12V relay reliability?

[BruceS]

Over on CF I noticed a 2 year old thread about "fridge switching".
Geoff made comment on using a relay that operates off the vehicles key switch or similar.
I've found 40A & 80A relays but am concerned about how long they would last out in the real world.
My plan is to use it more as a thermostat controlled switch so in effect it would be switching on and off much more often than was referred to in that other thread.
((( http://caravanersforum.com/viewtopic.php?f=2&t=7121 )))
Would the contacts last long switching one of those 12/24/240V 121cube fridge/freezers (Dometic I think?)
80A version here.
http://www.ebay.com.au/itm/170929879538 ... 1423.l2649
Would it be advisable to run it through a 40A relay & then to a solenoid like on an outboard startermotor? I doubt they would run longer than a minute before over heating?
Comments? Geoff? Anyone?

[PeterM]

Can you find a solid state relay (mosfet)? Jaycar has some, but I have not checked their current capacity. Surely you don't need 40A!

[T1 Terry]

the cheap mechanical one fall appart, the good stuff like Bosch or Hella are 40 amp rated but a lot of switching will ash the contact faces after a while, every time a DC circuit is broken an arc is formed, slowly the contact faces burn away.
as Peter suggests a solid state relay would be a better choice, for the small amount of current draw you need the chinese rubbish ones would work fine http://www.ebay.com.au/itm/Solid-State- ... 43a6d99730 they are rubbish for any serious long "on" applications because they get very hot but for the short run period of a fridge they would be ok mounted to a bit of heat sink some where that a breeze would move the heat away.

[oldtrack123]

Hi I assume you mean for switching the 12V heater circuit
My Dometic 3way is thermo controlled on 12v [yes ,many are]
To take the load off the thermo switch contacts, I use the thermo to switch the relay coil of a cheap Jaycar 40A relay,
no problems.
The contacts do not get much arcing damage with a 3way as the load is resistive,not INDUCTIVE [ very little spark, compared to an inductiv load]
The frequency of operation is quite low . perhap 20 times a day MAX.
Absolutely no need to have complicated fancy switching in THAT situation.

Peter

[oldtrack123]

Can you find a solid state relay (mosfet)? Jaycar has some, but I have not checked their current capacity. Surely you don't need 40A!

Hi
The price is little different , for a 40A to say a 20A electro mechanical relay[which would be the minimum]
Larger contact surface is better.

. Again absolutely no justification for mosfet switching in THIS situation .
IF the mechanical relay does fail, very simple & easy to find a replacement @ any auto shop
Peter

[BruceS]

Peter it is a compressor fridge. Danfoss I believe? Does that alter your advice at all?
Thankyou all for suggestions.

[Izabarack]

I've found 40A & 80A relays but am concerned about how long they would last out in the real world.

I have and do use 80A contact relays for powering fridge and for switching Alternator output to a second battery. The relays in the Landcruiser lasted 14 years and were operational when that vehicle went. Current setup is 6 years old and fine. I have installed similar in several other vehicles and have had no problems. Maybe it is because I do carry a spare, JIC.

80A version here.
http://www.ebay.com.au/itm/170929879538 ... 1423.l2649

That is pretty much what I use. Momentary on pushbutton on the dash and a self-latching relay in the middle to ensure everything drops out when the ignition turns off and is not on when the ignition is turned on again.

Would it be advisable to run it through a 40A relay & then to a solenoid like on an outboard startermotor?

I just use the relay contacts and do not use anything else, such as a RedArc or a solenoid. Never had any problems. KISS works for me, including the manual management and the use of prospective memory (e.g. remembering to do stuff).

Iza

[T1 Terry]

I may have misinterpreted what you wanted to do with the relay, I though you were switching the relay with the thermostat, lots of switching every day, but it it's only to switch the power to the fridge off with the ignition, an automotive relay such as Bosch or Hella or Narva, will do the job just fine, these are avaible for around $5 from any automotive shop.

T1 Terry

[TigerMK1]

What about the solid state relay's from Jcar. I have been using a 50A unit to switch my 700W+ solar @12V to my batteries switched very quickly by the PL20 solar controller. Has been working (on a suitable heatsink) for 6 months or more. T1 may elaborate, they have been promoted for high amp switching on the other forum for some time. I also have a chinese copy spare if the Jcar unit karks it.

[BruceS]

Yes, sorry T1...... I inferred both ways in the original post.
I've been working on a 'problem' for running Bernie's new fridge with the existing setup he has.
Bus on 24V.
House battery is big AGM, 12V.
Fridge is compressor 12/24/240AC.
Fridge appears to have 'strange' thermostat.
Would like to run fridge on bus battery while mobile and on house one when engine off.

Considering ignition activated changeover relay to switch power source.
Install electronic thermostat activated relay in electrical cable between switchover relay and fridge.
When 240V switched on it auto turns off DC power. (apparently?)
Sounds complicated but attempting to avoid "I forgot" syndrome!
The switchover one wouldn't get much use but the other one would get a lot more use depending on differential settings.