The simple UPS circuit discussed here is a PWM based design, so becomes perfectly safe for operating sophisticated electronic equipment like computers, music system etc. The entire unit will cost you around $3. A built in charger is also included in the design for keeping the battery always in a topped up condition and in a stand by mode. Let’s study the whole concept and the circuit.
The circuit concept is quite basic, it’s all about making the output devices switch according to the applied well optimized PWM pulses, which in turn switches the transformer to generate an equivalent induced AC mains voltage having identical parameters to a standard AC Sine wave-form.
The circuit diagram can be understood with the help of the following points:
The PWM circuit utilizes the very popular IC 555 for the required generation of the PWM pulses.
The presets P1 and P2 can be set precisely as required for feeding the output devices.
The output devices will respond exactly to the applied PWM pulses from the 555 circuit, therefore a carefull optimization of the presets should result in almost an ideal PWM ratio that can be considered quite equivalent to a standard AC waveform.
However since the above discussed pWM pulses are applied to the bases of the both the transistors positioned for switching two separate chennels would mean a total mess, as we will never want to switch both the windings of the transformer together.
Using NOT gates for Inducing the 50Hz Switching
Therefore another stage consisting a few NOT gates from the IC 4049 has been introduced, which ensures that the devices conduct or switch alternately and never all at a time.
The oscillator made from N1 and N2; execute perfect square wave pulses, which are further buffered by N3---N6. The diodes D3 and D4 also plays an important role by making the devices respond only to the negative pulses from the NOT gates.
These pulses switch OFF the devices alternately, allowing only one channel to conduct at any particular instant.
The preset associated with N1 and N2 is used to set the output AC frequency of the UPS. For 220 volts, it must be set at 50 Hz and for 120 volts, it must be set at 60 Hz.
Parts List for the UPS
R1, R2, R3 R4, R5 = 1K,
P1, P2 = as per formula,
P3 = 100K preset
D1, D2 = 1N4148,
D3, D4 = 1N4007,
D5, D6 = 1N5402,
D7, D8 = 3v zener diode
C1 = 1uF/ 25V
C2 = 10n,
C3 = 2200uF/25V
T1, T2 = TIP31C,
T3, T4 = BDY29
IC1 = 555,
N1…N6 = IC 4049, please consult the datasheet for the pin out numbers.
Transformer = 12-0-12V, 15 Amps
The Battery charger circuit:
If it’s an UPS, the inclusion of a battery charger circuit becomes imperative.
Keeping the low cost and simplicity of the design in mind, a very simple yet reasonably accurate battery charger design has been incorporated in this uninterruptible power supply circuit.
You can get the entire explanation in this battery charger circuit article The two relays RL1 and RL2 are positioned to make the circuit completely automatic.When mains power is available, the relays energize and switch the AC mains directly to the load via there N/O contacts. In the meantime, the battery also gets charged through the charger circuit.The moment AC power fails, the relays revert and disconnect the mains line and replaces it with the inverter transformer so that now the inverter takes charge of supplying the mains voltage to the load, within milliseconds.
Another relay RL4 is introduced to flip its contacts during power failure, so that the battery which was kept in the charging mode is shifted to the inverter mode for the required generation of the back up AC power.
Parts List for the Charger
R1 = 1K,
P1 = 10K
T1 = BC547B,
C1 = 100uF/25V
D1---D4 = 1N5402
D5, 6, 7 = 1N4007,
All relays = 12 volt, 400 Ohm, SPDT
Transformer = 0-12V, 3 Amps