I have an old UPS unit that I found in a trash pile at a Hamfest = probably 5=20 or more years ago, and I figured I might be able to use it as a 12 VDC = to=20
120 VAC inverter. So I opened it up, and removed the two 6V SLA = batteries=20 (which had cracked, probably from freezing). I had thought the weight of = the=20 unit (about 22 lb.) was from the batteries, but there is also a rather = large=20 (3.5" x 4" x 4") transformer which probably weighs at least 10 lb. I had =expected a much smaller ferrite transformer such as found in most = inverters.
So I thought maybe it was used in such a way as to create a good = waveform,=20 but when I powered the unit with a 12V supply (1.08A), I found that it = was a=20 "modified sine wave" which was just a series of positive and negative = pulses=20
175 V peak and 2.6 mSec wide with a period of 17.2 mSec (57 Hz), and it = has=20 an overshoot at turn-off. This indicates that it is not being driven by = a=20 true bridge but just a very simple PWM push-pull drive. I found two heat =sinks, each of which have four TO-220 devices, and I found a rectangular =
pulse train 11.5 volts and 3.6 mSec wide, so they must be MOSFETs.
It has quite a large and complex PCB, with a 4" x 6" section having = about 10=20 DIP ICs and associated components. No SMDs. The unit has a DB-9 serial = port=20 for computer connection. The date on the PCB is 1997. There does not = appear=20 to be any sort of microprocessor or "smarts", and the only IC I did not=20 recognize was an MF4CN-50 which is a low pass filter=20
I don't think there's much value to this thing, except maybe the big = tranny=20 which might make a good high current low voltage supply. Probably about =
15=20 VCT at 30 amps or so.Are more modern UPS units made this way? Seems strange not to use high=20 frequency magnetics to generate a 175 VDC rail and then use a bridge to=20 generate the output waveform. And now that it's easy to make a PWM = inverter=20 with a high frequency carrier like a motor control, a fairly good sine = wave=20 could be made. And a crystal could be used to get a very accurate 50 or =
60=20 Hz waveform, although it's usually not much needed except for an alarm=20 clock. Actually, most computer equipment uses switching supplies and=20 probably would operate just as well, if not better, with the straight DC =rail (although perhaps at less than the 175 volts).
Any thoughts? Thanks.
Paul=20