In this post we learn how to trick an inverter's "no load auto-shutdown" feature through an external circuit so that the inverter may be kept running even with minor, below permissible loads at the output. The idea was requested by Mr. Em.

Technical Specifications

Thanks for your prompt responses and guidance.

I was wondering If you could enlighten me regarding a peculiar problem related to No Load/Low Load Auto Shutdown phenomenon of UPS Circuits.

I have two UPS'es that are rated for 360 Watts. Unfortunately both have a load sensing mechanism thanks to similar micro controller circuit.

The battery back up time is fine if load is greater than at least 60-80 Watts. However, I only tend to use a humble 10 watt power WiFi Router with the UPS.

This tiny load is considered insignificant and ignored by the UPS circuit during power cut and it initializes a no load auto shut countdown of 300 seconds (5 Minutes). After the auto shutdown I can again restart the UPS, it beeps a little and gives another extension of 5 minutes .. and so on..

Collectively it can give back up of at least an hour, except for the 5 minute interrupt sequence which causes inconvenience.

Can I trick the circuit somehow to extend the time during no load ? I read in an "Ancient book" that No-Load Auto Shutdown could be disabled by biting off a resistor in the load sensing circuit of UPS. I have no idea which resistor could it be ..

Pardon me for the multiple shots,

I tried to give a better view..

Solving the Circuit Request

Thanks Em, yes you can trick it through an external circuit arrangement, because trying to modify the internal circuitry could be risky, unless you are absolutely sure about the proceedings.

you can make a 4 minute 555 IC relay timer circuit with a 2 second ON time and 4.8 minute off time. and connect a 25 watt bulb load at its output relay via the UPS AC...

so this timer will switch ON for a second or two and connect the load to the UPS, forcing the UPS to extend and reset the output switch ON timing every after 4 minutes for the next 5 minutes.

I hope this might help.


Hi Swagatam,
Thanks for the guidance, it worked..
I followed the second half of your suggestion about using a 25 watt load and it served my purpose to a significant extent. I connected a 25 Watt Bulb to the UPS, now  it gives a run time of almost one hour and 10 minutes,
 Have been using cum testing it for the past 10 days..and feel highly pleased..despite it being little inefficient.

Now coming back to the 555 timer, I found the IC on ebay, How do I program the timer for 2 second ON and 4.8 Minutes off ??
Any special equipment needed ?

Adjusting the IC 555 Output

Im glad it worked Em! for adjusting the 555 circuit you can take the help of any "online 555 calculator" and set the values of R1, R2 and the C of your circuit accordingly by matching the results presented by the software, through some trial and error.

There's another similar but more efficient method that can be tried using an IC 555 astable and a triac, as shown below:

The above circuit will rapidly switch ON and OFF the load (25 watt bulb) at the indicated rate (2.4 seconds OFF, 0.03 seconds ON) keeping the load just dimly lit, and also tricking the inverter to "think" that the inverter is loaded, preventing the auto shut down.

Adding a Capacitor

One of the readers commented, saying why not add an appropriately rated capacitor across the output of the inveter, so that it will trick the inverter simulating it as a small load.

The idea looks simple, smart and effective,

So, instead of going through all the complex designs explained in the above sections, you can simply try adding a high value capacitor such as a 1uF/400V across the inverter transformer output, and solve the no load inverter shut down problem within minutes.

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