Simple Online UPS Circuit

In this post we learn about the making of a simple online uninterruptible power supply (UPS) which guarantees a seamless transfer of AC mains supply to inverter mains supply for the load, due to the absence of cumbersome transfer switches or relays.

What is an Online UPS

As the name suggests, an online UPS system stays continuously online, and never goes offline even for a split second, since the battery supply to the UPS inverter is held continuously connected, regardless of the mains AC situation.

During the period the mains AC input is available, it is first converted to DC and stepped down to the battery level.

This DC charges the battery and also takes precedence over the battery to simultaneously power the inverter due to its higher power rating than the battery. The inverter converts this DC back to the mains AC for powering the connected load.

In an event that AC mains fails, the stepped down AC to DC supply gets cut off, and the battery being continuously connected in line, now begins powering the inverter seamlessly, without any interruption of power to the load.

Online UPS vs Offline UPS

The main difference between an online UPS and an offline UPS is that, unlike offline UPS, the online UPS does not depend of mechanical changeover relays or transfer switches for transiting from AC mains to inverter mains AC during an AC mains failure (as shown below).

On the other hand, Offline UPS systems as shown in the below block diagram, rely on mechanical relays for transferring the UPS to the inverter mode, during the absence of mains AC supply.

In these systems when mains AC is available the supply is directly supplied to the load via a set of relay contacts, and the battery is held in the charging mode through another set of relay contacts.

As soon as AC mains fails, the relevant relay contacts deactivate and switch the battery from the charging mode to inverter mode, and the load from grid AC to inverter AC.

This implies that the transfer process tends to involve a slight delay, albeit in milliseconds while changing over from the grid mains to the inverter main.

This delay though small could be critical for sensitive electronic equipment such as computers or micro-controller based systems.

Therefore the online UPS system seems to be more efficient than an offline UPS in terms of speed and smoothness, during the changeover process from grid AC to inverter AC for all types of appliances.

Designing a Simple Online UPS/Inverter Circuit

As discussed in th above sections, making a simple online UPS actually looks quite easy.

We will ignore the EMI filter for simplicity sake and also because the inverter in our design will be a low frequency (50 Hz) iron-core transformer based inverter, and the SMPS would already include built in EMI filters for the necessary rectifications.

We will need the following materials for the basic online UPS design:

• A ready made Mains AC to DC 14 V 5 Amp SMPS module.
• A battery over charge cut-off system with constant current charger circuitry.
• A battery over discharge cut-off circuit stage.
• A battery 12 V / 7Ah
• Any simple Inverter circuit from this website.

Circuit Diagrams and Stages

The various circuit stages for the proposed online UPS circuit can be learned from the following details:

1) Battery Cut-off Circuits: The circuit below shows the very important battery over-charge cut off circuit, built around a couple of op amp stages.

The left side op amp stage is configured to control the over charging of the battery. The pin#3 of the op amp is connected with the battery positive for sensing its voltage level. When this battery voltage at pin#3 exceeds the corresponding pin#2 zener value, the op amp output pin#6 turns high.

This activates the relay via the BC547 driver transistor causing the relay contacts to shift from the N/C to N/O, which cuts off the charging supply to the battery, preventing over charging of the battery.

The feedback hysteresis resistor across pin#6 and pin#3 of the left op amp causes the relay to latch for certain period of time, until the battery voltage drops to a level below the holding threshold of the hysteresis, which causes the pin#3 to go low, and correspondingly pin#6 also goes low, switching off the relay. The relay contacts now switches back to the N/C, restoring the charging supply to the battery.

Over Discharge Cut OFF Circuit

The right side op amp controls the over discharge limit of the battery or the low battery situation. As long as the pin#3 voltage of this op amp stays above the pin#2 reference level (as set by the pin#3 preset), the op amp output continues to be high.

This high output at pin#6 enables the attached MOSFET to remain in the conduction mode, which allows the inverter to be switched ON through the negative line.

In an even that the battery is over-drained by the inverter load, the op amp pin#3 level drops below the pin#2 reference voltage, causing pin#6 of the IC to go low, which cuts off the MOSFET and the inverter.

Current Control Stage

The BJT associated with the MOSFET forms a current control circuit for the online UPS, which allows the battery to be charged through a constant current level.

R2 must be calculated to set the maximum current control level for the battery and the inverter. It may be implemented using the following formula:

R2 = 0.7 / Max Current

2) Inverter Circuit: The inverter circuit for online UPS system, which needs to be connected with the above battery controller circuit is shown below.

We have selected an IC 555 based circuit for simplicity sake and also for ensuring adequate power output range.

This inverter will remain online as long as the charger circuit and the battery remains functional, and the grid AC mains is fed appropriately to the system via a AC to DC SMPS circuit rated at 14V, 5 amp, or as per the particular power rating of the system, which is fully customizable.

The BJT feedback across the gates of the inverter MOSFETs ensures that the output voltage of the inverter never exceeds above the safe level, and is fed in a controlled manner.

This conclude our simple online UPS circuit design, which ensures a continuous uninterruptible online power to any AC load, which needs to be functional without any interruption regardless of the input AC availability.