Latching relay question

Relay contacts will bounce, although not so much that they go all the way back to the opposite state. Google for "relay contact bounce" to turn up lots of discussions on this. Even firm, precise on/off switches can bounce.

You should be fine. You could add some capacitance to the power lines to smooth over the bounces but be aware that slowly rising power can also cause some circuits to mis-behave so don't go overboard on monster caps.

I haven't decided on a power supply yet, but I suppose a small switched type (as used with computers) is the best way to go. It'll be mounted in the same 1U 19" metal cabinet as the two SATA drives. Such a power supply must surely have the needed capacitors already to be able to deliver clean and stable power? Or should I (in addition to this) add some capacitors across the

The power supply will be on the upstream side of the relays and won't help to smooth the effects of any contact bounce on the load side of the relays. But, there's probably enough capacitance on the drive's power rails already.

The duration of the bouncing is typically around 10 msec and since it is from "on" to "open," not "on" to "ground," the drives should be perfectly happy. The digital portion will (should?) have a power-on reset circuit that holds itself in reset long enough to wait through the startup transients.

Yes, that makes sense. Should I connect something like a caramic capacitor across each of the relay's switch connections? Which capacitance values are we talking?

I would hope so ;-)

Just swagging this: The 12 V shouldn't care; the drive motor isn't spun until after the drive electronics start up. On the 5 V rail, it looks like a typical modern drive initially pulls about 300 mA. Hand-waving a power interrupt time during contact bounce at around 1 ms, to hold the droop on 5 V to < 10% would require about 600 uF.

So a 600uF ceramic capacitor (or similar) across the 5V switched lines of the relay? Sounds good.

Something else comes to mind. Since I'll have two separate on/off switches there is a chance that I may power up the enclosure without putting much load on the power supply. I've heard that this is a bad thing for switched mode power supplies. The dual-SATA to Firewire/USB bridge board will of course be directly connected to the PSU, so when I flip the enclosure's power switch it'll draw some power (I'm not sure how much, but I do know that it only uses the +5V line, which leaves the +12V line unused). I do however plan to put a fan inside the enclosure, but through some sort of temperature controlling circuitry so as to keep the noise level down, so I don't know how this will affect the PSU. Is this something I should be concerned about?