Automatic Micro UPS Circuit

The following article discusses a simple automatic micro UPS circuit which can be used with modems for acquiring uninterrupted power from a DC source, and battery during mains power failures. The circuit also incorporates an automatic over charge cut off, and a low battery indication feature. The circuit was requested by Mr. Kapil Goel.

Technical Specifications

Hi Swagatam, How are you, and was really happy to read your blog as I was scrolling through circuit sites for my requirement.If you can help me out for that, I have a requirement:

This is exactly what my requirement is: I have a 12volt operated device, it consumes approx 35 watts right now I power it up using a 12volt adapter, but when main power fails its get rebooted..

I wanted to use 12volt 2200mh Li-ion battery pack so that whenever there’s a power cut it will automatically shift to battery.

Also, the circuit should have over charge protection, and low battery indicator.

At last I am not asking this circuit for free, as I am ready to pay for it. Many thanks in advance

Regards, Kapil Goel

UPDATE: New Simplified Micro-UPS Circuit

No need to build the op-amp based complex circuits explained in the following article. You can simply build and the implement the below shown simple micro-UPS circuit for your modem UPS application:

The zener diode D1 value must be selected such that the output voltage across R3 is slightly less than the maximum full-charge level of the battery. For example, if the maximum full charge level of the battery is 14.3V, then select the D1 value appropriately which generates around 14V across R3.

The R2 value can be calculated using the following formula:

R2 = 0.6/Battery charging current.

For lead acid battery, the suitable battery charging current can be around 10% of the battery Ah, and for Li-Ion battery this can be around 50% of the battery Ah.

How it Works

The design was actually presented in one of my earlier posts also, however it does not include an automatic over charge cut off feature.The present design has similar functions, but has an added protection feature in the form of an automatic battery over charge cut off and also an under voltage indicator.

The proposed circuit diagram of an automatic micro UPS may be understood with the following points:

The input supply is acquired from any standard AC/DC adapter rated anywhere within 15 and 19V DC, current at anything above 1.5 amps.

The above supply is regulated via a 7812 IC whose ground pin is elevated to about 2.4V so that the output from the IC gets raised to about 14.4V rather than the normal 12V.

This is required because the attached 12V battery needs to be supplied with a slightly higher potential than its rated value.

How the IC 741 is Configured

The 741 IC stage is configured as a comparator.

Its pin#2 is clamped to a fixed reference voltage of 4.7V using a suitably rated zener diode.

Pin#3 is rigged as the sensing input if the IC via an adjustable preset.

The preset is adjusted such that the potential at pin#3 just exceeds the potential at pin#2 when the battery voltage crosses the 13.5V mark.

As long as the above situation is not sensed, the output of the IC at pin#6 sticks to it initial zero voltage level which in turn keeps the BC547 transistor switched OFF. With BC547 being switched OFF, the TIP122 gets a chance to conduct via the 1K resistor and charges the connected battery.

The battery terminals are directly connected with the modem which is being used for some application.

This allows the modem to remain powered via the external AC/DC adapter while the battery gets charged simultaneously.

The battery is allowed to charge freely until it reaches the over charge threshold when the output at pin#6 of the IC goes high, switching ON the connected BC547 transistor.

The above switching cuts off the base bias to the TIP122 transistor and stops the battery from getting further charged. This does not affect the modem as it continues to acquire power from the external power supply.

During mains failure, the supply from the external adapter gets inhibited, and the modem starts receiving back-up supply from the battery.

Since no relays are used the transition is within micro seconds which keeps the supply to the modem interrupted during power failures or even under heavy power fluctuations.

If the mains stays absent for long, and the battery reaches its over discharge threshold, the situation is immediately indicated with the green LED, which can be also replaced with a buzzer. The modem should be switched OFF then, to stop damage to the battery due to over discharge.

The adjustment of the 100K preset determines the low voltage threshold mark or the lower indication. level.

Once the green LED is lit, it will remain lit until the battery is fully charged, similarly once the red LED illuminates, it will stay illuminated until the green LED lights up or when the battery voltage level falls below the set lower threshold.

Using a PNP BJT for the above Charger Circuit

The above circuit can be also configured in the following manner, here the LED indications get reversed, meaning red LED shows low voltage while the green LED indicates high voltage threshold.

The following circuit also incorporates a current limiting facility which can be used for providing a current controlled charging to the connected battery.

FEEDBACK from Mr. Kapil

Hi Swagat,
Thanks for the circuit.. I really appreciated your swift and kind response..
I have couple of questions on the same.
1) What will be the max current it will support, my device requires atleast 5 amps 12 volts, will this be able to handle that.

2)As per the circuit, I can see, you have directly connected the modem to the battery, but if I am not wrong, this means that modem will keep on taking the power from battery, and battery will not get charged?
Please I clear out this confusion.

Also I am using a li-ion battery, which has a voltage of 12.6 volts on full charge and 11 when discharged.

Also my input volt is also 12volts, I cannot use a higher volt rated adapter.. will it be able to charge my battery at fullest.
Regards,

Kapil Goel

My Reply

Hi Kapil,

Presently the above shown circuit is rated at 3 amps maximum, so I may have to amend the design to suit your requirements, however the input voltage will need to be above 13V otherwise the battery will never get optimally charged.

The direct connection of the battery with the modem will not affect the battery charging as long as the input source power is active....both outputs will be simultaneously taken care of.Regards.

The modified 5 AMP micro UPS circuit design: