Relay contact ratings.

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.

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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.

Reply to
Sylvia Else
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"Stupider than Anyone Else"

** 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

Reply to
Phil Allison

Care to point, in the data sheet, to where that distinction is drawn.

Sylvia.

Reply to
Sylvia Else

**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

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Reply to
Trevor Wilson

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.

Reply to
Sylvia Else

You can often get more life out of a mechanical defrost timer by opening up the motor and oiling the bearings.

Reply to
mike

I hope you are not being sarcastic.

Reply to
F Murtz

It could be a reference to the absence of bearings - except to the extent that the metal axle rotates in a hole in the plastic housing. At least, that's the situation in the one I have.

Perhaps I shouldn't be concerned about having a "genuine" part, because the original looks about as cheaply made as it could get while still working at all.

Sylvia.

Reply to
Sylvia Else

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

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

2.SPECIFICATIONS
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Reply to
John Fields

That makes more sense. RS Components don't help by reversing the order of the power ratings relative to the voltage and current ratings.

Quoting power ratings seems redundant.

Sylvia

Reply to
Sylvia Else

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"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Reply to
Spehro Pefhany

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use.

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.

Reply to
kreed

There are aftermarket universals better than the OEM part

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Reply to
Fred Bloggs

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Reply to
linnix

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

Reply to
ehsjr

d
.

No, you don't want to use auto relay. If your house burn down, insurance will not pay. It doesn't mater whether the part can handle it or not.

Reply to
linnix

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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

Reply to
NT

That could be a problem. Mine has a standard clock motor with metal case and bronze-looking bearings.

A clock motor has about zero torque. Takes very little friction in the first few gear reduction stages to stop it. hardened grease can easily do it. Just cleaning out the dried up grease can make it work.

Mine ran for another decade after I cleaned/oiled it. Replacement was about $13, but it failed after a week. Second replacement timer lasted years before I retired the fridge.

Reply to
mike

"kreed"

"TW"

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

Reply to
Phil Allison

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