Relay contact ratings.

linnix · Dec 10, 2011

On Dec 10, 2:28 am, Sylvia Else <syl...@not.here.invalid> wrote:

On 10/12/2011 10:14 AM, David Eather wrote:

On 9/12/2011 10:56 AM, Sylvia Else wrote:
On 9/12/2011 4:23 AM, linnix wrote:
On Dec 8, 9:01 am, Fred Bloggs<bloggs.fredbloggs.f...@gmail.com
wrote:
On Dec 8, 1:50 am, Sylvia Else<syl...@not.here.invalid> 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.

There are aftermarket universals better than the OEM
parthttp://compare.ebay.com/like/290638915750?var=lv<yp=AllFixedPriceIt...

There are plenty of relays you can salvage out of garage opener (12V),
microwave (12V) or furnace (24V). Add a micro or 555 for less than $5
and you are done. I am doing one for my furnace timer.

Not sure about getting a 555 to run at 1 cycle per six hours.

A PIC would no doubt do the job easily, but then I'd have to invest in
hardware to program it.

Sylvia.

Sylvia,

Try a PICAXE 08M2 - it costs $2.50 in one off's and programs in BASIC.
Software and examples are free fromwww.picaxe.co.uk
It is not uncommon for people totally new to the PICAXE have had them
running programs within 30 minutes of getting their first chip.

The only stuff you need to program one is a serial port (RS232 or TTL
level) and a resistor.

Interesting. Thanks.

Yes, good to hide the chip arch. But go with a PIC32 (calling all ARM
fans) and you don't have to.

I'm tempted to combine the timer with the thermostat, a replacement for
which I've been cursin..., er, I mean testing, today. (Sod's law
subsection 3.1 - a fridge must always enter defrost mode while a person
is attempting to adjust the upper and lower settings of a thermostat.)

Is that thermostat for the fridge or for the room?

Either way, easy enough to do with a micro. I am currently working on
one for the room thermostat. My wife always complaint that i am
setting the the room temp too high (because the old thermostat reading
is too low). I got a new one with reading too high, and she is
complainting that i am setting the temp. too low. I am going to build
one that would fit me right, unless i can find an adjustable temp.
sensor thermostat. The reading must be 68F, doesn't matter what the
real temp. is.

Of course, I could just buy a new fridge.

NO, NO, NO. save the world from another junk fridge.

linnix · Dec 10, 2011

On Dec 10, 8:46 am, "Michael A. Terrell" <mike.terr...@earthlink.net>
wrote:

Sylvia Else wrote:

There are aftermarket universals better than the OEM part
http://compare.ebay.com/like/290638915750?var=lv<yp=AllFixedPriceIt...

Though not 240V versions as far as I can see. Pity.

Can't you find a cheap 240 to 120 VAC transformer?

Yes, power the whole house with 240V to 120V T. I know someone who
does that, because 120V applicants are safer and sometime cheaper.

David Eather · Dec 10, 2011

On 10/12/2011 10:55 AM, Jim Thompson wrote:

On Sat, 10 Dec 2011 09:14:27 +1000, David Eather<eather@tpg.com.au
wrote:

We have failed to address the fundamental truth that endless growth is
impossible in a finite world.

Kill off non-productive socialists and you're left with LOTS of space

...Jim Thompson

Yes, but you have fewer stimulating arguments.

--
We have failed to address the fundamental truth that endless growth is
impossible in a finite world.

David Eather · Dec 10, 2011

On 10/12/2011 8:28 PM, Sylvia Else wrote:

On 10/12/2011 10:14 AM, David Eather wrote:
On 9/12/2011 10:56 AM, Sylvia Else wrote:
On 9/12/2011 4:23 AM, linnix wrote:
On Dec 8, 9:01 am, Fred Bloggs<bloggs.fredbloggs.f...@gmail.com
wrote:
On Dec 8, 1:50 am, Sylvia Else<syl...@not.here.invalid> 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.

There are aftermarket universals better than the OEM
parthttp://compare.ebay.com/like/290638915750?var=lv<yp=AllFixedPriceIt...

There are plenty of relays you can salvage out of garage opener (12V),
microwave (12V) or furnace (24V). Add a micro or 555 for less than $5
and you are done. I am doing one for my furnace timer.

Not sure about getting a 555 to run at 1 cycle per six hours.

A PIC would no doubt do the job easily, but then I'd have to invest in
hardware to program it.

Sylvia.

Sylvia,

Try a PICAXE 08M2 - it costs $2.50 in one off's and programs in BASIC.
Software and examples are free from www.picaxe.co.uk
It is not uncommon for people totally new to the PICAXE have had them
running programs within 30 minutes of getting their first chip.

The only stuff you need to program one is a serial port (RS232 or TTL
level) and a resistor.

Interesting. Thanks.

I'm tempted to combine the timer with the thermostat, a replacement for
which I've been cursin..., er, I mean testing, today. (Sod's law
subsection 3.1 - a fridge must always enter defrost mode while a person
is attempting to adjust the upper and lower settings of a thermostat.)

Of course, I could just buy a new fridge.

Sylvia.

Yes, but it strikes me that you much prefer to fix it yourself. BTW, The
PICAXE directly supports the DS18S20 one wire thermometer.

Just thinking further, depending on energy ratings it may be a money
saver to get a new fridge.

--
We have failed to address the fundamental truth that endless growth is
impossible in a finite world.

David Eather · Dec 10, 2011

On 10/12/2011 10:32 AM, John Fields wrote:

On Sat, 10 Dec 2011 09:14:27 +1000, David Eather<eather@tpg.com.au
wrote:

We have failed to address the fundamental truth that endless growth is
impossible in a finite world.

---
Not true; consider Moore's law.

Consider that Moore's Law is not a law.

--
We have failed to address the fundamental truth that endless growth is
impossible in a finite world.

Jim Thompson · Dec 10, 2011

On Sun, 11 Dec 2011 07:24:20 +1000, David Eather <eather@tpg.com.au>
wrote:

On 10/12/2011 10:55 AM, Jim Thompson wrote:
On Sat, 10 Dec 2011 09:14:27 +1000, David Eather<eather@tpg.com.au
wrote:

We have failed to address the fundamental truth that endless growth is
impossible in a finite world.

Kill off non-productive socialists and you're left with LOTS of space

...Jim Thompson

Yes, but you have fewer stimulating arguments.

Jim Thompson · Dec 10, 2011

On Sun, 11 Dec 2011 07:29:25 +1000, David Eather <eather@tpg.com.au>
wrote:

On 10/12/2011 8:28 PM, Sylvia Else wrote:
On 10/12/2011 10:14 AM, David Eather wrote:
On 9/12/2011 10:56 AM, Sylvia Else wrote:
On 9/12/2011 4:23 AM, linnix wrote:
On Dec 8, 9:01 am, Fred Bloggs<bloggs.fredbloggs.f...@gmail.com
wrote:
On Dec 8, 1:50 am, Sylvia Else<syl...@not.here.invalid> 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.

There are aftermarket universals better than the OEM
parthttp://compare.ebay.com/like/290638915750?var=lv<yp=AllFixedPriceIt...

There are plenty of relays you can salvage out of garage opener (12V),
microwave (12V) or furnace (24V). Add a micro or 555 for less than $5
and you are done. I am doing one for my furnace timer.

Not sure about getting a 555 to run at 1 cycle per six hours.

A PIC would no doubt do the job easily, but then I'd have to invest in
hardware to program it.

Sylvia.

Sylvia,

Try a PICAXE 08M2 - it costs $2.50 in one off's and programs in BASIC.
Software and examples are free from www.picaxe.co.uk
It is not uncommon for people totally new to the PICAXE have had them
running programs within 30 minutes of getting their first chip.

The only stuff you need to program one is a serial port (RS232 or TTL
level) and a resistor.

Interesting. Thanks.

I'm tempted to combine the timer with the thermostat, a replacement for
which I've been cursin..., er, I mean testing, today. (Sod's law
subsection 3.1 - a fridge must always enter defrost mode while a person
is attempting to adjust the upper and lower settings of a thermostat.)

Of course, I could just buy a new fridge.

Sylvia.

Yes, but it strikes me that you much prefer to fix it yourself. BTW, The
PICAXE directly supports the DS18S20 one wire thermometer.

Just thinking further, depending on energy ratings it may be a money
saver to get a new fridge.

David Eather · Dec 10, 2011

On 11/12/2011 7:31 AM, Jim Thompson wrote:

On Sun, 11 Dec 2011 07:24:20 +1000, David Eather<eather@tpg.com.au
wrote:

On 10/12/2011 10:55 AM, Jim Thompson wrote:
On Sat, 10 Dec 2011 09:14:27 +1000, David Eather<eather@tpg.com.au
wrote:

We have failed to address the fundamental truth that endless growth is
impossible in a finite world.

Kill off non-productive socialists and you're left with LOTS of space

...Jim Thompson

Yes, but you have fewer stimulating arguments.

The silence would be exhilarating ;-)

...Jim Thompson

Wouldn't it get boring after a while?

--
We have failed to address the fundamental truth that endless growth is
impossible in a finite world.

Jim Thompson · Dec 10, 2011

On Sun, 11 Dec 2011 07:48:28 +1000, David Eather <eather@tpg.com.au>
wrote:

On 11/12/2011 7:31 AM, Jim Thompson wrote:
On Sun, 11 Dec 2011 07:24:20 +1000, David Eather<eather@tpg.com.au
wrote:

On 10/12/2011 10:55 AM, Jim Thompson wrote:
On Sat, 10 Dec 2011 09:14:27 +1000, David Eather<eather@tpg.com.au
wrote:

We have failed to address the fundamental truth that endless growth is
impossible in a finite world.

Kill off non-productive socialists and you're left with LOTS of space

...Jim Thompson

Yes, but you have fewer stimulating arguments.

The silence would be exhilarating ;-)

...Jim Thompson

Wouldn't it get boring after a while?

kreed · Dec 10, 2011

On Dec 10, 9:00 pm, Sylvia Else <syl...@not.here.invalid> wrote:

On 10/12/2011 9:33 PM, kreed wrote:

On Dec 10, 8:28 pm, Sylvia Else<syl...@not.here.invalid> wrote:
On 10/12/2011 10:14 AM, David Eather wrote:

On 9/12/2011 10:56 AM, Sylvia Else wrote:
On 9/12/2011 4:23 AM, linnix wrote:
On Dec 8, 9:01 am, Fred Bloggs<bloggs.fredbloggs.f...@gmail.com
wrote:
On Dec 8, 1:50 am, Sylvia Else<syl...@not.here.invalid> 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.

There are aftermarket universals better than the OEM
parthttp://compare.ebay.com/like/290638915750?var=lv<yp=AllFixedPriceIt...

There are plenty of relays you can salvage out of garage opener (12V),
microwave (12V) or furnace (24V). Add a micro or 555 for less than $5
and you are done. I am doing one for my furnace timer.

Not sure about getting a 555 to run at 1 cycle per six hours.

A PIC would no doubt do the job easily, but then I'd have to invest in
hardware to program it.

Sylvia.

Sylvia,

Try a PICAXE 08M2 - it costs $2.50 in one off's and programs in BASIC..
Software and examples are free fromwww.picaxe.co.uk
It is not uncommon for people totally new to the PICAXE have had them
running programs within 30 minutes of getting their first chip.

The only stuff you need to program one is a serial port (RS232 or TTL
level) and a resistor.

Interesting. Thanks.

I'm tempted to combine the timer with the thermostat, a replacement for
which I've been cursin..., er, I mean testing, today. (Sod's law
subsection 3.1 - a fridge must always enter defrost mode while a person
is attempting to adjust the upper and lower settings of a thermostat.)

Of course, I could just buy a new fridge.

Sylvia.

Depends on how old, and how good the current one is other than this
current problem. New ones might be even worse

It would have to be at least ten, and it's been running all that time.

The big question, of course, is how much longer the compressor will
last, since it's probably not going to possible to source a replacement,
even if it would otherwise be economically worthwhile.

BTW, the wires are attached to the (mains-voltage) lamp socket by way of
spring clips. Seems a bit iffy to me.

Sylvia.

I remember a few years back there was one of these educational
programs on how things work, in particular household appliances. They
actually went to a dump site where were were dozens of junked fridges,
and connected each one to power. Almost all still cooled, but had
been dumped due to rust, broken shelves, seals etc or other similar
damage. They said that the compressors were extremely reliable.

If your fridge is only 10 and hasnt gor rust or any of these problems,
odds are that it should continue working for a long time yet, and
probably is quite efficient compared to older ones. Probably worth
fixing, especially if you can DIY. New ones aren't cheap

I don't know what the climate is where you are, but we have had 2
fridges without auto defrost since the 90's and they rarely ice up.
Might defrost once a year, and isnt a big drama. If you are opening
the freezer section a lot though (especially with single door fridges)
it might be different as that would let more humid air in, that would
condense on the cold surface and form ice.

As for that electronic aftermarket timer from Ebay that is 120v - if
you get one and open it up, you might find that it has either a
switchmode power supply or a series capacitor type supply. You can
modify these fairly easily to run from 240v, or make up a small power
supply that you can mount in the back of the fridge. For something
like this a linear power supply might be best, as linear ones are very
reliable.

kreed · Dec 10, 2011

On Dec 10, 8:34 pm, Sylvia Else <syl...@not.here.invalid> wrote:

On 9/12/2011 4:01 AM, Fred Bloggs wrote:

On Dec 8, 1:50 am, Sylvia Else<syl...@not.here.invalid> 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.

There are aftermarket universals better than the OEM part
http://compare.ebay.com/like/290638915750?var=lv&ltyp=AllFixedPriceIt...

Though not 240V versions as far as I can see. Pity.

Sylvia.

Try here Sylvia

http://www.ebay.com.au/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=defrost+timer&_sacat=See-All-Categories

Rheilly Phoull · Dec 10, 2011

On 12/10/2011 6:34 PM, Sylvia Else wrote:

On 9/12/2011 4:01 AM, Fred Bloggs wrote:
On Dec 8, 1:50 am, Sylvia Else<syl...@not.here.invalid> 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.

There are aftermarket universals better than the OEM part
http://compare.ebay.com/like/290638915750?var=lv&ltyp=AllFixedPriceItemTypes&var=sbar

Though not 240V versions as far as I can see. Pity.

Sylvia.

Plenty on ebay, pretty complicated though.
You have to search for "fridge defrost timer'

Rheilly

John Fields · Dec 11, 2011

On Sun, 11 Dec 2011 07:30:25 +1000, David Eather <eather@tpg.com.au>
wrote:

On 10/12/2011 10:32 AM, John Fields wrote:
On Sat, 10 Dec 2011 09:14:27 +1000, David Eather<eather@tpg.com.au
wrote:

We have failed to address the fundamental truth that endless growth is
impossible in a finite world.

---
Not true; consider Moore's law.

Consider that Moore's Law is not a law.

---
OK, Moore's conjecture then, if that pleases you.

The point is that we _have_ addressed the fundamental truth that
endless growth is impossible in a finite world by stalling the
inevitable using miniaturization.

--
JF

linnix · Dec 11, 2011

On Dec 10, 2:39 pm, kreed <kenreed1...@gmail.com> wrote:

On Dec 10, 9:00 pm, Sylvia Else <syl...@not.here.invalid> wrote:

On 10/12/2011 9:33 PM, kreed wrote:

On Dec 10, 8:28 pm, Sylvia Else<syl...@not.here.invalid> wrote:
On 10/12/2011 10:14 AM, David Eather wrote:

On 9/12/2011 10:56 AM, Sylvia Else wrote:
On 9/12/2011 4:23 AM, linnix wrote:
On Dec 8, 9:01 am, Fred Bloggs<bloggs.fredbloggs.f...@gmail.com
wrote:
On Dec 8, 1:50 am, Sylvia Else<syl...@not.here.invalid> 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.

There are aftermarket universals better than the OEM
parthttp://compare.ebay.com/like/290638915750?var=lv<yp=AllFixedPriceIt...

There are plenty of relays you can salvage out of garage opener (12V),
microwave (12V) or furnace (24V). Add a micro or 555 for less than $5
and you are done. I am doing one for my furnace timer.

Not sure about getting a 555 to run at 1 cycle per six hours.

A PIC would no doubt do the job easily, but then I'd have to invest in
hardware to program it.

Sylvia.

Sylvia,

Try a PICAXE 08M2 - it costs $2.50 in one off's and programs in BASIC.
Software and examples are free fromwww.picaxe.co.uk
It is not uncommon for people totally new to the PICAXE have had them
running programs within 30 minutes of getting their first chip.

The only stuff you need to program one is a serial port (RS232 or TTL
level) and a resistor.

Interesting. Thanks.

I'm tempted to combine the timer with the thermostat, a replacement for
which I've been cursin..., er, I mean testing, today. (Sod's law
subsection 3.1 - a fridge must always enter defrost mode while a person
is attempting to adjust the upper and lower settings of a thermostat..)

Of course, I could just buy a new fridge.

Sylvia.

Depends on how old, and how good the current one is other than this
current problem. New ones might be even worse

It would have to be at least ten, and it's been running all that time.

The big question, of course, is how much longer the compressor will
last, since it's probably not going to possible to source a replacement,
even if it would otherwise be economically worthwhile.

BTW, the wires are attached to the (mains-voltage) lamp socket by way of
spring clips. Seems a bit iffy to me.

Sylvia.

I remember a few years back there was one of these educational
programs on how things work, in particular household appliances. They
actually went to a dump site where were were dozens of junked fridges,
and connected each one to power. Almost all still cooled, but had
been dumped due to rust, broken shelves, seals etc or other similar
damage. They said that the compressors were extremely reliable.

OK, i am going dumper diving.

...

As for that electronic aftermarket timer from Ebay that is 120v - if
you get one and open it up, you might find that it has either a
switchmode power supply or a series capacitor type supply. You can
modify these fairly easily to run from 240v, or make up a small power
supply that you can mount in the back of the fridge. For something
like this a linear power supply might be best, as linear ones are very
reliable.

Actually, a good source of relays are garage openers. I have a spare
board with 3 240V/5A w 24V/5A coil. I guess it needs open/close/on.

John Fields · Dec 11, 2011

On Thu, 08 Dec 2011 21:28:28 +1100, 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.

From your description, it appears that your fridge looks like this:
(View using a fixed-pitch font)

---

MAINS>--+---------------+----------------------------+
| | |
| O C |
[DEFROST MOTOR]- - - -|S1 |
| O-> |<-O-[THERMOSTAT]-+ |
| |NO NC | O C
| | [COIL}- - - -|
| [HEATER] | O-> |
| | | |NO
| | | [COMPRESSOR]
| | | |
MAINS>--+-----------+---------------------+------+

Am I right?

If I am and you're having problems with the defrost timer and its
switches, then it seems you could replace the motor with a timer and
the timer motor's set of switches with a SPDT relay, like this:

MAINS>-+-----------------+----------------------------+
| | |
| O C |
[TIMER]-[COIL]- - - -|K1 |
| O-> |<-O-[THERMOSTAT]-+ |
| |NO NC | O C
| | [COIL]- - - -|
| [HEATER] | NO O-> |
| | | |
| | | [COMPRESSOR]
| | | |
MAINS>-+-------------+---------------------+------+

You've said the defrost only happens every six hours, but for how long
is the heater energized?

knowing that will allow us to determine the duty cycle of the timer
and cause it to mimic the motor timer.

--
JF

linnix · Dec 11, 2011

On Dec 10, 5:06 pm, John Fields <jfie...@austininstruments.com> wrote:

On Thu, 08 Dec 2011 21:28:28 +1100, 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.

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.

From your description, it appears that your fridge looks like this:
(View using a fixed-pitch font)

---

MAINS>--+---------------+----------------------------+
| | |
| O C |
[DEFROST MOTOR]- - - -|S1 |
| O-> |<-O-[THERMOSTAT]-+ |
| |NO NC | O C
| | [COIL}- - - -|
| [HEATER] | O-> |
| | | |NO
| | | [COMPRESSOR]
| | | |
MAINS>--+-----------+---------------------+------+

Am I right?

If I am and you're having problems with the defrost timer and its
switches, then it seems you could replace the motor with a timer and
the timer motor's set of switches with a SPDT relay, like this:

MAINS>-+-----------------+----------------------------+
| | |
| O C |
[TIMER]-[COIL]- - - -|K1 |
| O-> |<-O-[THERMOSTAT]-+ |
| |NO NC | O C
| | [COIL]- - - -|
| [HEATER] | NO O-> |
| | | |
| | | [COMPRESSOR]
| | | |
MAINS>-+-------------+---------------------+------+

You've said the defrost only happens every six hours, but for how long
is the heater energized?

knowing that will allow us to determine the duty cycle of the timer
and cause it to mimic the motor timer.

--
JF

It should be long enough to melt the ice and let the water drain off.
It can easily be programmable in a micro, at least during
development. Perhaps a 5 to 10 minutes range.

Sylvia Else · Dec 11, 2011

On 11/12/2011 8:33 AM, Jim Thompson wrote:

On Sun, 11 Dec 2011 07:29:25 +1000, David Eather<eather@tpg.com.au
wrote:

On 10/12/2011 8:28 PM, Sylvia Else wrote:
On 10/12/2011 10:14 AM, David Eather wrote:
On 9/12/2011 10:56 AM, Sylvia Else wrote:
On 9/12/2011 4:23 AM, linnix wrote:
On Dec 8, 9:01 am, Fred Bloggs<bloggs.fredbloggs.f...@gmail.com
wrote:
On Dec 8, 1:50 am, Sylvia Else<syl...@not.here.invalid> 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.

There are aftermarket universals better than the OEM
parthttp://compare.ebay.com/like/290638915750?var=lv<yp=AllFixedPriceIt...

There are plenty of relays you can salvage out of garage opener (12V),
microwave (12V) or furnace (24V). Add a micro or 555 for less than $5
and you are done. I am doing one for my furnace timer.

Not sure about getting a 555 to run at 1 cycle per six hours.

A PIC would no doubt do the job easily, but then I'd have to invest in
hardware to program it.

Sylvia.

Sylvia,

Try a PICAXE 08M2 - it costs $2.50 in one off's and programs in BASIC.
Software and examples are free from www.picaxe.co.uk
It is not uncommon for people totally new to the PICAXE have had them
running programs within 30 minutes of getting their first chip.

The only stuff you need to program one is a serial port (RS232 or TTL
level) and a resistor.

Interesting. Thanks.

I'm tempted to combine the timer with the thermostat, a replacement for
which I've been cursin..., er, I mean testing, today. (Sod's law
subsection 3.1 - a fridge must always enter defrost mode while a person
is attempting to adjust the upper and lower settings of a thermostat.)

Of course, I could just buy a new fridge.

Sylvia.

Yes, but it strikes me that you much prefer to fix it yourself. BTW, The
PICAXE directly supports the DS18S20 one wire thermometer.

Just thinking further, depending on energy ratings it may be a money
saver to get a new fridge.

Or just replace the relay with a switch. Must of these automatic
defrosting fridges defrost so often that they freezer-burn and ruin
your food.

It's hard to tell whether it's 'defrosting' an element that is in any
case more or less ice free. The element isn't visible without detaching
the backplate of the freezer, and running the fridge without the
backplate in place would so change the airflow as to make any
observations irrelevant to the normal operation of the fridge.

The reason I got started on this whole exercise was the fact that at
times the bread in the fridge was clearly not frozen, even though the
freezer temperature certainly drops to less than -15C at times. I hadn't
previously looked into the defrost mechanism, and indeed, didn't realise
that there is actually a quite powerful heater element involved. Nor did
I realise that the defrosting occurs every six hours.

A particular issue with the defrost mechanism is that it has a defrost
termination thermostat mounted on the cooling element (set to open at
+11 degrees celsius, closes again at 0C), but regardless of what it
does, the fridge remains in its non-cooling mode for about half an hour
because of the defrost timer. I put a temperature sensor onto the
thermostat, and it shows that the actual temperature there rises to
nearly 20C, presumably because even after the heater is turned off, it's
still very hot.

In addition, there's no link between the defrost timing and the
thermostat. Defrosting may start just as fridge has reached its highest
normal temperature and the termostat is about to turn the compressor on.
So the day to day outcome will vary considerably, even ignoring ambient
temperature changes, making assessing whether the thing is working
properly rather problematic.

Now the bread is probably the thing in the freezer that has the least
thermal inertia. Maybe the fridge has been working correctly (in the
sense of "as designed") all along, and I've just been overreacting to
the occasional thawing of the bread.

I've now learnt stuff about self defrosting fridges that I might have
been happier not knowing.

Sylvia.

Sylvia Else · Dec 11, 2011

On 11/12/2011 3:46 AM, Michael A. Terrell wrote:

Sylvia Else wrote:

There are aftermarket universals better than the OEM part
http://compare.ebay.com/like/290638915750?var=lv&ltyp=AllFixedPriceItemTypes&var=sbar

Though not 240V versions as far as I can see. Pity.

Can't you find a cheap 240 to 120 VAC transformer?

I could. Using it wouldn't be so straight forward. The timer has a
single pin for input, which is both the source of power for the clock,
and for running either the compressor and heater.

Sylvia.

Sylvia Else · Dec 11, 2011

On 11/12/2011 9:47 AM, kreed wrote:

On Dec 10, 8:34 pm, Sylvia Else<syl...@not.here.invalid> wrote:
On 9/12/2011 4:01 AM, Fred Bloggs wrote:

On Dec 8, 1:50 am, Sylvia Else<syl...@not.here.invalid> 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.

There are aftermarket universals better than the OEM part
http://compare.ebay.com/like/290638915750?var=lv&ltyp=AllFixedPriceIt...

Though not 240V versions as far as I can see. Pity.

Sylvia.

Try here Sylvia

http://www.ebay.com.au/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=defrost+timer&_sacat=See-All-Categories

Strange that I didn't find them. It's not as if I didn't look.

Sylvia.

Sylvia Else · Dec 11, 2011

On 11/12/2011 3:55 AM, krw@att.bizzzzzzzzzzzz wrote:

On Fri, 09 Dec 2011 12:53:26 -0800, Joerg<invalid@invalid.invalid> 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?

2500VA? Wow, that is one heck of a fridge!

How many big cans of Foster's can it hold?

Being a purely inductive load, it can't cool many. ;-)

I should buy one of them new-fangled phase-correcting "power savers" and
see how much my bill drops by when I reduce the load from zero to zero.

Sylvia.

Relay contact ratings.

linnix

Guest

linnix

Guest

David Eather

Guest

David Eather

Guest

David Eather

Guest

Jim Thompson

Guest

Jim Thompson

Guest

David Eather

Guest

Jim Thompson

Guest

kreed

Guest

kreed

Guest

Rheilly Phoull

Guest

John Fields

Guest

linnix

Guest

John Fields

Guest

linnix

Guest

Sylvia Else

Guest

Sylvia Else

Guest

Sylvia Else

Guest

Sylvia Else

Guest

Log in

Welcome to EDABoard.com

Sponsor