Question:

When designing a power supply system for a machine, we would like to include an Uninterruptible Power Supply, or UPS, to protect that machine against the problems associated with power supply issues.

How do we determine which UPS to use for the machine in question?

Answer:

When selecting a UPS, such as the Omron S8BA UPS, there are two primary characteristics to consider, namely Output Capacity and Backup Time.

Output Capacity:

Determine the maximum power consumption of the devices that will be receiving power through the S8BA UPS. This can be done by locating the power consumption specification in the datasheets of those devices, and adding them all together. Note that if a value is note provide in Watts, this can be calculated by multiplying the supplied voltage by the current drawn by the device.

The output capacity of the S8BA UPS must be greater than this maximum power consumption figure.

Note that the switch mode power supply that provides power to the S8BA UPS and the connected devices will require an output capacity greater than the sum of the maximum power consumption of the connected devices and the power consumption of the S8BA UPS itself.

For example, if we were connecting an Industrial Computer (IPC) with a maximum power consumption of 70W to the S8BA UPS, we would require an S8BA UPS with an output capacity of greater than 70W. As such, we can select the 120W model of the S8BA UPS. Now, we also need to select an appropriate switch mode power supply. To do this, we find that the maximum power consumption of the 120W output capacity S8BA, which is 22W, and add it to the 70W from the IPC, giving a new maximum power consumption of 92W. Therefore, an appropriate switch mode power supply for this system will have an output capacity greater than 92W.

Backup Time:

In order to select an Omron S8BA UPS with an appropriate backup time, we first need to decide how much backup time is required. Many power supply issues are extremely brief, while others can last minutes or even hours. Further, we need to consider if the role of the S8BA UPS is to keep the connected machine running, or to supply power to critical elements of the machine for long enough to enable a safe and routine shutdown.

The result of considering these factors is highly dependant on the specific machine, including its use and surrounding environment.

To calculate the actual backup time, first find the total power consumption of the connected devices, then compare it to the below graph, where total power consumption is on the horizontal axis, and the backup time is on the vertical axis.

The different curves on this graph refer to different models of S8BA UPS units, with different maximum output capacities.

Note that the real backup time may vary depending on the age of the UPS, as well as the ambient temperature. The above graph is for an unused S8BA at an ambient temperature of 25 degrees Centigrade.

For example, if we continue from the case used when determining output capacity, we have a total power consumption from the connected devices of 70W, and we have determined that the 120W S8BA would be appropriate. Comparing these values to the graph, we find that the approximate backup time would be 10 minutes. If we required a longer backup time, we can see that the 240W S8BA can provide approximately 25 minutes of backup time, and so on as the capacity of the S8BA increase.