A 'SuperCap' UPS

I must learn to keep my mouth shut! I mentioned, in passing, an idea I had for a relatively cheap and simple maintenance free single-PC UPS, and before one knows it, it's apparently being promoted around the country as someone else's idea.

If one investigates the standard UPS for stand-alone PCs, the number of DC-AC-DC conversions that take place are phenomenal. What's also frightening is each process takes a bunch of electronics, each capable of failing. If one could reduce this, then reliability is increased.

The following exceedingly simple UPS has become extremely suitable with the advent of LCD monitors that are powered from switch-mode power supplies (SMPS) that effectively use rectified mains. They no longer require the degaussing coil to operate on power-up (i.e. no need for the 50/60Hz).

The UPS is also not used very often, and most times it is simply needed to keep the PC alive during a quick dip in the mains. All that electronics when all one needs is a bigger capacitor! And, if the site has a generator then all one needs is a few seconds longer and mains is restored in any event.

Circuit Description

SuperCap UPS

D1..D4 and C1 rectify the incoming mains feeding this directly to the output of the UPS. This effectively replaces the rectifier in the SMPS in both the PC and monitor. R1 takes a portion of this rectified mains and slowly charges the bank of SuperCaps.

During power failure (which the PC and monitor will not even know has happened), D5 will conduct supplying the PC and monitor from the SuperCaps.


  • D1..D5 must be large current diodes as they handle the full inrush current of C1, the PC, and monitor.

  • For a 230V AC supply it would be advisable to cater for up to 360VDC, e.g. 20 x 18V caps.

  • The zeners must be able to handle full reverse current required by the PC and monitor as each cap will not discharge at the same rate.


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© 02.03.04