Relay contact ratings.

[Phil Allison]

"John Fields" .

The contacts are rated at 10A for either 250VAC or 30VDC, so 250VAC *
10A = 2500VA, and 30VDC * 30A = 300W.

The higher AC voltage is permitted because the arc on opening will
quench in, at most, 1/2 cycle when the voltage across the contacts
crosses zero, while with DC through the contacts the arc will persist
until the gap between the contact is large enough to quench the arc.

** Problem being, there is simply not enough gap available to break the arc
that forms if the DC ratings are exceeded.

For the type of relay in question, a permanent arc will form with DC if the
current flow is say 10 amps and the voltage across the contacts is over 40
volts. This means there is over 400 watts of heat, mostly being dissipated
into the contacts and destroying them in a few seconds.

In the world of high powered audio amplifiers it is still common to see such
relays used to protect speakers from DC high fault currents and turn on/off
transients. In the case of former, the relay is not capable of doing the
job.



.... Phil

 

[Trevor Wilson]

On 12/9/2011 12:35 AM, kreed wrote:

On Dec 8, 8:28 pm, Sylvia Else<syl...@not.here.invalid> wrote:
On 8/12/2011 9:04 PM, Trevor Wilson wrote:



On 12/8/2011 5:50 PM, Sylvia Else wrote:
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.

**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

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.



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,

**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

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).

**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.

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.

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

--
Trevor Wilson
www.rageaudio.com.au

 

[Trevor Wilson]

On 12/9/2011 7:59 AM, Phil Allison wrote:

"John Fields" .

The contacts are rated at 10A for either 250VAC or 30VDC, so 250VAC *
10A = 2500VA, and 30VDC * 30A = 300W.

The higher AC voltage is permitted because the arc on opening will
quench in, at most, 1/2 cycle when the voltage across the contacts
crosses zero, while with DC through the contacts the arc will persist
until the gap between the contact is large enough to quench the arc.


** Problem being, there is simply not enough gap available to break the arc
that forms if the DC ratings are exceeded.

For the type of relay in question, a permanent arc will form with DC if the
current flow is say 10 amps and the voltage across the contacts is over 40
volts. This means there is over 400 watts of heat, mostly being dissipated
into the contacts and destroying them in a few seconds.

In the world of high powered audio amplifiers it is still common to see such
relays used to protect speakers from DC high fault currents and turn on/off
transients. In the case of former, the relay is not capable of doing the
job.

**Tapco CP500 anyone? Crap design, with useless relay protection.

--
Trevor Wilson
www.rageaudio.com.au

 

[Phil Allison]

"Trevor Wilson"

**IME, the vast majority of lightning problems occur via TV antennas.

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

Power line issues are massively over-stated.

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

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

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



.... Phil

 

[Spehro Pefhany]

On Thu, 08 Dec 2011 12:48:13 -0500, ehsjr <ehsjr@nospamverizon.net>
wrote:

Sylvia Else wrote:
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.


The price of the relay at the url you posted is ridiculous.
Surely you can get something much cheaper and equally robust.
There are relays made for appliances like air conditioners
available from Digikey for < $5.00 so it's likely you can
get something where you are for much less than the ~ $82.00 at
the RS Australia site. You might be able to use an automotive
relay. Generally their contacts are rated at 30 or 40 amps at
12 VDC, and they'll handle 240 AC nicely.

Ed

I would not use a 12V automotive relay to switch mains voltage,
particularly inductive loads.. coil-to-contact dielectric strength of
those POS relays is << 1kV. I like to see 4kV or so. Are the housings
required to be flame retardant?

 

[Trevor Wilson]

On 12/9/2011 8:28 AM, Phil Allison wrote:

"Trevor Wilson"


**IME, the vast majority of lightning problems occur via TV antennas.


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

**Quite true.

Power line issues are massively over-stated.


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

**I'm a city boy. I don't see many country products.

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


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

**Might be something to do with the different failure modes of the two
devices.


--
Trevor Wilson
www.rageaudio.com.au

 

[Phil Allison]

"Trevor Wilson"

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

**Might be something to do with the different failure modes of the two
devices.

** Thinking about it - it is more to do with failure mode in the oven
itself.

If a diode or high voltage cap goes west, or there is an internal short the
HRC fuse in the AC will blow.

The current surge during such events would destroy any low cost triac - but
not a relay.

Also, a relay provides high isolation from the control PCB to the AC supply.



.... Phil

 

[Trevor Wilson]

On 12/9/2011 9:20 AM, Phil Allison wrote:

"Trevor Wilson"


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

**Might be something to do with the different failure modes of the two
devices.


** Thinking about it - it is more to do with failure mode in the oven
itself.

If a diode or high voltage cap goes west, or there is an internal short the
HRC fuse in the AC will blow.

The current surge during such events would destroy any low cost triac - but
not a relay.

**Good points.

Also, a relay provides high isolation from the control PCB to the AC supply.

**All academic now anyway. I've noticed that MOs are starting to employ
SMPS nowadays. I scored a broken, high end model from a neighbour. Lots
of IGBTs, but no relays to be seen. Naturally, the fault lies with the
IGBTs. :-(

--
Trevor Wilson
www.rageaudio.com.au

 

[Trevor Wilson]

On 12/9/2011 8:43 AM, linnix wrote:

On Dec 8, 1:30 pm, Spehro Pefhany<speffS...@interlogDOTyou.knowwhat
wrote:
On Thu, 08 Dec 2011 12:48:13 -0500, ehsjr<eh...@nospamverizon.net
wrote:









Sylvia Else wrote:
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.

The price of the relay at the url you posted is ridiculous.
Surely you can get something much cheaper and equally robust.
There are relays made for appliances like air conditioners
available from Digikey for< $5.00 so it's likely you can
get something where you are for much less than the ~ $82.00 at
the RS Australia site. You might be able to use an automotive
relay. Generally their contacts are rated at 30 or 40 amps at
12 VDC, and they'll handle 240 AC nicely.

Ed

I would not use a 12V automotive relay to switch mains voltage,
particularly inductive loads.. coil-to-contact dielectric strength of
those POS relays is<< 1kV. I like to see 4kV or so. Are the housings
required to be flame retardant?

Is the house (hosting the fridge) required to be fire retarded?

**Irrelvant. Whilst I don't know the specific legislation pertaining to
relays, here in Australia, ALL mains connected components (from
transformers to power leads) MUST have the requisite approval numbers
and, usually, fire retardant protection, or specific materials used in
their construction. Other jurisdictions may not have such legislation,
for whatever reason. Sylvia resides in Australia. As such, connecting
non-approved components may carry an insurance liability risk.

--
Trevor Wilson
www.rageaudio.com.au

 

[Spehro Pefhany]

On Thu, 08 Dec 2011 08:30:00 -0500, the renowned Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> wrote:

On Thu, 08 Dec 2011 21:28:28 +1100, the renowned Sylvia Else
sylvia@not.here.invalid> wrote:

On 8/12/2011 9:04 PM, Trevor Wilson wrote:
On 12/8/2011 5:50 PM, Sylvia Else wrote:
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.


**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

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.

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

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"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

 

[linnix]

On Dec 8, 1:30 pm, Spehro Pefhany <speffS...@interlogDOTyou.knowwhat>
wrote:

On Thu, 08 Dec 2011 12:48:13 -0500, ehsjr <eh...@nospamverizon.net
wrote:









Sylvia Else wrote:
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.

The price of the relay at the url you posted is ridiculous.
Surely you can get something much cheaper and equally robust.
There are relays made for appliances like air conditioners
available from Digikey for < $5.00 so it's likely you can
get something where you are for much less than the ~ $82.00 at
the RS Australia site.  You might be able to use an automotive
relay. Generally their contacts are rated at 30 or 40 amps at
12 VDC, and they'll handle 240 AC nicely.

Ed

I would not use a 12V automotive relay to switch mains voltage,
particularly inductive loads.. coil-to-contact dielectric strength of
those POS relays is << 1kV. I like to see 4kV or so. Are the housings
required to be flame retardant?

Is the house (hosting the fridge) required to be fire retarded?

 

[Trevor Wilson]

On 12/9/2011 10:08 AM, linnix wrote:

On Dec 8, 2:35 pm, Trevor Wilson<tre...@SPAMBLOCKrageaudio.com.au
wrote:
On 12/9/2011 8:43 AM, linnix wrote:









On Dec 8, 1:30 pm, Spehro Pefhany<speffS...@interlogDOTyou.knowwhat
wrote:
On Thu, 08 Dec 2011 12:48:13 -0500, ehsjr<eh...@nospamverizon.net
wrote:

Sylvia Else wrote:
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.

The price of the relay at the url you posted is ridiculous.
Surely you can get something much cheaper and equally robust.
There are relays made for appliances like air conditioners
available from Digikey for< $5.00 so it's likely you can
get something where you are for much less than the ~ $82.00 at
the RS Australia site. You might be able to use an automotive
relay. Generally their contacts are rated at 30 or 40 amps at
12 VDC, and they'll handle 240 AC nicely.

Ed

I would not use a 12V automotive relay to switch mains voltage,
particularly inductive loads.. coil-to-contact dielectric strength of
those POS relays is<< 1kV. I like to see 4kV or so. Are the housings
required to be flame retardant?

Is the house (hosting the fridge) required to be fire retarded?

**Irrelvant. Whilst I don't know the specific legislation pertaining to
relays, here in Australia, ALL mains connected components (from
transformers to power leads) MUST have the requisite approval numbers
and, usually, fire retardant protection, or specific materials used in
their construction. Other jurisdictions may not have such legislation,
for whatever reason. Sylvia resides in Australia. As such, connecting
non-approved components may carry an insurance liability risk.


You are missing the point.

**No, I am not, but continue....


Once they (insurance co) determined that

the fire is caused by mis-speced component, they will deny the whole
claim. They will tell you to sue the manufacturer, or whoever made
the modification. It is utterly irrespondable to use auto components
for house main.

**I believe that is pretty much what I said.

--
Trevor Wilson
www.rageaudio.com.au

 

[Jim Thompson]

On Thu, 8 Dec 2011 15:08:05 -0800 (PST), linnix
<me@linnix.info-for.us> wrote:

On Dec 8, 2:35 pm, Trevor Wilson <tre...@SPAMBLOCKrageaudio.com.au
wrote:
On 12/9/2011 8:43 AM, linnix wrote:









On Dec 8, 1:30 pm, Spehro Pefhany<speffS...@interlogDOTyou.knowwhat
wrote:
On Thu, 08 Dec 2011 12:48:13 -0500, ehsjr<eh...@nospamverizon.net
wrote:

Sylvia Else wrote:
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.

The price of the relay at the url you posted is ridiculous.
Surely you can get something much cheaper and equally robust.
There are relays made for appliances like air conditioners
available from Digikey for<  $5.00 so it's likely you can
get something where you are for much less than the ~ $82.00 at
the RS Australia site.  You might be able to use an automotive
relay. Generally their contacts are rated at 30 or 40 amps at
12 VDC, and they'll handle 240 AC nicely.

Ed

I would not use a 12V automotive relay to switch mains voltage,
particularly inductive loads.. coil-to-contact dielectric strength of
those POS relays is<<  1kV. I like to see 4kV or so. Are the housings
required to be flame retardant?

Is the house (hosting the fridge) required to be fire retarded?

**Irrelvant. Whilst I don't know the specific legislation pertaining to
relays, here in Australia, ALL mains connected components (from
transformers to power leads) MUST have the requisite approval numbers
and, usually, fire retardant protection, or specific materials used in
their construction. Other jurisdictions may not have such legislation,
for whatever reason. Sylvia resides in Australia. As such, connecting
non-approved components may carry an insurance liability risk.


You are missing the point. Once they (insurance co) determined that
the fire is caused by mis-speced component, they will deny the whole
claim. They will tell you to sue the manufacturer, or whoever made
the modification. It is utterly irrespondable to use auto components
for house main.

In such socialist societies as Australia (and coming soon to the US),
you'd better use the exact replacement and have it done by a certified
technician... otherwise no insurance coverage at all.

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[Trevor Wilson]

On 12/9/2011 10:22 AM, Jim Thompson wrote:

On Thu, 8 Dec 2011 15:08:05 -0800 (PST), linnix
me@linnix.info-for.us> wrote:

On Dec 8, 2:35 pm, Trevor Wilson<tre...@SPAMBLOCKrageaudio.com.au
wrote:
On 12/9/2011 8:43 AM, linnix wrote:









On Dec 8, 1:30 pm, Spehro Pefhany<speffS...@interlogDOTyou.knowwhat
wrote:
On Thu, 08 Dec 2011 12:48:13 -0500, ehsjr<eh...@nospamverizon.net
wrote:

Sylvia Else wrote:
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.

The price of the relay at the url you posted is ridiculous.
Surely you can get something much cheaper and equally robust.
There are relays made for appliances like air conditioners
available from Digikey for< $5.00 so it's likely you can
get something where you are for much less than the ~ $82.00 at
the RS Australia site. You might be able to use an automotive
relay. Generally their contacts are rated at 30 or 40 amps at
12 VDC, and they'll handle 240 AC nicely.

Ed

I would not use a 12V automotive relay to switch mains voltage,
particularly inductive loads.. coil-to-contact dielectric strength of
those POS relays is<< 1kV. I like to see 4kV or so. Are the housings
required to be flame retardant?

Is the house (hosting the fridge) required to be fire retarded?

**Irrelvant. Whilst I don't know the specific legislation pertaining to
relays, here in Australia, ALL mains connected components (from
transformers to power leads) MUST have the requisite approval numbers
and, usually, fire retardant protection, or specific materials used in
their construction. Other jurisdictions may not have such legislation,
for whatever reason. Sylvia resides in Australia. As such, connecting
non-approved components may carry an insurance liability risk.


You are missing the point. Once they (insurance co) determined that
the fire is caused by mis-speced component, they will deny the whole
claim. They will tell you to sue the manufacturer, or whoever made
the modification. It is utterly irrespondable to use auto components
for house main.

In such socialist societies as Australia (and coming soon to the US),
you'd better use the exact replacement and have it done by a certified
technician... otherwise no insurance coverage at all.

**Idiot. EXACTLY the same situation exists in the US. And the US, like
all Western Democracies, is, in part, a socialist nation. Always has been.

--
Trevor Wilson
www.rageaudio.com.au

 

[Trevor Wilson]

On 12/9/2011 10:37 AM, linnix wrote:

On Dec 8, 3:13 pm, Trevor Wilson<tre...@SPAMBLOCKrageaudio.com.au
wrote:
On 12/9/2011 10:08 AM, linnix wrote:





On Dec 8, 2:35 pm, Trevor Wilson<tre...@SPAMBLOCKrageaudio.com.au
wrote:
On 12/9/2011 8:43 AM, linnix wrote:

On Dec 8, 1:30 pm, Spehro Pefhany<speffS...@interlogDOTyou.knowwhat
wrote:
On Thu, 08 Dec 2011 12:48:13 -0500, ehsjr<eh...@nospamverizon.net
wrote:

Sylvia Else wrote:
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.

The price of the relay at the url you posted is ridiculous.
Surely you can get something much cheaper and equally robust.
There are relays made for appliances like air conditioners
available from Digikey for< $5.00 so it's likely you can
get something where you are for much less than the ~ $82.00 at
the RS Australia site. You might be able to use an automotive
relay. Generally their contacts are rated at 30 or 40 amps at
12 VDC, and they'll handle 240 AC nicely.

Ed

I would not use a 12V automotive relay to switch mains voltage,
particularly inductive loads.. coil-to-contact dielectric strength of
those POS relays is<< 1kV. I like to see 4kV or so. Are the housings
required to be flame retardant?

Is the house (hosting the fridge) required to be fire retarded?

**Irrelvant. Whilst I don't know the specific legislation pertaining to
relays, here in Australia, ALL mains connected components (from
transformers to power leads) MUST have the requisite approval numbers
and, usually, fire retardant protection, or specific materials used in
their construction. Other jurisdictions may not have such legislation,
for whatever reason. Sylvia resides in Australia. As such, connecting
non-approved components may carry an insurance liability risk.

You are missing the point.

**No, I am not, but continue....

Once they (insurance co) determined that

the fire is caused by mis-speced component, they will deny the whole
claim. They will tell you to sue the manufacturer, or whoever made
the modification. It is utterly irrespondable to use auto components
for house main.

**I believe that is pretty much what I said.


Then, why is the fire/insurance issue "**Irrelvant"?

**I never said it was. I said that the flammability of the house was
(irrelevant).

--
Trevor Wilson
www.rageaudio.com.au

 

[Jim Thompson]

On Thu, 8 Dec 2011 15:47:34 -0800 (PST), linnix
<me@linnix.info-for.us> wrote:

On Dec 8, 3:25 pm, Trevor Wilson <tre...@SPAMBLOCKrageaudio.com.au
wrote:
On 12/9/2011 10:22 AM, Jim Thompson wrote:

[snip]

In such socialist societies as Australia (and coming soon to the US),
you'd better use the exact replacement and have it done by a certified
technician... otherwise no insurance coverage at all.

**Idiot. EXACTLY the same situation exists in the US. And the US, like
all Western Democracies, is, in part, a socialist nation. Always has been.

--

I think the problem is your language. You called my post irrevant
when we are agreeing on the same thing. You called JT Idiot (He may
or may not deserve it)

Flattery will get you nowhere with me ;-)

when he is really saying the same thing: The
US is getting more socialist, like AU.

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[linnix]

On Dec 8, 2:35 pm, Trevor Wilson <tre...@SPAMBLOCKrageaudio.com.au>
wrote:

On 12/9/2011 8:43 AM, linnix wrote:









On Dec 8, 1:30 pm, Spehro Pefhany<speffS...@interlogDOTyou.knowwhat
wrote:
On Thu, 08 Dec 2011 12:48:13 -0500, ehsjr<eh...@nospamverizon.net
wrote:

Sylvia Else wrote:
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.

The price of the relay at the url you posted is ridiculous.
Surely you can get something much cheaper and equally robust.
There are relays made for appliances like air conditioners
available from Digikey for<  $5.00 so it's likely you can
get something where you are for much less than the ~ $82.00 at
the RS Australia site.  You might be able to use an automotive
relay. Generally their contacts are rated at 30 or 40 amps at
12 VDC, and they'll handle 240 AC nicely.

Ed

I would not use a 12V automotive relay to switch mains voltage,
particularly inductive loads.. coil-to-contact dielectric strength of
those POS relays is<<  1kV. I like to see 4kV or so. Are the housings
required to be flame retardant?

Is the house (hosting the fridge) required to be fire retarded?

**Irrelvant. Whilst I don't know the specific legislation pertaining to
relays, here in Australia, ALL mains connected components (from
transformers to power leads) MUST have the requisite approval numbers
and, usually, fire retardant protection, or specific materials used in
their construction. Other jurisdictions may not have such legislation,
for whatever reason. Sylvia resides in Australia. As such, connecting
non-approved components may carry an insurance liability risk.

You are missing the point. Once they (insurance co) determined that
the fire is caused by mis-speced component, they will deny the whole
claim. They will tell you to sue the manufacturer, or whoever made
the modification. It is utterly irrespondable to use auto components
for house main.

 

[Sylvia Else]

On 9/12/2011 4:48 AM, ehsjr wrote:

Sylvia Else wrote:
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.


The price of the relay at the url you posted is ridiculous.

Er, yes. It was only a randomly selected item for the purpose of the
discussion about power ratings. There are indeed much cheaper relays
available that would, on the face of it, be up to the task.

Sylvia.

 

[Sylvia Else]

On 9/12/2011 4:48 AM, ehsjr wrote:

Sylvia Else wrote:
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.


The price of the relay at the url you posted is ridiculous.
Surely you can get something much cheaper and equally robust.
There are relays made for appliances like air conditioners
available from Digikey for < $5.00 so it's likely you can
get something where you are for much less than the ~ $82.00 at
the RS Australia site. You might be able to use an automotive
relay. Generally their contacts are rated at 30 or 40 amps at
12 VDC, and they'll handle 240 AC nicely.

I'd never use something that isn't expressly rated for the voltages and
currents I intend to use. There's nothing about a relay rated at 12VDC
that would guarantee that the insulation is up to use for mains voltages.

Sylvia.

 

[Sylvia Else]

On 9/12/2011 9:38 AM, Spehro Pefhany wrote:

On Thu, 08 Dec 2011 08:30:00 -0500, the renowned Spehro Pefhany
speffSNIP@interlogDOTyou.knowwhat> wrote:

On Thu, 08 Dec 2011 21:28:28 +1100, the renowned Sylvia Else
sylvia@not.here.invalid> wrote:

On 8/12/2011 9:04 PM, Trevor Wilson wrote:
On 12/8/2011 5:50 PM, Sylvia Else wrote:
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.


**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

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.

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

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.

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.