Small Project of Mini UPS system

This is a simple circuit that’s provides an uninterrupted  power  supply  (UPS) to  operate  12V,  9V  and  5V  DC-powered at up  to 1A current. The backup battery takes up the load without spikes or delay when the mains power gets interrupted.  It can also be used as a workbench power supply that provides 12V, 9V and 5V operating voltages. 

The circuit automatically disconnects the load when the battery voltage reduces to 10.5V to prevent deep discharge of the battery. LED1 indication is provided to show the full charge voltage level of the battery. Miniature white LEDs (LED2 and LED3) are used as emergency lamps during power failure at night.

step-down transformer provides 12V of AC, which is rectified by diodes D1 and D2. Capacitor C1 provides ripple-free DC to charge the battery and to the remaining circuit. 

When the mains power is on, diode D3 gets forward biased to charge the battery. Resistor R1 limits the charging current.  Potentiometer VR1  (10k) with  transistor T1  acts  as the  voltage  comparator  to  indicate the voltage level. VR1  is so adjusted that LED1 is in the ‘off’ mode. When the  battery  is  fully  charged,  LED1 glows  indicating a  full voltage  level of 12V.

When  the  mains  power  fails, diode  D3  gets  reverse  biased  and D4  gets  forward  biased  so  that  the battery  can  automatically  take  up the  load  without  any  delay.

When the battery voltage or input voltage falls below 10.5V, a cut-off circuit is used to prevent deep discharging of the battery.

Resistor R3, zener diode ZD1  (10.5V)  and  transistor  T2  form the  cut-off  circuit.  When  the  volt- age  level  is  above  10.5V,  transistor T2  conducts  and  its  base  becomes  negative (as set by R3, VR2 and ZD1).

But when the voltage reduces below 10.5V, the zener diode stops conduction and  the base voltage of  transistor T2 becomes positive. It goes  into  the  ‘cut-off’ mode  and  prevents  the  current  in  the  output  stage.  Preset VR2  (22k) adjusts  the voltage below  0.6V  to make T2 work  if  the voltage  is above 10.5V.

When  power  from  the  mains  is  available,  all  output  voltages—12V,  9V  and  5V—are  ready  to  run  the  load.  On  the  other  hand,  when  the  mains  power  is  down,  output  voltages can run the load only when the  battery is fully charged (as indicated  by LED1).

For  the partially  charged  battery, only 9V and 5V are available.

Also, no output is available when the voltage goes below 10.5V.  If battery  voltage  varies  between  10.5V  and  13V,  output  at  terminal A may  also vary  between  10.5V  and  12V, when the UPS system is in battery mode.

Outputs at points B and C provide 9V and 5V, respectively, through regulator  ICs  (IC1 and  IC2), while output A  provides  12V  through  the  zener diode. The emergency  lamp uses  two ultra-bright  white  LEDs  (LED2  and LED3) with  current  limiting  resistors R5 and R6. The lamp can be manually switched ‘on’ and ‘off’ by S1.

The circuit  is assembled on a general-purpose  PCB.

There  is  adequate space  between  the  components  to avoid overlapping. heat sinks for transistor T2 and  regulator  ICs (7809 and 7805) to dissipate heat are used. The  positive  and  negative  rails  should  be  strong  enough  to  handle  high  current. Before  connecting  the  circuit to the battery and transformer,  connect it to a variable power supply. 

 Provide  12V  DC  and  adjust  VR1  till  LED1  glows. After  setting  the  high  voltage  level,  reduce  the  voltage  to  10.5V  and  adjust  VR2  till  the  output  trips  off.  After  the  settings  are  complete, remove the variable power supply and connect a fully-charged battery  to  the  terminals and  see  that LED1  is  on. After making  all  the  adjustments  connect  the  circuit  to  the battery  and  transformer. The battery used in  the  circuit is a 12V, 4.5Ah UPS battery.  

Resistor :

R1= 68 ohm

R2= 1k

R3= 1k

R4=47 ohm

R5= 390 ohm

R6= 390 ohm

Variable Resistor:

VR1= 10k

VR2= 22k

Diode:

D1= 1N4007

D2=1N4007

D3=1N4007

D4= 1N4007

Zener Diode :

ZD1= 10.5V, 0.5W

ZD2= 12V, 1W

LED:

LED1= Red light (normal)

LED2= White

LED3= White

Capacitor:

C1= 470µF ,

Transistor :

T1=BC548

T2= TIP127

IC :

IC1= 7809

IC2=7805

Transformer = 230V AC 50Hz Output 12V, 1A