UPS with too much boost

[John Canon]

I have a MinuteMan standby UPS 1500W unit for my entertainment center. It has a single stage buck/boost. Recently, several times per day, it would click and the output voltage would read 134 volts AC. Most SMPS would accept that, but I noticed my box fan revving up speed.

I think the unit is going into boost mode at the wrong voltage. When it clicks the input voltage is around 109 volts AC.

Is there usually a trim pot on the UPS board to set the threshold/trigger voltage? I won't open it up until next weekend, right now it is on my workbench,

 

[kellys_eye]

Many UPS devices only have relays and 'taps' on an auto transformer to cater for changes in input voltage. It sounds as if yours has a switching level of around 110V before it 'clicks' to the next tap on the transformer, boosting the output to 134V.

Sounds as if you are NOT under any significant load therefore the voltage is a little high - it will likely drop to around 120V when the unit is operating at its designed load of 1500VA.

You 'might' be able to alter the switching point but if it ain't broke, don't fix it.

 

[John Canon]

Over the past week I run the UPS on my test bench, specifically monitoring the input and output voltages. The unit seems normal, and the line voltage has stayed around 116 to 118 vAC. I have not seen 109 vAC since that one day. When it happened the voltage would go down to 109 volts for about 20 seconds every 5 minutes. So I guess it was a glitch in the power company or building.