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Latching relay question

N

NoSp

Jan 1, 1970
0
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.
 
R

Rich Webb

Jan 1, 1970
0
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?

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.
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.

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.
 
N

NoSp

Jan 1, 1970
0
Rich said:
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
+12V/GND and +5V/GND lines close to each relay as well?
 
R

Rich Webb

Jan 1, 1970
0
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?

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.
 
N

NoSp

Jan 1, 1970
0
Rich said:
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.

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?

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.

I would hope so ;-)
 
R

Rich Webb

Jan 1, 1970
0
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?

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.
 

neon

Oct 21, 2006
1,325
Joined
Oct 21, 2006
Messages
1,325
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.
 
N

NoSp

Jan 1, 1970
0
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?
 
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