Turn Your Old Inverter Into An Emergency Power System

An inverter turned into emergency power system, which turns on when the mains supply fails, and more importantly doesn’t turn on when the main supply is available -- Robin Chalana & Sani Theo

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Fig. 3: Rear panel of APC 800VA inverter showing input cord and output socket

An inverter turned into emergency power system, which turns on when the mains supply fails, and more importantly doesn’t turn on when the main supply is available

Can you give me a setup (emergency lighting system) where few of my lights will turn on when the supply from mains is unavailable, and more importantly, will not turn on when the supply from mains is available? That’s the question a reader asked us, who wanted to set up an emergency lighting system in his factory.

Fig. 1: Auto on/off circuit for inverter
Fig. 1: Auto on/off circuit for inverter

Our firt reaction was “why doesn’t he use an inverter?” But then we realised that a normal inverter would mean that the emergency light would be turned on when the power from main is there too—something the reader did not require. Quickly, we dished out an old inverter and made a few changes. We hooked our customised inverter to the wire powering his emergency light. It took us less than two hours to reconfigurethe inverter, and we got a happier reader. Now we share with you the changes made by us, in the hope to get even more happy readers.

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Construction
There are several ways to make the automatic on/off switching circuit for the inverter, but to make it simple and universal is slightly challenging. To make a universal circuit, a transformer, a regulator, a relay and a few discrete components are required. Fig. 1 shows the circuit of the home-made auto-on/off switch. The circuit assembled on a general-purpose PCB is shown in Fig. 2.

Fig. 2: Circuit assembled on a general-purpose PCB
Fig. 2: Circuit assembled on a general-purpose PCB
Fig. 3: Rear panel of APC 800VA inverter showing input cord and output socket
Fig. 3: Rear panel of APC 800VA inverter showing input cord and output socket

The circuit was tested on an APC 800VA inverter, which has 230V AC input and output sockets (refer Fig. 3). The primary side of the transformer is connected to 230V AC input line of the inverter. The 12V-0-12V, 250mA secondary output of the transformer is rectifiedby two diodes, filteredby a 1000μF, 35V capacitor and regulated by IC 7812. The 12V regulated output is fed to a 12V change-over relay through which the switching action takes place.

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Fig. 4: Connections of home-made on/off circuit inside APC’s 800VA inverter
Fig. 4: Connections of home-made on/off circuit inside APC’s 800VA inverter

When the 230V AC mains is available, the relay energises and its common pole shifts to the normally-open (N/O) contact. The 230V AC output from the inverter is not available at its output socket. When mains goes off, the relay de-energises and its pole shifts towards the normally-closed (N/C) contact. The 230V AC output from the inverter is now available through the N/C contact. Fig. 4 shows connection of the on/off switching circuit to the main inverter circuit.

If the inverter output load is known, this simple circuit can be inserted in any home inverter for auto-on/off switching. For a higher load, use a relay having a higher current rating.


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