output power transistor (BJT) of a fan motor driver

[chu]

bjt driver

hi,

can anybody tell me why the output power transistor (BJT) of a fan motor driver need to be in saturation ??
example in datasheet uses Vce(sat).

to be able to drive a load, shouldn't the transistor in fwd active region so that it will give max stable Ic for the output ??


thanks in up-front.

rgds,
chu

 

[IanP]

fan motor driver

This is not only when driving a fan, but in all applications where a transistor is working as a switch, it should be driven to saturation (ON), as the Vce is the lowest and therefore power lost in this transistor (Vce * Ic) is the lowest, and in this case power = heat, so the transistor will not overheat ..

To drive the transistor to saturation also means that most of the voltage appears across the load, and that in saturation it can conduct its maximum current ..

Regards,
IanP

 

[chu]

bjt drive load

hi IanP,
thx a lot, now i understand more , i hav a bit more questions ...

To drive the transistor to saturation also means that most of the voltage appears across the load, and that in saturation it can conduct its maximum current ..

from the IV curve, BJT in saturation has lower Ic current compared to fwd active region, why u say it can conduct its max current ? please explain a bit more on this, thx.

for audio amp, the output transistor is in fwd active, why is this different from fan motor driver ?? (cause both also need to drive huge load)
anything to do with the driving of capacitive load (speaker) ?

thx again.

rgds,
chu

 

[IanP]

power transistors, base drivers

There is huge difference between driving a transistor to saturation and a transistor in audio application, where it suppose to operate it in its linear region ..
By comparison, try to imagine a transistor as a tap, but instead of water you have electrons ..
What happens if you fully open this tap: all possible water will flow through; you can say it is saturated; you can also say, that no more can flow through - is it not the maximum? (see picture below) ..
In audio amplifier (class A for example) you will never fully open or close this "tap" ..

Regards,
IanP

 

[hossein samani]

transistor audio motor

hi chuin the output of audio amp u want to fortificate a signal so the transistors should be biased in active mode...but when u want to drive a fan u just want to controll the fan just like a swich...so the transistor should be in saturation and cutoff mode....it means in one mode the fan is on and in another mode the fan is off...but if u want to controll the speed of a fan it is diffrent and it should be in three mode(active..saturation...cutoff)

 

[chu]

analog ic power driver fan

hi IanP,
thank u so much, i'm now clear on the issue.


hi hussein,
thx for the info ...

..but if u want to controll the speed of a fan it is diffrent and it should be in three mode(active..saturation...cutoff)

i would like to know more abt how the 3 mode can control the speed of a fan, please explain or is there any article or paper i can refer to ? thx again.

rgds,
chu

 

[hossein samani]

power fan transistor

hi
u know controlling the speed of fan is in three bias:
1.cutoff mode:in this mode the fan is off because all the supply voltage is beetwin C and E
2.active mode:in this mode u can change the speed by changing the BASE CURRENT ..u can use a potansiometer or volume to control it
3.saturation mode:in this mode the fan is in the highest speed because all the supply voltage is on the load(fan)and VCE is almost 0v(0.2v)
u can controll these three mode by just one potansiometer in the BASE of transistor by controlling BASE current.
1-IB is too much=transistor is in saturation mode(VCE is almost 0v)
2-IB is too low(almost 0mA):transistor is in cutoff mode(VCE is almost as VCC or power supply
3-IB is not too much and not too low and it is normal:transistor is in active mode and u can controll the speed by changing this current(IB).this current is beetwin cutoff and saturation current.


GOOD LUCK
hossein