I recently designed a system based on toradex iris carrier board. Application was simply using a UART channel to interact with other systems. There are 10 digital IR sensors are used and interfaced with GPIOs. As they are digital, so when they get activated, it gives 3.3v and when low, it shows 0v. Based on this data and algorithm, a final data is send to UART and ethernet. So a UART port and ethernet is also used. This is about working of the application.

Now we need to use UPS which can give it a power backup upto 20-30mins. UPS comes in different ratings like: 1KVA or 3KVA. I am facing few problem in selecting the proper rating as I do not have much knowledge about it. I started searching and found out that, lets say 1KVA UPS can hold a load of 0.8Kw (given that power factor is 0.8). That means 800W. We have 220V mains power supply here, so it can give upto 3.6A current.

Is this current sufficient for the system. I searched alot and summed up all the current consumption in my system:

Toradex    3A ( Datasheets say it can take upto 6A but I will not be using all the components like display, sd cad, hdmi port. So assuming 2A)

Sensors    50mA * 10 = 0.5A

Led, Coupler, Diode  = 0.5A

If I roughly calculate the total current, then it comes out to be 4A-5A which I think is too high for this kind of application. So I want to know how exactly everyone calculate the total current and then how to select the UPS to provide it a backup

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    \$\begingroup\$ Your calculation gives the required current at 5 volts. The UPS is producing 220 volts, so the current your system draws from the UPS will be roughly 5/220 * 5 Amp = 0.1 Amp. It is best to think of power required, rather than current drawn, when dealing with power at different voltages. \$\endgroup\$ – Peter Bennett Aug 20 '16 at 16:13

The data sheet says the Toradex needs 17.5 watts from the 5V rail and 8.25 watts from a 3V3 rail. Total power is circa 30 watts including inefficiences of a typical switching power supply (that you'll have to buy or design).

If the sensors run from 5V (guess) and take a total of 0.5 amps that's another 2.5 watts. If the LEDs take the same (assuming a 5V drive and current limiting resistor) you are looking for at a total power well below 50 watts.

Your UPS has to supply this power.

It beats me how you came up with 800 watts but maybe you can clear this up.

  • \$\begingroup\$ Thanks. Well I did'nt came up with 800 watt. 800 watt is the total load 1KVA ups can hold. \$\endgroup\$ – S Andrew Aug 20 '16 at 10:14
  • \$\begingroup\$ The ups will supply the power needed by the load so, if only 50 watts is required then you can estimate from the ups documentation how long the battery will last. You are looking for a kWhour number. \$\endgroup\$ – Andy aka Aug 20 '16 at 11:02

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