UPS as the name
suggests means Uninterrupted Power Supply. The function of a UPS is to keep the
supply voltage as it is even during power cuts.
Let me explain
how it works. In side a UPS ( which we use for personal computer) there is a
storage cell or battery, an electronic switch, an inverter and a battery
charger. Normally, when we start a UPS the main supply is present. The out put
line is directly connected to the main supply and the battery is charged if
necessary. During power failure, the main supply suddenly goes to zero then the
electronic switch which is mainly a power transistor, changes over to the
inverter. The inverter turns on immediately and the stored charge in the battery
is inverted to get the required A.C. supply. This voltage obtained by the
inverter is fed to the load.
Parts List for Simple UPS
B1- 6V lantern battery
C1- 100µF, 16 V DC electrolytic capacitor
D1, D2- Silicon rectifier rated 25 PIV at 1 A or higher
Ry1- Relay, 6V DC approximately 20mA (Potter & Brumfield RS5D or
T1- 12.6V center-tapped filament transformer rated 1A or higher.
R1- 250kΩ, potentiometer for testing then replace with appropiate
R2- The output load
L2- Inductor. 22 Gauge, 10 -25 turns on a 1cm. Dia Soft Iron core.
Operation:- In this project we
are constructing a simple UPS an the basis of the idea behind a real UPS. This
simple UPS does not contain any charger and inverter. So, DC voltage is obtained
and there is no rechargeable battery.
The simple UPS
contain a step down transformer, a rectifier and a filter to get a stable 6V DC
supply from 230V AC mains. Then instead of a electronic switch we use a
mechanical switch, a relay. The normally closed (NC) terminal is
connected to the 6V battery and the normally open (NO) terminal is
connected to the out of the 6V DC regulated power supply. The out put load is
taken from the common terminal. We add an inductor at the out put to maintain
the current during cut-off.
When the UPS is
turned on the main 230 V power is present. So the AC supply is step downed then
rectified then filtered to get a ripple free 6V DC supply. This voltage is fed
to energies the relay coil. The relay switch comm is now connected to the
normally open terminal and the load is directly connected to the DC source. When
power goes off the relay coil is de-energized and the comm terminal
is switched over to the normally closed terminal. That means now the
current is taken from the battery. In the mean time if main power is restored,
the relay coil again be energized and the comm terminal will be connected to the
rectified DC terminal and the battery will be disconnected. As we all know
inductance prevents the current to change rapidly in a circuit, we add an
inductance in series with load. We may add a small value capacitor across the NC
and Comm and NO and comm terminal of relay to prevent sparks.
A high value resistance should be connected in series with the relay coil. So,
that the standby current would be as minimum as possible. The resistance will be
chosen such that it must pass the least current required for the relay to
energized its input coil. We may use a pot of 100kΩ. The inductor is obtained by
a thick copper wire wound on a soft iron core. The more the value of inductor
the more the stable will be the current in the load during disturbance.
Application:- We can use the
simple UPS with any electronic devices operated in DC power supply like radio
walkman, audio systems or lights. You can construct the circuit for 3-18V DC if
you get proper step down transformer and battery.