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'[EE:] 5 Volt relays and 12 Volt relays'
2005\06\21@022439 by laura capaldi

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Is there a good reason why one should pick 12 Volt relays? We have a system where we have 5 Volt power and 12 Volt relays. Is it worth designing in a DC-DC step-up in order to use the 12 Volt relay or is it preferable to use 5 Volt relays? I'm interested in hearing experiences with this. Thanks.


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2005\06\21@024824 by Chen Xiao Fan

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If the two relays are of the same family, they would have similar
nominal coil power. Say, if the nominal coil power is 200mW,
the 5V version will consume 40mA and the 12V version will
consume 16.7mA. Therefore it is not worth the effort to
add the DC-DC converter.

If they are of different family and the nominal coil power
of the 12V version is much lower than the 5V version, then
it is another story.

-----Original Message-----
From: laura capaldi [spam_OUTlauracapaldi01TakeThisOuTspamyahoo.com]
Sent: Tuesday, June 21, 2005 2:25 PM
To: .....piclistKILLspamspam@spam@mit.edu
Subject: [EE:] 5 Volt relays and 12 Volt relays


Is there a good reason why one should pick 12 Volt relays? We have
a system where we have 5 Volt power and 12 Volt relays. Is it worth
designing in a DC-DC step-up in order to use the 12 Volt relay or
is it preferable to use 5 Volt relays? I'm interested in hearing
experiences with this. Thanks.

2005\06\21@042750 by Alan B. Pearce

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>Is there a good reason why one should pick 12 Volt relays? We have
>a system where we have 5 Volt power and 12 Volt relays. Is it worth
>designing in a DC-DC step-up in order to use the 12 Volt relay or
>is it preferable to use 5 Volt relays? I'm interested in hearing
>experiences with this. Thanks.

How is the system being powered? Is the 12V available all the time, or are
there things you need to switch with a relay when the 12V is not available
(like turning on the 12V)?

My gut feeling would be that it is generally nicer to use the higher voltage
to drive relays, especially as there will often be some form of buffer (such
as a transistor or ULN2003/2803 chip) to handle the relay current anyway. So
it makes sense to remove the load from whatever regulator is supplying the
5V to the logic, as this may save in heat dissipation, reducing other
requirements of the system.

However there may be other considerations in the picture - a specific area
of the device may not want the 12V power as it may be considered "noisy"
because it is unregulated and may bring in all sorts of other conducted
electrical noise, so it may be more prudent to use the 5V supply for a
relay, and if it needs to be kept clean of electrical noise, then pay
particular attention to the inductive spikes which occur when the relay is
operated.

In short the reasons for using different voltages may have specific reasons,
or may not - it may be just the designers whim.

2005\06\21@042937 by vasile surducan

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On 6/21/05, laura capaldi <lauracapaldi01spamKILLspamyahoo.com> wrote:
> Is there a good reason why one should pick 12 Volt relays? We have a system where we have 5 Volt power and 12 Volt relays. Is it worth designing in a DC-DC step-up in order to use the 12 Volt relay or is it preferable to use 5 Volt relays? I'm interested in hearing experiences with this. Thanks.

The reason why the relays have usualy bigger voltage than the logic is
only the noise which may be induced into the logic section if many
relays are on the board.
Also as high is the current through the relays switch as much current
(or high feromagnetic technology on the core) on the driving coil. If
you plan to switch 40A, then a relay with a coil on 5V will be quite
hard to find.
DC to DC step up for driving some relays is anyway funny.

best,
Vasile


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2005\06\21@082711 by Spehro Pefhany

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At 11:24 PM 6/20/2005 -0700, you wrote:
>Is there a good reason why one should pick 12 Volt relays? We have a
>system where we have 5 Volt power and 12 Volt relays.

Yes, that's quite common. It's better not to try to run the relays off the
same supply as the logic. As someone has already pointed out, the coil
power for DC relays is about the same regardless of coil voltage, and the
price tends to be the same (up to 12V, sometimes small higher voltage relays
are more expensive than lower voltage ones, especially above 24V, because
they need a lot of very fine wire). Thus, for a given type of relay, the
coil current will be higher by almost 2.5:1. The voltage drop of the driver
will be more important at 5V than at 12V-- something like a Darlington
or Sziklai pair may not be acceptable, and you may not be able to use a
really cheap TO-92 MOSFET as a driver because of Rds(on) (and note that
Rds(on) is high at high temperature--- just where the relay needs more
voltage to pull in smartly because of the +4000ppm/K positive tempco of
copper).

>Is it worth designing in a DC-DC step-up in order to use the 12 Volt relay
>or is it preferable to use 5 Volt relays?

No, probably not, but you may have to take precautions to keep noise from
the relay supply from getting into the logic supply. In some extreme cases
creating an isolated 12V supply from the 5V supply might make sense, but
it would be a fairly rare situation.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam.....interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
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2005\06\21@084332 by Jinx

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>Is there a good reason why one should pick 12 Volt relays? We have a
>system where we have 5 Volt power and 12 Volt relays

Is that what you're wanting to use or is that what's in use ?

> Thus, for a given type of relay, the coil current will be higher by almost
> 2.5:1

So wouldn't the pull-in pulse be more likely to cause noise with a 5V
supply than a 12V (assuming they're rated at the same amperage) ?

2005\06\21@100956 by olin piclist

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> So wouldn't the pull-in pulse be more likely to cause noise with a 5V
> supply than a 12V (assuming they're rated at the same amperage) ?

Actually pull in isn't so much the problem.  Relay coils are inductors too,
so the current takes some time to ramp up.  Nasty things happen when you try
to turn off this current quickly, just like with any inductor.  This is why
a reverse diode accross the relay coil is so important.

Generally you'd want to use 12V relays if you have a system with rough 12V
available and a 5V logic supply.  If the power only comes at you as 5V, then
I'd split the logic supply off with a separate filter.  It doesn't make
sense to boost this up to 12V just to run relays.  In the end it will take
more power because the relays will take about the same power and the
switcher will eat some too.


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2005\06\21@200550 by Jinx

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> Nasty things happen when you try to turn off this current quickly,
> just like with any inductor

That's true. I've had one occassion when the turn-off pulse was
picked up by a ribbon cable passing across the relay, causing
general mayhem in the rest of the circuit, despite usual noise-
reduction


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