Add active cooling to UPS

[naftaliin]

Hello,

I have an PowerMust Office 650 UPS device that normally has a 12V 7Ah battery. https://www.mustek.de/product/powermust-office-650/

The device is is not designed for longer use and has passive cooling that relies on high thermal mass aluminium blocks and transformer steel to sink heat for short period of time. I want to use larger battery for longer use. To do this, I have to add active cooling. I want to use 12V fan(s) up to 3W that turn on when UPS is switched to battery power. The fan should not run when on mains supply.

Unfortunetly I do not know where I should connect the fan to get 12V only when UPS switches to battery power. I figure there has to be some point in the circuitry where 12V DC can be tapped of when UPS uses battery power but has no potential when only mains is used.

I cant reverse engineer myself and was hoping someone could help me to find the right spot to connect the fan. I took pictures of the board and I possibly found a schematic for it (do not know if it is correct).

Can someone please comment whether such 12V tap-off point exists and where it is?

Due to file size limitations I give a link to all images. https://drive.google.com/open?id=1O1mWk4T2zwJHPPILiI7xB0r4VOUHErdV

The thick red and black wires go to the battery.

[robmobz]

Looking in the manual I see that it has a light called "backup mode".  I am going to assume that lights up wen you are running off battery.
I couldn't find the LEDs in the schematic so I cannot check but I will also assume that the LED and the battery share a common ground.
If both of my above assumptions are correct then I would use the LED to drive the gate of a mosfet.

Attached is the circuit I was thinking of, Vsw is the signal to control the LED. The IRF540N is overkill in this application but would work fine.

[naftaliin]

Thanks robmobz. In theory should work. Only thing is that the LED turns off for one second every 10 seconds (same time the battery operation buzzer sounds). I guess I can add a capacitor in parallel with the fan to keep it spinning full speed when the LED switches off. I have some old soviet 2200mkф (µF ?) electrolytics.  Should I limit how quickly the capacitor charges with a resistor?

[robmobz]

I would suggest just grabbing a fan and a battery and seeing how noticeable the slowdown actually is (using a switch manually) before bothering with capacitors as, from a cooling perspective, the effect is probably minimal.
If you use the IRF540N then it is rated fot 110A puls so shouldn't need a resistor but, if you decide to add one then I would suggest something in the 5-20R range.
Be aware that if you use one the resistor may end up dumping a moderate amount of power so I would recommend one rated 5-10W.  Here is an example of a suitable resistor: https://uk.farnell.com/multicomp/mcknp05sj0100aa9/res-10r-5-5w-axial-wirewound/dp/1903916

[robmobz]

Attached are circuits showing (left) the capacitor installed on my previous circuit and (right) the out of circuit test to see how noticeable the slowdown is with no or varying size capacitor.

[nAyPDJ]

Adding a resistor on the gate & placing a capacitor between ground and gate of the mosfet would get you similar results, but with a much smaller value capacitor.

[nali]

Unless you want the do this as a learning/fun exercise, I'd suggest using the KISS approach - just connect up a SPDT (or DPDT etc) relay with suitable coil voltage to the incoming mains then use the Normally Closed contacts to switch in the fan. Simples!

[naftaliin]

Unless you want the do this as a learning/fun exercise, I'd suggest using the KISS approach - just connect up a SPDT (or DPDT etc) relay with suitable coil voltage to the incoming mains then use the Normally Closed contacts to switch in the fan. Simples!

Relays with high AC coil voltage are expensive compared to other solutions. Locally I can get cheapest one for 6,5€. If I can´t get it to work then I might try this.

I think I will try nAyPDJ`s suggestion with the capacitor supplying MOSFET instead of fan.

I have a salvaged KHB2D0N60P high voltage MOSFET. This should also work? It has only up to 2A drain current but should be enough for fans, especially with adequate airflow.

Any suggestions for the gate supply capacitor and resistor values? I think the gate current draw is going to be very small and pretty much any capacitor is going to be enough? For example I have a salvaged 2a152j. That is only 1.5 nF. For such a small capacitor might not even need resistor?

[naftaliin]

I have an update on the project.

Added finned heat-sinks and a 40mm server fan. Also used a 15 ohm resistor to slow down the fan so it runs at ~7.5V and is more quiet.

Everything works except using the orange LED signal to drive the MOSFET does not work. It seems that the LED is turned on when the cathode is pulled low (probably connected to the ground) and +5V is always supplied to the LED circuitry when UPS is turned on. This means I still do not have a way to turn on the MOSFET when UPS switches to battery mode. I don´t want go with the relay option because then the fan will run even if I switch off the UPS and I would have to disconnect the battery if I want to stop the fan. Not ideal.

Any other suggestions to make it work?