Hi, a friend of mine gave me an older APC uninterruptable power supply (Backups 400) unit. It's working quite alright, it just needed a new 12V battery and all was well or so I thought. I measured the output of the plugs when running from mains and it's 120VAC like it should be. When I disconnect the mains it lowers itself to about 100VAC. I tried adjusting the resistor for v out and it only raises it about a volt or two. Is this normal for a UPS or should I investigate further?
Check battery voltage,Check (i asume mosfet half-bridge,H-bridge)one of the mosfets is not switching properly or check for dry connections from xformer to pcboard and around transistors or open drive resistor(s).If oscilloscope is available do a wave check on either side of the xformar(centre tapped)from the centre tap.I am not familiar with that specific ups but at some stage there should be a circuit regulating the ac output to about 110V to compensate for battery voltage drop.Check around there as well for dry electrolytic caps. Cheers.
Thanks for the reply, I'm awaiting my new digital scope for a few more days. Yes, the battery voltage is a good 13 volts :) So all I can really do at the moment is look around and ponder. When you say dry capacitors, I'm assuming you mean the solder joints as well?
I was reading anoth thread about a 220/230VAC inverter and saw audioguru's reply stating that the waveform generated by the inverter is more complicated than a regular multimeter may be able to analyze. Is it possible the UPS may be outputting a modified sine wave of 120VAC but my multimeter is only reading 100VAC http://www.aaroncake.net/forum/topic.asp?TOPIC_ID=7698
Most ups output a square wave wich is more acurate than a sinus wave the reason for that is i think that if you look at a sinus wave the efficiancy is at about 95%and that you can actually fit a square wave into it.It all comes down to usable power and should not be less than 5% and by the looks of it you got about 20% loss there.You can do another experiment and install a bridge rectifier(without filter cap)and then check the voltage but i doubt if there is gonna be any(much) difference.What i noticed as well is that some ups use a 7-10V(14-20V centre tapped) winding on either side and by chopping the square (regulating the square to 7V) you get a more reliable(regulation) output when the battery starts to drain.
Ok, so I checked out the black wire (Hot) and got this picture on my scope. I'm actually amazed than anything runs off this waveform but then again looking to the output of a ups is all new for me. Any tips I can get on correctly grounding the scope to the ups? I'd really like to not kill my oscilloscope by trial and error
Old UPS not delivering 120VAC
Posted - Sep 29 2008 : 6:10:39 PM
SINE.BMP
scope640475.jpg