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Relay contact ratings.

S

swanny

Jan 1, 1970
0
"swanny"
"Phil Allison"

** I have made a bit of a study of this in an attempt to find a solution.

Conclusions are:

1. There is no readily available relay capable of breaking the voltages and
currents involved if a 1000 wpc amp goes DC.

2. If you wire a changeover relay so it shorts the speaker and disconnects
the amp at the same time - chances are good the speakers will be OK, but
the relay contacts will be burnt by the resulting arc to ground. The amp
must have DC rail fuses if this is to work.

3. A large ( ie octal base ) relay with 4mm clearance contacts PLUS a
strong magnet next to the contacts CAN
work with amps up to about 400wpc. The magnet pulls at the arc and helps
break it.

4. A triac "crowbar" will also work but gives no on/off transient
suppression. The amp must be able to take a dead short without internal
failure and not have response down to subsonics or DC.




... Phil

A big capacitor in series with the output :)
 
E

ehsjr

Jan 1, 1970
0
linnix said:
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.

I'm laughing at the turn this thread took. If she installs her
own circuit, she is modifying the thing with a non-oem part,
regardless of what parts she uses in her circuit. Thus any concerns
about fire/liability/insurance/local regulations apply. Doesn't
matter if it's flame retardant, 240VAC rated, gold plated, whatever,
the exposure is still there.

She has already indicated she would not use an automotive relay,
so it's moot as far as her project is concerned. But I wonder,
has anyone else here actually tested one of those relays at 240VAC?
I have, and it worked fine, but I did not test to destruction,
nor for dielectric strength or flammability.

Ed
 
J

Jasen Betts

Jan 1, 1970
0
"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.

yeah, over voltage, or excessive dV/dt, will turn them on,
but couldn't subsequent over-current cause them to fail closed?
 
J

Jasen Betts

Jan 1, 1970
0
Not sure about getting a 555 to run at 1 cycle per six hours.

CD4060, CMOS logic, an R-C astable and a 14-stage divider chain in
one DIP package, but not strong enough to drive a relay directly like
an LM555 would be.
 
J

Jasen Betts

Jan 1, 1970
0
And to get the issue back on topic, the US regulatory authorities have
considerable influence of what can and can not be connected to US mains
power. In the US, however, due to the low mains Voltage, there is far
less risk to consumers than there is here in Australia.


less risk of fatal electric shock, more risk of fire from overheating
conductors.
 
P

Phil Allison

Jan 1, 1970
0
"Jasen Betts Sheep Shagger "
yeah, over voltage, or excessive dV/dt, will turn them on,


** LOL .....

but couldn't subsequent over-current cause them to fail closed?


** So could an errant meteorite.

You fucking, autistic Kiwi moron !!!



.... Phil
 
J

John Devereux

Jan 1, 1970
0
Phil Allison said:
"Trevor Wilson"




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




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




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



... Phil

Hey Phil, he's stolen your **'s tag! :)
 
J

Joerg

Jan 1, 1970
0
Sylvia said:
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? :)
 
S

Sylvia Else

Jan 1, 1970
0
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.
 
S

Sylvia Else

Jan 1, 1970
0
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.
 
S

Sylvia Else

Jan 1, 1970
0
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.
 
S

Sylvia Else

Jan 1, 1970
0
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.
 
S

Sylvia Else

Jan 1, 1970
0
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.
 
S

Sylvia Else

Jan 1, 1970
0
There is no neutral connection to the timer?
OK, I suppose that would work, though the thing would still be switching
voltage which is double its design rating.

Sylvia.
 
S

Sylvia Else

Jan 1, 1970
0

Since I'm in Australia, both the defrost heater and the compressor are
rated at 240V. Anything that switches them on and off has to switch 240V.

Sylvia.
 
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