine how
much current it uses (if there's a multimeter handy) or the
more conservative (higher) equipment current rating, or the
overly conservative and probably less useful original PSU
(wall wart?) current rating.

I seriously doubt it comes remotely close to 1A, none of the
external modems I've had got that hot for one thing, maybe
closer to 250mA would be reasonable.

Anyway, the easiest thing to to here is probably put an
LM7809 linear regulator between the 12V PSU rail and the
modem. It'll produce closer to a watt of heat, more than
you really want on a TO220 sized part so put a heatsink on
it. It need not be a large 'sink, particularly if there's
some ventilation.




could I power this from the 12v line of my PC PSU? (assuming that the modem will run on 12v)

It should be ok voltage wise, it should be regulated in the modem.

BUT there may be only enough heatsink on the regulator
to be able to handle 9V and not 12V without overheating
on the hottest days of summer if you arent airconditioned.

It would be easier if we knew what modem this was, perhaps a
link to a reasonably high resolution picture of the inside,
top-down.



And it isnt a great idea to be taking the 12V rail outside
the case either, its a very high current rail in modern ATX
supplys, so the effect of a short can be rather spectacular
and electronically fatal if its a cheap power supply.


So?
Use
A
Fuse

kony wrote
Rod Speed wrote
Noozer wrote

If I have an external phone modem that uses a 1amp 9vDC tranformer,

That would be unusual.

True, but if it is really 9VDC, next is to determine how
much current it uses (if there's a multimeter handy) or the
more conservative (higher) equipment current rating, or the
overly conservative and probably less useful original PSU
(wall wart?) current rating.

I seriously doubt it comes remotely close to 1A,

Plenty of mine are of that order.

none of the external modems I've had got that hot for
one thing, maybe closer to 250mA would be reasonable.

I doubt it.

Anyway, the easiest thing to to here is probably put an LM7809
linear regulator between the 12V PSU rail and the modem.

Makes a lot more sense to keep using an external wall wart instead.

It'll produce closer to a watt of heat, more than you really
want on a TO220 sized part so put a heatsink on it. It need
not be a large 'sink, particularly if there's some ventilation.

What is the point over just using a wall wart ? A switch mode one if
you dont like the constant power used of a transformer based one.

could I power this from the 12v line of my PC PSU?
(assuming that the modem will run on 12v)

It should be ok voltage wise, it should be regulated in the modem.

BUT there may be only enough heatsink on the regulator
to be able to handle 9V and not 12V without overheating
on the hottest days of summer if you arent airconditioned.

It would be easier if we knew what modem this was, perhaps a
link to a reasonably high resolution picture of the inside, top-down.

And it isnt a great idea to be taking the 12V rail outside
the case either, its a very high current rail in modern ATX
supplys, so the effect of a short can be rather spectacular
and electronically fatal if its a cheap power supply.

So?

So, its too much farting around.

Use
A
Fuse

Too
Much
Farting
Around

Makes a lot more sense to stick with a wall wart.

Skeleton Man wrote:

If I have an external phone modem that uses a 1amp 9vDC tranformer,
could I power this from the 12v line of my PC PSU? (assuming that
the modem will runon 12v)

Might I ask what the purpose of this is ? (vs a PCI modem instead)

External modems are much more useful. The presence of activity
lights alone gives you clues as to what is going on.

I still like my earlier suggestion of 4 rectifier diodes.
Including a 1 amp fuse in the line would be valuable. Everything
goes in series, and the result can be in line enclosed in some
shrinkable tubing. The suggestion got somewhat lost in my
rambling.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
http://cbfalconer.home.att.net

CBFalconer wrote:
Skeleton Man wrote:

If I have an external phone modem that uses a 1amp 9vDC tranformer,
could I power this from the 12v line of my PC PSU? (assuming that
the modem will runon 12v)

Might I ask what the purpose of this is ? (vs a PCI modem instead)

External modems are much more useful. The presence of activity
lights alone gives you clues as to what is going on.

I still like my earlier suggestion of 4 rectifier diodes.
Including a 1 amp fuse in the line would be valuable. Everything
goes in series, and the result can be in line enclosed in some
shrinkable tubing. The suggestion got somewhat lost in my rambling.

Can't see that being very viable, 3W in heatshrink isnt a great idea.

On Sun, 28 Jan 2007 14:36:53 +1100, "Rod Speed"
wrote:

kony wrote
Rod Speed wrote
Noozer wrote

If I have an external phone modem that uses a 1amp 9vDC tranformer,

That would be unusual.

True, but if it is really 9VDC, next is to determine how
much current it uses (if there's a multimeter handy) or the
more conservative (higher) equipment current rating, or the
overly conservative and probably less useful original PSU
(wall wart?) current rating.

I seriously doubt it comes remotely close to 1A,

Plenty of mine are of that order.

The wart might be, but I doubt the modem itself is.
If it used 1A @ 9V, that's of course 9W (practically all
heat generated internally), and nothing gets that hot to the
touch or needs significant heatsinking. On the other hand
consider a router using 9V @ 500mA, it may easily need a
'sink just to run stable.



none of the external modems I've had got that hot for
one thing, maybe closer to 250mA would be reasonable.

I doubt it.


Measure, it's the only way. It is not typical for a wart to
be spec'd for 100% duty, very doubtful the modem uses much
over 500mA if that.



Anyway, the easiest thing to to here is probably put an LM7809
linear regulator between the 12V PSU rail and the modem.

Makes a lot more sense to keep using an external wall wart instead.

If he's keeping the modem external, yes. If not, it depends
on how much work he's willing to put into it as a wart is
easier to source but more expensive to replace than adding
a 30 cent linear regulator. We don't know exactly what the
goal is and what concessions would be made soonest if that
goal entails significant, undesirable measures.




It'll produce closer to a watt of heat, more than you really
want on a TO220 sized part so put a heatsink on it. It need
not be a large 'sink, particularly if there's some ventilation.

What is the point over just using a wall wart ? A switch mode one if
you dont like the constant power used of a transformer based one.

Switch mode wart has higher failure rate
It may look ghetto to have a cord wedged, running into the
case from outside to get the power in.
There is already an (after dropped) acceptible power source
inside.

It's really a matter of time, esthetically use of the
internal supply is better, but takes longer to implement.
If he happened to have a molex plug, a suitable linear
regulator and an idea for securing it all, it would be a
pretty quick job, but if unaccustomed to this kind of work
and doing it from scratch it could certainly be as
reasonable to use the external supply still... only with
enough details is an informed choice made.