Enable voltage regulation ONLY when below certan voltage

[Da9eI]

Hi!

I recently got an UPS, that has a fan inside it. Its a 24V fan, and the ups controls it by two speeds; low (12V) and high (24V).
The problem, there is no off, the ups is placed in the other end of my bedroom and the fan is loud enough for me to not use the ups (Uninteruptable power supply, or battery backup). I have tested the ups without the cover and fan removed, and nothing gets hotter than 35*C under normal operation (IR thermometer). I also like to have just a small amount of airflow at standby, just to be "safe". During inverter On and Rapid charging the fan run on high, and once the charger goes back to standby\tickle\float charge, the fan slows down to low.

So my question is there a way to "modify" the low speed to "low-er", without modifying the unit itself, but still keep the high speed near or at high with only simple "external" components?

At first I thought of using a relay (or transistor) with a zener diode of high enough voltage to not tick the relay in low setting sending power trough its -NC- contacts trough a dropper resistor (or 5V regulator). Once the fan is running at high speed the voltage + current through the zener is high enough to energize the relay and bypassing the dropper resistor and the fan runs at high speed..?
Could OP-amps work? comparator? any voltage regulator?
...Just a theory, however the "PTO" to the fan has a limited current of maximum 250mA, but the UPS does not sense if the fan is running or not.

Any ideas?

The ups is a APC 2.2kva unit. (which has enough power to power my alarm clock all night long in case of power outage lol)
On general forums of this unit, its a common "problem". The older model had OFF-LOW-HIGH, if I remember correctly.

The fan runs fine on as low as 5V (able to start itself on 4.5V). Plenty fast enough for low setting, but using a simple drop resistor makes it not run fast enough on high, and the UPS could over-heat.

Thanks for any info and help!

-Dan

[Marco]

First thought :

A resistor from ground to fan, a NPN transistor across it with the collector at the the side of the fan and the emitter at ground, and a ~18 volt zener with a base resistor in series between supply and the base.

At 12 Volt the resistor will slow the fan down, at 24 volt the Zener will pass a base current to drive the transistor into saturation bypassing the resistor.

PS. second thought, I'm too stupid to put the transistor the right way around on the first try.

PPS. MOSFET works too, but instead of the base resistor you get a bleed resistor from gate to ground to deal with the Zener leakage current.