Reversing the Function of a UPS transformer for a DC Power Supply!

I had a UPS which had gone out of order. Took out the transformer for repurposing into a DC power supply as it was a heavy duty one. But I did not know whether it is a 6V or 12 V one. In the UPS, the transformer steps up a low voltage to a high voltage for the computer. I am going to use the transformer in the reverse order as a step down transformer to convert mains supply to a lower voltage for a DC power supply. This is how I proceeded.