Thread: UPS Inverters

Hi All,

Came across a recent AULRO thread re: turning old UPS (uninterruptable power supply) into a 12/24v - 240V inverter for use in 4WD, caravan and motorhomes.

As an Electrical & Electronics technician, I would advise extreme caution using these devices for anything other than their intended use because there is a real possibility of ending up very dead (either by electrocution or fire)

Why...?

Because the vast majority of them do not have sufficient isolation between the output and the battery terminals that supply the drive current. In other words, if installed in a vehicle/caravan/motorhome, it is possible for other 12/24V equipment in the local environment becoming live and deadly if you happen to be hanging on to the low voltage supply for any reason.(and that occurs far more often than you realise)

Whereas purpose built 12v/240v inverters mostly have better than 1kV isolation, UPS devices generally don't because the unit is pretty much sealed so the assumption is that access to the LV side is non existant.

Additionally, most UPS are designed for a very low duty cycle, which is usually directly proportional to the current capacity of the enclosed battery.

For this reason, heatsinking of the switching MOSFETS is generally very limited and designed pretty much for the pre-programmed off time of the UPS or alternatively when the enclosed battery reaches a low voltage condition (mostly around 11.8 volts)

I opened a 600VA UPS this morning and it is immediatley evident that the heatsinking at even 50% duty cycle is woefully inadequate. To confirm this, I loaded the UPS with a 300W non inductive load and it crapped out under thermal overload after just 20 minutes.(100% duty cycle) Heatsink temperature was 135C.

The real concern is that not all UPS have thermal overload protection, with many of them relying on low battery voltage shutdown. If you bypass the battery with a constant voltage supply (or larger capacity battery), the unit may overheat and start a fire.

The irony of it all, is that the vast majority of UPS are modified sine wave and not pure sine wave. The reason for this is really quite logical, because they only need to feed the switched mode power supply in the computer or server which in turn converts everything to DC before switching/chopping it back and recifying it to the desired LV supply rails.

You can buy a 600W modified (quasi) sine wave inverter with >1kV isolation for under $200. Even 1500W modified sine wave are not that expensive, so the question has to be asked....what is your life worth?

I hope the above prevents an untimely death!

regards
Marty
VK6KCI

Very good advice thanks for that.