SPCR

I'm trying to hook up my DVD combo drive and a spare Maxtor HDD through firewire so I can turn them off to lower power consumption and noise when not in use. I bought a small Oxford 911 firewire bridge (here), and managed to get it to work with both devices (separately - don't need them both at once).

Now I'm trying to wire up a DPDT center-off switch with three positions - HDD/Off/DVD. This is pretty easy to do for the drives - I hard-wired the grounds and just switched the 5V & 12V. The bridge takes 5V, so my first attempt was to just wire this up to the 5V contact on both sides of the switch. Well, that didn't work right, for obvious reasons. Since the bridge is connected to both sides of the switch, both drives get 5V whenever the switch is not in the center-off position.

I suppose I could hard-wire the bridge power so it isn't switched, but that wastes power (~1W), something I've come to fanatically avoid. Also, the bridge requires rebooting when I change devices, so I would need an additional momentary (normally closed) switch to cut the power temporarily and reboot it. I'd rather keep it to one switch, if possible.

Which leads me to my question: Is there an electrical component I could wire up along the two +5V power wires to the bridge that only allows the electricity to go one way, without dropping the voltage out of spec (I'm assuming ~10%)?

I'm not an electronics genius, but isn't a zener diode basically a one-way power gate?

I'm not an electrical engineer, but I work with some. I think the thing you're looking for is a diode, however I personally know nothing about them. Hope that helps (a little).

Peter L.

wooglin wrote:I'm not an electronics genius, but isn't a zener diode basically a one-way power gate?

A zener diode will let current pass through it one was with a voltage drop of something like 0.7V, and will let current pass through it the other way if the voltage is above the zener voltage stated on the diode. What you want is a low voltage drop diode, but I'm not sure what they're called.

Okay, so I found this page with some background on diodes: AmericanMicroSemi Diode Tutorial. This page leads me to believe that Qwertyiopisme is right - I'm looking for a diode with a low voltage drop, or Vf, at 500mA (the highest current I would expect the bridge to draw). The reverse breakdown voltage only has to be over 6V, since my PSU will never output more than that.

I managed to find a Schottky diode that seems to fit the bill...Digi-Key part #10BQ015-ND. Can one of you electrical geniuses verify that this would work?

Sounds to me like you need a relay.

If I were to go with a relay, I assume I would need a dual-pole verson. Otherwise, the wires from each side of the switch would still be connected to the same contacts and both drives would be powered up.

I'm inclined away from relays because they're big, more expensive and often mechanically-based (and I'm assuming more prone to failure).

A Schottky diode is essentially a diode with a low voltage drop. Where most diodes have a voltage drop of about 0.7V, Schottkys are closer to 0.1~0.2v, putting you much closer to spec.

The part number you gave is for a Surface Mount diode. I generally don't recommend using Surface Mount devices to people who aren't that skilled in surface mount soldering. But if you're comfortable with it, then by all means, go for it.

I use a 1N5818 Schottky diode for my applications, but I work around 24V and use them with switching voltage regulators. The IN5817 will probably be enough for you.

Going up further, diodes are semiconductor devices designed to allow current to pass in one direction only, given a certain voltage. A diode is the answer to the question you asked, although I'm having a difficult time picturing how you intend to incorporate a diode into your plans (though this is due to the fact that I'm having a difficult time picturing what your switch looks like and what problems you've having).

sthayashi: Thanks for the informative post. Let me see if I can clarify this circuit for you...

The devices I want to switch between are a 3.5" Maxtor HDD (not the boot drive) and a laptop DVD combo drive. The HDD needs both 5V & 12V, while the DVD drive and firewire bridge need only 5V power.

The switch is a DPDT with a center-off position. I have 5V going into one pole and 12V going into the other (the grounds aren't switched). Here's which voltages I want going to which devices at each switch position: Originally, I wired the +5V input on the bridge directly to both Up and Down positions on the switch. Unfortunately, this meant that the DVD got 5V even when the switch was in the Up position (electricity went from PSU to switch to Up contact to Bridge to Down contact to DVD). If I add a diode to each +5V wire to the bridge, this can't happen.

Besides powering up the DVD drive and wasting power, it doesn't work without the diodes because the bridge can only deal with one device at a time. They can both be connected to the same IDE cable, but neither works if both are given power (hence, why I have them wired one-or-the-other). All you have to do to switch between them is turn off the old device, turn on the new device, and temporarily cut power to the switch (it still works if the last two are done at the same time).

Thanks for the heads-up on the Surface Mount stuff - DO-41 packaging should be fine, right?

Would most electronics parts stores carry 1N5817's?

It took me a while, but I understand how things are working. The diodes ought to work, at least in theory.

I don't know if most electronics stores will have the Schottky 1N5817 or not, since the only electronics store I have around me is a Radio Shack (and I can't recall if THEY carry that diode). But most will carry a Schottky diode. A rectifying diode might work as well, but you'd have to look at the specifications for that.

Failing that, I'd recommend Futurlec for a cheap shipping and low volume orders (though I have to admit that the cheapest shipping takes longer than normal).

Well, I got a diode (1N5817), and it works pretty much as planned. I found that I had to put the firewire drives on a different PSU cable from the boot drive to prevent the boot drive from spinning down with the sudden load. And the startup timing is also somewhat of an issue with the laptop DVD - I have to flip the switch to turn on the drive and then just nudge the switch to reset the bridge again for it to connect properly. Overall, though, I'm psyched - no noise or heat from those drives when they're not in use! It also seems to have affected my over/under-clocking stability, possibly because of the stock 220W power supply (?).

For the record, this is my system:

Case: Silverstone Lascala SST-LC02
Motherboard: Biostar M7NCG 400
Processor: AMD Athlon XP 2200+ 1.35v Barton
Memory: 2x Kingston 256MB DDR PC2700 CAS3
System HDD: Fujitsu MHT2060AT 2.5" 60GB
Storage HDD: Maxtor Diamondmax 9 160GB
DVD: Panasonic CW-8123-B laptop drive
Firewire bridge: Two-device Firewire 400 (1394a)