Backyard Aquaponics
A place to discuss aquaponics

Steve has a very valid point ,, the relay mentioned is fantastic for 12V and then backing a 12 volt system.
Putting 240 volts on this may be disasterous ,, the contacts are not made for that but even more importantly the "air gap" or insulation between the contacts are not designed to prevent 240 Volts from arcing across.
The battery charger and having a 12V coil are GREAT ,, now we need to find a 12V coiled relay with 240V rated contacts ....., yes they are very common and reasonably cheap. J-car , even Dick Smiths have them.

OOOPS d*ck Smiths was a no no . Wasn't swearing ..... AHH opk Richard Smiths ,, but you know what i mean.

yes steve MAY have a valid point but not necessrily in a basic AP application ( he might be thinkin about powering up a stargate)....but i reckon someone should try them ...i think u will find the airgap on these automotive relays would suffice theyre not overly delicate

The contacts for relays are rated and made completely differently for switching DC or AC voltages. In a DC application, the current (Amps) rises and stays high until the relay is turned off, opening the contacts and breaking the circuit. An electrical ark is formed as the contact points separate, but is extinguished as the contacts get farther apart. The contacts need to be very heavy to withstand the ark without blowing apart or worse, welding shut. How fast the relay opens and how heavily built the contacts are will tell you how long the relay will last switching DC voltages.


In an AC rated relay, the current (Amps) rises and falls at 60 Hz (50Hz in other areas), following the AC sine wave of the voltage. When the relay contact opens an ark is formed, but is extinguished as soon as the current crosses over the zero point on the sine wave. Remember the voltage and current are rising and falling 60 times a second. Distance between the contact points and how large the contact is become less of a factor. A DC rated contact will be much larger and heavier than an AC rated Contact for the equivalent amps and voltage.

Doesnt matter that it is not rated at 240V does it? I mean it is being triggered by the loss of 12V from the mobile phone charger and it is switching a 12V pmp on - so no 240V involved other than powering the mobile phone charger...

Hamish, the relay is " initially" holding240 volts to 240 V pumps etcand "when" the240 volts dissapears ( black-ou) it is then supply 12V to the 12 V pumps / aerator etc.

I though the relay was being held open by 12V from the mobile phone charger. When the 240V is lost (power cut) the 12V hold charge dissapears and the 12V backup pump is operated. I still cant see why this needs to be rated at 240V. I must be thick. I will shut up now

Hamish,, your not thick. In fact it could be wiredthatway,,but by using the one relay to supply 240 OR12 V you can never have bothatthe same time,,like a sort of insurance policy in caseyou get black-out/poweron/black/on etc several timesin afew seconds.

I use one of these:



Backup Power Switch, .

While there is 240 volt power the relay is held open and no 12 volt power flows to the 12 volt water or air pump.
When 240 volt goes off the relay closes and 12 volt power flows through from 12 volt battery to the aeration device.
The instant the 240 volt goes off the 12 volt system will start, and the instant 240 volt mains power is restored the 12 volt system switches off.
My choice of aeration device is a 12 volt marine bilge pump sending water through a small upright with a defuser spray head at the top creating a waterfall effect thereby breaking the water surface and providing very effective aeration of the fish tank.
The device will also switch a second pump in the sump to clear water back to the fish tank.
This function makes it worthwhile to set up this system. If the mains power fails and there is still a large volume of water in the grow beds that will continue to drain into the sump. A 12 volt sump pump is very necessary to move this water back to the Fish tank, otherwise the sump will overflow and precious water will be lost.

The bigger the battery the longer the standby pump/device will run. The size of the battery is important.

The purpose of this system is to hold the fish tank aerated for an hour or two, as a safety device if power went out due to a storm or a car hitting a power pole etc, during the night or while you are at work.
I have learned the hard way the effects of no areation for very short periods of time, especially with high density fish stocking in aquaponic systems.
If there was a long term power outage due to a cyclone or similar, that would have the power out for several hours, or a day or more then you would only expect this device to aerate the fish tank until one could get a 240 volt generator set into place to run your normal pumps etc
To run a small 12 volt water pump, say 500 gph, a battery costing around 100.00 would run such a pump for 5 or 6 hours, more battery = more running time.

you could use that switch to power an inverter as well, if you wanted 240v gear.