Temperature Controlled Fan For Laptop

teleman

Member
2008-07-08 3:24 pm
Hi All,
I want to build this circuit to install in a laptop tray in order to reduce & control the heat of a rather expensive laptop which I acquired recently.

I am finding it rather difficult to source exactly a 200ohm SPST relay as required in the circuit, hence wonder if a higher value of say, a 700 ohm will suffice? The fan has a current consumption of 250mA.

My knowledge of relays is rather limited, so any further advice or tips in choosing the right value would be of great help! I have not constructed the circuit yet, so any further comments on it's merits or flaws would be most welcome

Thanks
 

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Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Well... 2k is a general sort of value. The chip has an 'open collector' output which means it needs to see a load resistor from the output pin up to the positive supply. That resistor determines how much current is available for the transistor.

The circuit as drawn... erm... seems to omit that and so its hard to see how it could work correctly.

So options... add a resistor in the 2k to 4k7 region and use the circuit as drawn. Another option (and I think this is what I would do) would be to use an FET to drive the fan directly. Something like an IRFP240 or IRF240

So it would look like this.

Capture5.JPG

You could still use an ordinary transistor but the FET is an ideal solution. Keep the diode across the motor.

Most N channel power FET's would be suitable. You could use an ordinary transistor too but it would need to have a high enough gain to work with the available drive current available. Its not the elegant solution as it is wasteful of power in the resistor that we need to add, and that resistor would need to be lower in value to drive the transistor.
 

teleman

Member
2008-07-08 3:24 pm
Thanks Mooly for the timely advice! I'll try your suggestions. However, first I just want to try this circuit with the components on hand & also learn a little more about how these different devices work in real life! Eleminating the relay in a way is a relief, as I too noticed that many similar circuits didn't employ a relay at all!
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
No problem :) You can't beat actually playing around with these kind of things.

You can actually use an opamp for this type of application as well (with very minor changes) if you hadn't got the LM311.

The output of the comparator should change state (go from high to low and vice versa) whenever the variable voltage (produced by the thermistor) crosses the value set by the preset. So you can test that without the transistor, relay and fan being in place.
 
If using a BJT to switch on the fan motor, then transistor gain (hFE) is not important.
What is far more important is that the BJT operates in saturated mode.
You do that by supplying enough base current so that Ib:Ie ~ 1:10

If you use a 700ohm relay coil on a 12V supply then the coil current is 17mA, which is quite low, most 12V relays would pull 20mA to 30mA.

Or if the motor is switched directly, then again saturate the transistor.
A 1W motor will pull ~83mA once upto speed. But it will pull a starting current quite a bit higher possibly upto 3times that running current.
A 500mA transistor should start the motor reliably. Look for a Vce0 >25V
A 2n2222 has Ic max = 800mA and 30Vce0. It's OK
But you can use any To92 transistor with 500mA to 1A Ic rating.

If you go to a FET switch the Igate is near zero.
Just choose a fet that can easily pass the starting current.
The cheapest from Farnel is
http://uk.farnell.com/rohm/rhk005n03t146/mosfet-n-ch-30v-0-5a-sot-346/dp/1834057
 
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teleman

Member
2008-07-08 3:24 pm
If you use a 700ohm relay coil on a 12V supply then the coil current is 17mA, which is quite low, most 12V relays would pull 20mA to 30mA.

Or if the motor is switched directly, then again saturate the transistor.
A 1W motor will pull ~83mA once upto speed. But it will pull a starting current quite a bit higher possibly upto 3times that running current.
A 500mA transistor should start the motor reliably. Look for a Vce0 >25V
A 2n2222 has Ic max = 800mA and 30Vce0. It's OK /QUOTE]


Thank you both for the invaluable inputs!This is the kind of information what I am looking for!

AndrewT, just to clarify on your points, do then mean that the 700 ohm relay would not supply enough current to the fan motor in this instance?
 
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Mark Johnson

Member
Paid Member
2011-05-27 3:27 pm
Silicon Valley
This is a "bang bang" control system; the fan is either 100% on, or else 0% on. It's binary.

Another way to skin the cat is to build a "proportional" control system, in which the fan speed is proportional to the excess temperature above setpoint. If the setpoint is, for example, 30C, then the fan spins half as fast when the temperature is 32C as it spins when the temperature is 34C.

Long ago I designed a proportional fan speed controller for a pizza box desktop computer workstation (remember those?), and it contained surprisingly few parts. Mine had an 8 pin IC (LM358), an NPN power transistor (TIP something), a 10K NTC thermistor to measure the temperature, and assorted bias resistors. The only "trick" is to arrange the circuit so you can get the full 12V across the fan when it's really hot. Not (12V - VBE), Not (12V - DarlingtonVCEsat), the full 12V. Everything about this is easy except the frequency compensation that guarantees stability. Give it a try, you'll have some fun.

These days when microcontrollers are cheaper than dual opamps, you'd just use an 8 pin uC, power transistor, thermistor, and PWM software. Easy.

And (here) is a proportional controller using a thermistor, two transistors, and zero ICs. For the minimalists.
 

teleman

Member
2008-07-08 3:24 pm
The relay contact/switch supplies current to the motor. The relay coil is a solenoid that pulls in the contacts. The coil does NOT pass output current.

Yes, I got that part..it's just I got a bit confused about what the minimum current the relay should sink/source! I want to first try this circuit without the relay as Mooly has suggested, as this'll save me one component if not anything else! I've ordered a few missing parts.

Mark Johnson's circuit is very interesting as it has a speed controller too! However, the picture is a bit misleading as it doesn't match the component list, especially, apart from the missing trannies! The T220 like transistor is actually a V3170P? voltage regulator of polish origin, from data sheet & if the picture is anything to go by!

Waiting patiently for the parts to arrive! Thank you guys!
 
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teleman

Member
2008-07-08 3:24 pm
I've already have one like this for my 15" laptop! The reason why I'm making this fan temp controller is 'cause the original stock tray fan gets it's supply through the USB @ 5V! A waste of power as it will be running continuously & creating unnecessary noise too! I'll be using mine exclusively for recording! I've replaced the original fan with a 20 cm 12v unit which is perfect for my need!

The price shown too is rather steep i.m.h.o, as one could buy this much cheaper on Amazon or e-bay I think! Thanks anyway for showing this posibility to me.
 
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