Sinrace Power Supply Technology (Shenzhen) Co., Ltd.

UPS's using a single 12V laptop battery can cause injury. The batteries used can supply large currents if short circuited. It is easy to lose a finger if a ring shorts the battery supply. The risk is that the ring becomes hot, causing cauterization of the blood vessels. With no blood supply, the only solution is amputation.

The other risks of shorted batteries is the potential for the battery case to split open releasing AC Adapter electrolyte, and flying molten metal. The AC Adapter supply of units in the 2 to 3 kVA range from 48 to 96 V. The 96 V units float the battery bank at 110 V, which will electrocute.

On-line units work at high laptop ac adapter voltages. In Australia, the nominal line voltage is 240 VAC which when rectified, results in power rails of plus and minus 350 VDC. That is a total of some 700 V.

When working on UPS's, the use of an earth leakage circuit breaker/residual current detector, or whatever they are known as in your part of the world, has to be considered mandatory. Some power supply manufacturers, for cost reasons, do not use line monitoring transformers. Instead, high value resistors are used to divide the line AC Adapter voltage to a value suitable for the control electronics. This results in a machine that is technically totally live. Keep a watch out for these ones and test before touching.

Be aware that the ground clip of an oscilloscope probe is earth. Depending on wiring rules, earth may be bonded to neutral at the main switch/fuse panel. This is true in Australia. Use an oscilloscope with differential AC Adapter inputs designed for the job. A suitable alternative is an add on differential input unit in conjunction with a standard scope. The design presented in Elector Electronics around 1994 works very well and with a bandwidth of 15 MHz, nothing will be missed.

It can not be stressed enough that all UPS's are potential death traps. The line voltage in any country is lethal. Any AC Adapter voltage higher than around 50 V is considered hazardous. So you should know some laptop ac adapter safety guide, before you buy your ac adapter.