PSUs + battery backup = buzzing

[NoDitchDigging]

I've been experiencing weirdness with my power supplies and BBU. Here is my setup:

• Two PCs running on a single BBU - a brand-new CyberPower BC1200D (1200VA)
  
• PCs are connected to the 'battery' outlets on the BBU (not the 'surge only' outlets).
  
• PSUs are of different manufacturers (Enermax 500W and Thermaltake 400W).
  
• Both PSUs are less than 6 months old.
  
• CyberPower software reports the BBU is at about 40% load when both PCs are running.
  

The problem: both PCs' power supplies were emitting a noticable electrical buzzing sound the other night. I powered down both systems but the buzzing continued. The only way to stop it was to either unplug them outright, or flip the hard power switch on the PSU to 'off'. After about 30 min, I powered everything back on and all was well, but I'm very concerned about this problem returning.

I've heard this kind of buzzing before, but it only happens when the BBU switches to battery. Even then, it is sort of alarming to hear any sort of buzzing sound coming from the PSU. However, in this case, the PSU was not in battery mode.

Has anyone had any experience with this sort of thing? Any insights would be appreciated. Thanks!

[Zxian]

It kinda sounds to me like the inverter in your BBU is running at all times. These devices sometimes produce a very simple AC signal (not the sine wave that it should be) that can cause the ringing that you speak of.

Does the tone of the buzzing change if you unplug the BBU from the wall?

[NoDitchDigging]

Thanks for replying. I didn't unplug the BBU from the wall at the time. I probably should have tried that to see what happened.

Don't modern BBUs not run their inverters at all times? I was under the impression that they did, for the switchover 'gap' in the current would be enough to cause connected PCs to fail anyway.

[rendrag]

Don't modern BBUs not run their inverters at all times? I was under the impression that they did, for the switchover 'gap' in the current would be enough to cause connected PCs to fail anyway.

Not necessarily. The UPS units that are line interactive have power running through the inverter at all times. All of the big name UPS manufacturers (APC, Belkin, CyberPower) still make stand-by systems. In those systems, you are running of wall power at all times unless the power goes out. At that point you have the switch-over (appx 30ms I believe) which is still fine for most computers. Line-interactive UPS's are usually in the range of 2-10ms.

How were you able to determine that it was the PSU's that were emitting the buzzing? It's odd that psu's from 2 different manufacturers would emit the same buzz.

[NoDitchDigging]

I stuck my ear right up behind the back side of the PSUs which is where the buzzing was the loudest. The sound wasn't coming from lower down where the motherboard is. As soon as I switched off the rocker switch on the PSU, the buzzing would stop.

That kind of sucks that the feed is still straight from the AC line and therefore has to 'switch over' to backup power. I was under the impression that AVR-type units (like mine) were better than standby-type and fed their outlets like so:

AC Line In -> AVR/switching logic -> battery -> outlets

Oh well,...

[rendrag]

if it has automatic voltage regulation, then it is a line-interactive UPS.

if you want a good explanation on how that works, check this out:

http://nam-en.apc.com/cgi-bin/nam_en.cfg/p...amp;p_topview=1