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Latching relay question

Discussion in 'Electronic Components' started by NoSp, Jun 23, 2008.

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  1. NoSp

    NoSp Guest

    Do latched relays in general "wiggle" back and forth (if even for a
    fraction of a second), or do they work as a firm and precise on/off switch?

    I'm planning to cut/supply power to an external SATA hard drive by
    having the +12V and +5V lines through a relay (dual pole), controlled by
    a momentary push-button. But if the relay "wiggles" from one position to
    another when activated I'm worried that this will in the long run harm
    the drive (from what I've been told it's harmful to most electronic
    devices if you switch it on-off-on-off... quickly), so like a mechanical
    switch I want it to be in either a definite on or off position when I
    activate it.
  2. Rich Webb

    Rich Webb Guest

    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.
  3. NoSp

    NoSp Guest

    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
    +12V/GND and +5V/GND lines close to each relay as well?
  4. Rich Webb

    Rich Webb Guest

    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.
  5. NoSp

    NoSp Guest

    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 ;-)
  6. Rich Webb

    Rich Webb Guest

    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.
  7. neon


    Oct 21, 2006
    adding caps on the power lines will do nothing about a mechanical bounce. why stupid old fashion relays? aslow turn on transistor is much better.
  8. NoSp

    NoSp Guest

    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?
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