Noisy Fan 

 Back in October 2023 I decided that my old UPS was becoming too unreliable and as we regularly get power cuts I decided to buy a new one. For those that haven't used a UPS let me explain what it is...

If you're using a PC and the incoming mains supply suddenly fails the PC will instantly turn off losing whatever you're working on and also perhaps getting so corrupted it will fail to boot up. An "Uninterruptible Power Supply" develops a mains equivalent voltage from internal batteries at a power level of your choice. Connect your PC plus display etc to your UPS and you'll have enough time to finish what you're doing and close down correctly. The more you invest the longer the time you can rely on your UPS for power. In fact with one rated at say 2KVA you can also connect your router, a table lamp and whatever you deem necessary.

A UPS doesn't last for ever though because its lead acid batteries will eventually degrade. This will be progressive... meaning that you may lose UPS power fairly quickly after several years of use.

My last UPS blew up because of a surge in the mains supply during its loss. Fortunately my protestations to Scottish Power, who oddly are in charge of distributing power on the South coast, paid me compensation to cover new batteries and new power transistors so I was able to repair it without forking out for a new one.

By October I'd lost the display and, before then, both the USB and RJ45 comms had failed... plus the batteries failed to charge properly so I bought a brand new one.

 

 The new UPS is a Powercool 2000VA and worked perfectly well when I'd installed it.

After a week or so I noticed slight fan noise was now definitely not slightly noisy.. it was getting distracting. I had several options.. get the UPS replaced (it would take about a month through my supplier and the replacement may not be any different), unplug the fan (but the UPS might get too hot), insert a resistor in the 2-wire fan lead to reduce its speed (a bit hit and miss), or add a thermal device to control the fan (my chosen option).

 

 I opened the case and unplugged the fan. I'd done some rough calculations and purchased a couple of thermal switches type KSD9700 rated at 55 degrees Centigrade with normally open contacts.

I glued one to each of the two heat sinks accommodating its power transistors (the UPS has just two large heatsinks).

Because my plan was to turn on the fan if either heatsink reached 55 degrees I connected the devices in parallel in the fan's opened red lead. That means if either heatsink got too warm the fan would power up. The aim was to silence the unit when the PC was off or when the ambient temperature wasn't too warm.

Now the UPS is generally silent. A fall back option I'd considered was to add a resistor across the thermal cut-outs which would allow a slow and maybe an almost silent fan.

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