Combined AC and DC switch

[NoSp]

I need a couple of power switches which can simultaneously switch on
AC 230V and DC 12V power. I guess that would make it a DPST switch.
I would assume that using a normal DPST switch (rated at say, AC 240V)
wouldn't be classified as safe enough for this, and that I would need
something specifically made for this purpose?

 

[Lostgallifreyan]

I need a couple of power switches which can simultaneously switch on
AC 230V and DC 12V power. I guess that would make it a DPST switch.
I would assume that using a normal DPST switch (rated at say, AC 240V)
wouldn't be classified as safe enough for this, and that I would need
something specifically made for this purpose?

Choose for current as well as voltage, low voltage stuff might have very
high currents that can destroy switch contacts. Consider things that can
make strong surges too, like capacitors or inductors on the equipment side
of the DC switch. You might have to choose a higher current switch than
you'd normally expect.

 

[Lostgallifreyan]

Choose for current as well as voltage, low voltage stuff might have
very high currents that can destroy switch contacts. Consider things
that can make strong surges too, like capacitors or inductors on the
equipment side of the DC switch. You might have to choose a higher
current switch than you'd normally expect.

Also, if isolation of the high and low voltage systems is critical (it
usually is), maybe consider a high-volt low current switch for the mains,
driving a mains voltage relay coil with heavier contacts for low volt DC.

 

[NoSp]

Choose for current as well as voltage, low voltage stuff might have very
high currents that can destroy switch contacts.

That sounds like overkill for my purpose.
I'm building an external enclosure for two hard drives which are
totally independent of each other (they just happen to share the same
enclosure and power supply) and I'd like to be able to switch them on/
off independantly.
So instead of having a mains switch (for the power supply) as well as
switches for each drive's power (I suppose cutting off one of the
power lines will do) I'm thinking that two switches will do where one
half of each switch is wired up to the AC whilst the other half is for
supplying DC power to the drive itself.
This way I can turn on the power supply and the drive I want all at
once.

 

[Michael A. Terrell]

That sounds like overkill for my purpose.
I'm building an external enclosure for two hard drives which are
totally independent of each other (they just happen to share the same
enclosure and power supply) and I'd like to be able to switch them on/
off independantly.
So instead of having a mains switch (for the power supply) as well as
switches for each drive's power (I suppose cutting off one of the
power lines will do) I'm thinking that two switches will do where one
half of each switch is wired up to the AC whilst the other half is for
supplying DC power to the drive itself.
This way I can turn on the power supply and the drive I want all at
once.

That isn't a good idea. The switches aren't designed for that
application, and flashover between circuits can destroy the drives.
Why not use a power switch, and another switch to select the drive? On
second thought, selecting a drive by powering the other down can damage
the drives, the controller, or both.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[NoSp]

NoSp wrote:

That isn't a good idea. The switches aren't designed for that
application, and flashover between circuits can destroy the drives.

I was afraid of something like that :-(

Why not use a power switch, and another switch to select the drive? On
second thought, selecting a drive by powering the other down can damage
the drives, the controller, or both.

The idea is to prevent unnecessary tear, wear and noise from the drive
I'm not using, which is why I thought I could solve this simply by
routing each drive's power line from the Firewire/SATA bridge board
(say +12V or whatever the drives use) to its own switch would solve
this.

 

[Lostgallifreyan]

That sounds like overkill for my purpose.
I'm building an external enclosure for two hard drives

Not overkill at all. That's exactly the kind of DC load I had in mind, hard
drives have LARGE startup current surges. You'll probably want a decoupling
or smoothing capacitor on the line too, which will increase that surge.

The simplest way is a mains 10 amp switch, the kind you might find in many
junk boxes. Get one that has a high dielectric strength of at least a few
KV for its insulators. Don't worry about using a big switch, it's better
that you do, it's by far the easiest way you'll get decent separation
between your 12V and mains lines. Get a double pole changeover type, wire
mains in to common terminal of one pole, mains out to both the other
terminals. 12V in to common terminal of the other pole, one drive to each
of the other terminals. And slather the connectors after soldering, with
plenty of VERY insulating goop, ideally an epoxy to make sure that only
brute force with a hard object can make an easy electrical path reappear
between your low and high volt lines.

 

[Lostgallifreyan]

Get a double pole changeover type

Forgot to say, it needs to be the centre-off type, or you won't be able to
switch off the mains, or switch off both drives with the arrangement I
described.

 

[Hal Murray]

The idea is to prevent unnecessary tear, wear and noise from the drive
I'm not using, which is why I thought I could solve this simply by
routing each drive's power line from the Firewire/SATA bridge board
(say +12V or whatever the drives use) to its own switch would solve
this.

Why not just tell the drives to spin down?

That way you can locate them where you can't get at the switch.

 

[Gary Tait]

Forgot to say, it needs to be the centre-off type, or you won't be
able to switch off the mains, or switch off both drives with the
arrangement I described.

Why not two switches, one for mains, one to switch power to either drive?

 

[Lostgallifreyan]

Why not two switches, one for mains, one to switch power to either drive?

Because the OP asked for one.

 

[NoSp]

Because the OP asked for one.

I actually asked about two switches. but not in the above
configuration, but rather in a way that I can individually choose
between powering up drive A, drive B or both.
I've also been wondering if there was a way to NOT spinup the drives
when power is applied, but instead have the computer (a Mac G4 running
MacOS 10.4) send a "spinup" signal at will. Alas I've asked around but
nobody seems to know about any such solution.

How do those professional rack mounted hard drive enclosures work with
several drives? Do they just have a single power switch and spin up
all drives?

 

[Lostgallifreyan]

I actually asked about two switches. but not in the above
configuration, but rather in a way that I can individually choose
between powering up drive A, drive B or both.
I've also been wondering if there was a way to NOT spinup the drives
when power is applied, but instead have the computer (a Mac G4 running
MacOS 10.4) send a "spinup" signal at will. Alas I've asked around but
nobody seems to know about any such solution.

How do those professional rack mounted hard drive enclosures work with
several drives? Do they just have a single power switch and spin up
all drives?

"I need a couple of power switches which can simultaneously switch on
AC 230V and DC 12V power. I guess that would make it a DPST switch."

Sounds like one switch to me... And you were right first time, you just
overlooked the need for centre-off, and for high current contacts for the
DC line, you DO need that to switch on a hard drive motor, unless you want
to risk data loss and annoying early replacement. Don't forget the high
dielectric strength and the thorough insulation of terminals...

Not sure about the spin switching, but that's a different thing entirely,
that will be down to the signals your drives accept (firmware controlled),
and the system that sends the signals. If you can find out what a PC sends
as a wakeup signal to a suspended drive, you can emulate that signal on
whatever system you want, probably.

 

[NoSp]

NoSp <[email protected]> wrote innews:[email protected]:

"I need a couple of power switches which can simultaneously switch on
AC 230V and DC 12V power. I guess that would make it a DPST switch."

Sounds like one switch to me... And you were right first time, you just
overlooked the need for centre-off, and for high current contacts for the
DC line, you DO need that to switch on a hard drive motor, unless you want
to risk data loss and annoying early replacement. Don't forget the high
dielectric strength and the thorough insulation of terminals...

I didn't know about the high power surge of hard drive motors, so I
learnt something new
As for the switch issue; a centre off switch won't do as I sometimes
might want to use both hard drives.
I could of course solve the whole issue with three switches:
a) AC switch for power supply
b) DC switch for powering drive A
c) DC switch for powering drive B

.... but not a very elegant and "pro" solution, which is what I'm
aiming for.
That's why I thought up this idea with just two switches (one for each
drive, but simultaneously switching power to the power supply). To
avoid any further misunderstanding, here's an illustration I've made:
http://img206.imageshack.us/img206/4231/dualdrivecircuitit2.png

I'm open for other solutions of course.

Not sure about the spin switching, but that's a different thing entirely,
that will be down to the signals your drives accept (firmware controlled),
and the system that sends the signals. If you can find out what a PC sends
as a wakeup signal to a suspended drive, you can emulate that signal on
whatever system you want, probably.

I've asked about this in Mac related forums in the past, but have
received no replies. I know from experience that I can easily *spin
down* a hard drive connected to my Mac (by dragging the drive icon on
the desktop to the trash/eject icon), so I'm sure there's a command
for "waking up" a drive as well. However, if software that takes
advantage of this is available or not is a totally different question,
and I'm no programmer :-(

 

[Lostgallifreyan]

As for the switch issue; a centre off switch won't do as I sometimes
might want to use both hard drives.

True, I forgot that bit. You could use two switches, DPST, but not centre
off, but all that stuff I said about strong dielectrics and insulators on
all terminals will apply, and route wires carefully so heat can't melt
something to allow mains and DC to meet. The scheme you drew is the easiest
to expand to more drives, too.

One drawback with it is that the power will switch on the moment you switch
ANY switch, so unless you're fast the boot sequence will be done before all
drives you might want enabled are powered up for that session. That single
fact might make you want to revise the whole deal.

The drive on standby thing might be easier than it looks, but it's hard to
know, software can easily be arbitrarily complex, way beyond need. In
Windows, if a drive was active but shuts itself down with its own firmware
controls, just trying to access it in Windows (which has no idea it has
shut down) will wake it up with no problems, just an annoying moment of
waiting. As your Mac OS can ask a drive to shut down, it might have just as
easy a way to ask it to wake up. It seems very odd, if that is not so.