Help with Mustek UPS

I have this Mustek PowerMust 1400 USB UPS.

Power distribution network:

AC 230 V ± 10 %, 50 Hz

I get voltages always above 240 V during the 24 hous of the day. Sometimes I get close to the superior limit of AC 253 V.

My UPS is lowering down the Voltage to almost 200 V.

Check these screenshots:

The above image is at AVR mode. Take a look to the input and output.

This one is at Battery mode. The output is fine because this is a 220 V UPS.

I am concerned with the output in AVR mode. I wonder if this UPS is defective. I am also very concerned if this voltage below 200 V can damage my computer equipment. It´s below the limit values my Power Suply is rated for!

Perhaps someone with Mustek UPSes could show me the screenshots of their UPS monitoring software, so we can compare with mine.

Please help.


I’m Not familiar with this Software, but on the one that came with my UPS, I can set the voltage output. Try looking in the program preferences and see if you can set the output voltage. It should be as close to your 220V as possible.

Its not good to run undervoltage to long.

Wich UPS and Software are you talking about?

I have searched the program for everything during the 6 months I used it and I found nothing like that.

Thanks for you reply.



I have exactly the same problem. As soon as input voltage gets up to 238V my PowerMust 1400 starts to lower output to 198-200V. At the same time I hear some click noise from UPS. If input is lower than 238V than output is around 230-233V.
I do not know if there is something I can fix myself because warranty expired just a few days ago. :frowning:

I use Winpower Manager ver. 2.0 software. There is no options to adjust output voltage in the program.

engine, did you fix the problem?

Thanks for any reply.


Welcome to CDFreaks.

I didn´t fix the problem as there wasn´t a problem to fix. Your UPS is fine. It´s the normal behaviour. After RMAing a good Mustek UPS and buy a new UPS of the same Line-interactive type of another brand I came to a conclusion. UPS Technical support is too damn incompetent. They don´t even understand the products they sell. And their mistake made me return a good working UPS. The product documentation isn´t adequate too.

Here is how an UPS of this type works on an European Union power network.

There are three modes with three distinct behaviours, [B]Normal Mode[/B], [B]AVR Mode[/B] and [B]Battery Mode[/B].

Let´s say [B]Input Voltage[/B] is the voltage at the wall, before the electric current enters the UPS. [B]Output Voltage[/B] is the voltage after the electric current has passed through the UPS. Output Voltage is therefore the same voltage that enters the electric equipment you have conected to the UPS.

The UPS will show disctinct behaviours depending on the [B]Input Voltage[/B].

When Input Voltage is between 200 V and 240 V the UPS enters in [B]Normal Mode[/B]. And the Input Voltage [B]equals[/B] the Output Voltage.

When the Input Voltage is greater than 240 V or inferior than 200 V it enters [B]AVR Mode[/B]. The Input Voltage and Output Voltage are different, but still the Output Voltage is always between 200 V and 240 V.

When there is no electric current on the wall i.e there is a power failure, the UPS enters in [B]Battery mode[/B]. The Output Voltage isn´t a real sine wave, but a modified one. It´s a set of square waves that aproach a sine wave. The delivered power is however the same as a real sine wave delivers. The power is drawn from the battery. If you measure the Output Voltage in this mode (I did it) it will be around 170 - 180 V. This is wrong because the multimeter isn´t considering the modified sine wave. With the right equipment you could measure or calculate the true voltage in this mode. So this doesn´t mean the UPS is defective.

You can see by this explanation the UPS was designed to keep the Output Voltage it delivers to your electronic equipment between 200 V and 240 V. This is a voltage range any computer power supply that sells in Europe can live with. If you didn´t had the UPS, your PSU could be working under conditions it wasn´t meant for and it´s lifespan shortened.

The 198 V on your UPS output is an acceptable margin of error. Your UPS is fine. Keep it.

It’s an Offline-UPS, right?!

Off-Line UPS don´t have an AVR Mode. Line-Interactive ones do.

Ah, ok. Learnt something new today.

Thank you for your help!

I did not measure output voltage with a multimeter until I saw 198-200V output in Winpower software. After that I checked the output voltage.
I contacted Mustek technical support and they send me some specifications see attachment) and dropped a hint that UPS might be defective if output voltage is 198-200V.
But if you look at the chart you can see that at around 240V input UPS lowers the output to almost 200V. So, this is normal I guess.

Quote from the mail:
If the input is 240 V but the output only 198- 200 V
then the UPS can be defective.

I would rather see that it is not broken so I will beleive you, engine. :wink:
At the time output voltage is 232-233V and input voltage is 233V. Both voltages measured with multimeter.

Thanks again for reply and sorry for bad english.

PowerMust600USB(Sample).XLS (56.5 KB)

Sorry. I can´t see the atachment.

The 198 V could mean it is defective. It is not suposed to be lower than 200 V. But it´s a negligible difference of only 2 V. Also the real voltage could be 200 V and above and you still see 198 V in WinPower because of an uncalibrated sensor. That´s nothing to worry about.

Switch it for another Mustek UPS is not an option. Every Mustek UPS works like that. The only other option would be asking for the return of your money. But like I said, you can live with a 2 V difference assuming the sensor is correct. Or the sensor is wrong and output voltage is 200 V. In both cases, there is nothing to worry abot. Just keep it.

After returnig the Mustek 1400 VA, I bought a Phasak 1000 VA, by about half the price of the Mustek. The design is virtually the same. Same Behaviour. Only difference is you never see less than 200 V in WinPower.

Ok, thanks. I will keep it.

Here is the attachment again: