Forum: Analog Circuits Replacing fans, attaching a resistor in parallel

von Calculating a resistor size for given power (Guest)

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Hello, everybody,

I have a UPS with two identical fans connected in parallel. Because they 
are loud, I want to replace them.

The nominal voltage of all fans is 12V.

Each of the old fans draws max 2.5W.
Each of the new fans draws max 0.96W.

If I only connect the new fans, the UPS reports a serious error (beeps 

If I add one of the old fans in parallel to the two new fans, the error 
goes away.

I think this is because the UPS recognizes a power consumption of only 
about 2W instead of 5W.

I don't think that a temperature sensor triggers the error, because the 
UPS works fine with two new fans and one old fan (all connected in 
parallel), even if the old fan is physically outside of the case and 
doesn't cool anything.

I consider connecting a resistor in parallel to the new fans.

If this is a good idea, the question is how big this resistor should be. 
If not, how can I get the new fans to run wihtout triggering errors?

An old fan has a measured resistance of 650 Ohm or 1320 Ohm, depending 
on the polarity.

Thank you very much.

von Dieter (Guest)

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Type, Model of UPS?
Type, Model of Fans?
Plug with two or three pins of every fan?

von Ntldr -. (ntldr)

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How many wires are connected to the UPS from the fans? Measuring current 
for detecting fan errors is fairly unusual in the PC space, even for 
enterprise gear. In most applications you will find at least a third 
wire, which indicates the fan's rotational speed back to the device.

Putting a resistor in parallel is unlikely to work, as the resistance of 
the motor changes when it spins.

von Ioan P. (ioan)

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Riello Dialog Plus Rack 150. This is an old online UPS.

The old fans have 2 wires. The plug on the PCB has, of course, only two 
wires. The new fans have 3 wires. I am using only two, obviously.

Old fan type: Sunon KDE1208PTB1
New fan type: be quiet Silent Wings 2 (BL060)

Without load the fan voltage is about 8V. I think it gradually increases 
once things start getting hot and I am assuming fan speed is regulated 
only by voltage, since there are only two wires.

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