Author Topic: Modifying a cheap fan control  (Read 3307 times)

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Offline 0xdeadbeefTopic starter

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Modifying a cheap fan control
« on: February 04, 2013, 06:52:15 pm »
Lately I decided to add a new fan with temperature control to an USB3 HDD enclosure and since I was lazy, I ordered a cheap SMD fan control branded H-Tronic together with a Sunon 0.8W fan. Anyway, while the fan control was assumed to lower the voltage down to <6V at room temperature, it lowered it only to ~8.5V with my Sunon fan. I figured it was designed for bigger fans with higher wattage, so I thought about modifying it a bit to fit my fan better.
Originally I thought the circuit would be something like this:
http://members.financial.com/~schmaus/luefter/luefterSchaltplan.gif

Yet when looking more closely at the circuit, I came up with another circuit though. I'm not sure about the transistors. I suppose that T1 (BLD 2H) is a NPN and T2 (BK2 2D) is a PNP. I measured the NTC as 7.6k at room temperature. Don't take the exact types too serious, I just selected them for the schematic (and simulation) without putting to much thought into it.
The resistors are obvious - except for R1, which is labeled "010" as far as I can tell, which doesn't seem to make much sense. Besides, I measure it as ~7k which seems to be plausible as this would fit the NTC.
I simulated the circuit in NI Multisim and it seems to be very close to the real thing. Based on this schematic, I think I should select  ~200Ohm for my Sunon ML 4010-12 S (12V, up to 0.065A ).

So do you think the schematic makes sense and fits the circuit? What puzzles me a little is that the 75Ohm resistor R4 is a 1206 package, which should be only 0.25W. Yet the manual claims that the circuit was meant for up to 400mA at 12V (4.8W). With T1 off, and assuming that the load resistance was around 30Ohm for a 4.8W fan, the current flowing through R4 would be ~114mA. This would be nearly 1W at R4 which is four times what it could take.
Even with a load resistance of ~75Ohm, the current would be 80mA resulting in ~0.5W which is still twice as much as the maximum value.
« Last Edit: February 04, 2013, 07:13:55 pm by 0xdeadbeef »
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Offline Simon

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Re: Modifying a cheap fan control
« Reply #1 on: February 04, 2013, 09:04:38 pm »
unussual
 

Offline 0xdeadbeefTopic starter

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Re: Modifying a cheap fan control
« Reply #2 on: February 04, 2013, 09:14:01 pm »
That's what I thought when I realized the 75 Ohm resistor was connected directly between 12V and the output. So it takes the complete current when T1 is off. This came a bit unexpected and I'm still not sure whether this is such a brilliant idea.
I first assumed that the concept was to split the current between transistor and resistor to distribute the dissipation, but indeed at maximum voltage, the main current flows through the transistor and the resistor takes only like 10mA.
« Last Edit: February 04, 2013, 11:25:59 pm by 0xdeadbeef »
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