I have only rudimentary knowledge of electronics.

I live in South Africa, where we are now experiencing rolling blackouts. (Euphemistically called "Load Shedding" here.) During the day it's just a matter of waiting for the blackout to end after about 2 & half hours. But, as you can imagine, at night it becomes more of a problem.

To lessen the impact, I have installed a parallel 12 V DC lighting circuit. I have run solid 20 A copper wires to each regular AC light fitting (isolated from the AC power of course) and stuck 3 W LED lights in each. These run from a 12 Ah sealed 12 V battery located on top of a kitchen cabinet, which, when there is power is kept on a float charger.

Although the LEDs don't provide enough illumination for general lighting, they do at least keep us from walking into the furniture.

At the moment, when necessary, I walk to the kitchen in the dark with a torch (flashlight) and turn on the emergency lights.

I would like to have the emergency lights come on automatically when the power fails but am not sure how to achieve this. I did manage to achieve an automatic switch-over when experimenting on the bench. I used a regular AC/DC relay and this seems to work OK but I have some concerns.

For instance, in my experiments, I used the interruption of the power to the float charger to trigger the relay. I'm concerned that I am playing around with 220 VAC current and that, on the DC side, the relays has to be powered full time, in order to keep the Normally Closed switch open. I'm worried that this may eventually lead to contact corrosion and eventually arcing with the resultant fire hazard.

I think there must be a simpler and safer solution.

Edit: -

Thanks for the comments so far and thanks to AnalogKid for setting my mind at ease regarding power relays.

I'm sorry, but I am not sufficiently knowledgable regarding electronics notation to use Cicuitlab, which I have never heard of before now.

It's been a while since I played around with the relay, so I don't remember exactly what I did at the time. (We have had various bouts of blackouts over the last few years.)

I will have to try the relay again and let you all know.

I have had a thought though that never occurred to me before: The wire running to the battery from the float charger, has to be DC and probably a bit over 12 Volts. So, I don't have to deal with AC power at all, as I did in the past.

The relay could be located between the wall wart of the charger and the battery terminals. When the charging current is interrupted, it could trigger the relay.

I'll give that a try and let ya'll know


  • \$\begingroup\$ Would you mind sharing a sketch or schematic of how you have the relay currently configured? It would help us to better understand your configuration and concerns. \$\endgroup\$
    – nanofarad
    Commented Mar 23, 2019 at 14:39
  • \$\begingroup\$ Note that when you use the CircuitLab button on the editor toolbar an editable schematic is saved in your post. That makes it easy for us to copy and edit in our answers. You don't need a CircuitLab account, no screengrabs, no image uploads, no background grid. Double-click a component to edit its properties. 'R' = rotate, 'H' = horizontal flip. 'V' = vertical flip. \$\endgroup\$
    – Transistor
    Commented Mar 23, 2019 at 14:40

1 Answer 1


A relay is the most simple solution, and very safe. Most power relays are designed for continuous operation. The coil warms up a bit, but not enough to cause insulation to break down, ignite something nearby, etc.

Many relays have contacts rated to switch both AC and DC. Note that often the ratings for the two types of current are different. Choose a relay with contacts that are rated for at least twice the maximum DC current in your circuit.

Once you have selected a relay, edit your question to include the manufacturer and part number. A PDF file of the datasheet (or a link) will help us evaluate it.


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