Relay contact ratings.

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The defrost timer in my fridge is failing, and given the cost of a
'genuine' replacement part, I was pondering the option of making
electronic timer driving an electromechanical relay.

It probably won't happen, but when I was looking at relay specs, I found
that their ratings are usually specified as a highish reactive power,
and a much lower real power.

http://australia.rs-online.com/web/p/electromechanical-relays/1279550 /

is typical, specifying in this case 300 W / 2500 VA.

I find this difficult to fathom. Imagine a purely inductive 2500VA load.
So no real power being switched, but it would arc like crazy. What am I
missing here?

Sylvia.


Re: Relay contact ratings.

"Stupider than Anyone Else"

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** The 300W figure refers to DC switching -  ie 10 amps at 30 volts.

   The 2500VA figure refers to resistive loads and 250VAC power.

   There is nothing about inductive load switching.

   See the data sheet.

   Fuckwit.



...  Phil



Re: Relay contact ratings.
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Care to point, in the data sheet, to where that distinction is drawn.

Sylvia.

Re: Relay contact ratings.
On Thu, 08 Dec 2011 20:36:08 +1100, Sylvia Else

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---
http://docs-asia.electrocomponents.com/webdocs/0b4c/0900766b80b4c467.pdf

2.SPECIFICATIONS

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Re: Relay contact ratings.

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When did they let you out of the asylum, Philly?
Your absence was not taken for granted.
--














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Re: Relay contact ratings.


That's what contactors are about. They're designed for motors, ie. non unity
PF load plus startup peak. Part of your problem will be determining what the
actual load is like, as it's certain to be unspecified in hte fridge
instruction manual. Assuming you can find out, or guess the Kw rating, a
suitably rated contactor is more than likely to work OK. But since you won't
get it from HN, it may not be cheap.



Re: Relay contact ratings.
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**Transformers and motors are notoriously nasty for relays to deal with.
TRIACs are a much better choice for such loads. Easy enough to design a
simple circuit, or, if you want a really easy way out, just buy a
suitably rated Solid State Relay (SSR). SSRs and VERY easy and safe to use.

--
Trevor Wilson
www.rageaudio.com.au

Re: Relay contact ratings.
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It's complicated by the fact that the defrost timer switch is a SPDT -
it switches between the compressor (via the thermostat, I presume) and
the defrost heater. In my relatively cursory search, I haven't found a
SPDT SSR. Could use two, I suppose, but a failure mode that leaves both
heater and compressor running seems more likely than with an
electromechanical relay.

The existing switch has contacts just as a relay would; they just don't
have an electromagnetic actuator.

The switch only cycles four times a day, so it's not so demanding in
terms of contact life.

Sylvia.

Re: Relay contact ratings.
On Thu, 08 Dec 2011 21:28:28 +1100, the renowned Sylvia Else

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Something like the Omron G7L is properly rated for motor loads. You
could use a second relay for the heater.




Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
snipped-for-privacy@interlog.com             Info for manufacturers: http://www.trexon.com
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Re: Relay contact ratings.
On Thu, 08 Dec 2011 08:30:00 -0500, the renowned Spehro Pefhany

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Come to think of it, do you really need two contacts? If the heater is
just a few watts maybe it could be connected in series with the
compressor and shorted out to turn the compressor on.



Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
snipped-for-privacy@interlog.com             Info for manufacturers: http://www.trexon.com
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Re: Relay contact ratings.
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It's not just a few watts. The rating plate says that in defrost mode
the fridge draws 450 watts. That may be the initial power when the
element is cold, but even when it's hot, it's clearly going to be
drawing significant power.

Sylvia.

Re: Relay contact ratings.
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A furnace relay should work:  8A @110V, 24V coil for around $10.  I
use a 5V relay to activate the 24V coil, but you can also use a
MOSFET.

http://www.zorotools.com/g/00009756/k-G0796406?utm_source3D%google_shopping =
&utm_medium3D%cpc&utm_campaign3D%Google_Shopping_Feed&kw3D%&gclid=
3D%CM3D-ZPd86wCFQJ8hwodjDwxUQ

Re: Relay contact ratings.
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use.
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we use 240 volts
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http://www.zorotools.com/g/00009756/k-G0796406?utm_source=google_shopping&utm_medium=cpc&utm_campaign=Google_Shopping_Feed&kw =&gclid=CM3D-ZPd86wCFQJ8hwodjDwxUQ


Re: Relay contact ratings.
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Yes, it will handle 8A at 240V.

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Even though SSR's and Triacs are quite rugged, something like a surge
from a lightning strike or similar event could cause one or both of
these triacs/SSR's to short out, and since fridges are turned on for
pretty much all their lives, this ensures that the fridge will be
connected to mains when something like this happens (unless you are
there to disconnect it at the first sign of lightning).

While same event might weld or vaporise relay contacts, its extremely
unlikely that both NO and NC could be connected to the common contact
simultaneously in any relay failure I can think of (short of the thing
being physically crushed).  This is one good reason to use a relay.

Even though it is a small risk, if you are going to keep this setup in
use for the next 10-20 years or so, since fridges rarely seem to die
from mechanical/electrical failure  it should be considered.

You could open up the timer, and look at the surface area of the
contacts used, and make an educated guess as to their ratings from
that info and also if there is any "quenching" capacitor across them,
use same on your relay contacts.

If you cannot find anything close to the Amperage you need, you could
get a suitable 2 or 4PDT relay and parallel the contacts to increase
the current it can handle.


Another reason for using a relay in this case is to make a "fail
safe"  system where only "one or the other" of the 2
devices will be allowed to come on at any one time - as well as
providing a delay (time it takes for the contacts
to move and physically change state from one contact to the other)
between one load turning off and the other turning on.

using other methods, without a reasonable delay changeover, or even an
overlap where both may be on together, If the heater is a reasonable
wattage, and it happens to be on as the compressor starts and is
pulling an enormous (relative) startup current, this combination might
be enough to cause nuisance fuse blowing and that is the last thing
you want in a fridge full of perishable and expensive food going
rotten,





If practical, it might even be worth your time to look around some
junked fridges, where the timer might still be ok (or even a better
build quality than yours) and somehow adapt it to fit it in your
fridge.  If you can get something at the rubbish dump shop, or similar
for a few bucks, and have the ability to do what is needed, then it
beats every other option on cost at least.




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Re: Relay contact ratings.
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An electromechanical relay is fine, no need to overcomplicate things.
I'd add a snubber to the compressor switch contacts, but its not
essential.


NT

Re: Relay contact ratings.

"kreed"

"TW"
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Even though SSR's and Triacs are quite rugged, something like a surge
from a lightning strike or similar event could cause one or both of
these triacs/SSR's to short out,

**  Nonsense.

If the max voltage rating of a triac is exceeded by a spike on the AC
upply  -  it simply turns on for the rest of that half cycle.



...  Phil







Re: Relay contact ratings.
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yeah, over voltage, or excessive dV/dt, will turn them on,
but couldn't subsequent over-current cause them to fail closed?

--
⚂⚃ 100% natural

---

Re: Relay contact ratings.
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**Unlikely. In fact, IME, TRIACs (appropriately rated ones) are vastly
more reliable than relays when driving highly inductive loads. In fact,
I've been using several for around 30 years, without issue. That is not
to say that TRIACs cannot fail. They can and do and usually shorted.
Which, in Sylvia's case, may prove inconvenient.

  and since fridges are turned on for
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**IME, the vast majority of lightning problems occur via TV antennas.
Power line issues are massively over-stated. Again, the only time I can
pin point a power line "surge" as the direct cause of a problem was a
very long time ago, when a 5kV bearer fell across the 240VAC main
overhead lines. The damage was considerable and affected several blocks.

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**I agree. However, in terms of longevity, TRIACs win hands down.

--
Trevor Wilson
www.rageaudio.com.au

Re: Relay contact ratings.

"Trevor Wilson"


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** Plus the telephone line -  modems and TAMs drop like flies when there is
a thunderstorm.


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**  It's an issue in many rural areas  -  the solution to which is fitting
varistors in the power box.


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**  Funny how microwave ovens all seem to have relays turning on the big
tranny.



...  Phil





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