x3oo wrote:
> the Fan is deisgned for 5V and 200mA: 1Watts

x3oo wrote:
> Lets say the TEG is producing 0.2W.

x3oo wrote:
> TEG is providing 0,2 volts. at 0.05W.

If the Fan needs a current running through the motor windings to turn 
the blades. And the windings have a resistance for the current.
With this fan, you need to apply 5 V across the motor, that 200 mA of 
current are forced against the resistance of the windings.

A lower voltage will not be able to force the same amount of current 
through the windings which is needed to turn the blades.

-----------------
Also:
If the motor hat 1 W of nominal power, this has to be satisfied. At 
least in part. you may be able to run the motor at 4 Volts and a bit 
lower current, but the blades will turn slower. 0.2 V is not nearly 
enough. Yes, you can step up the voltage from 0.2 V to 5 V, but the 
current is reduces by the same factor.

Say your source can provide 0,2 V and 0.05 W. This means it can provide 
0.25 A @ 0.2 V.
The wattage does not change, but you step up the Voltage to 5 V, then 
the source can provide ideally 0.01 A. Because 5 V times 0.01 A is the 
0.05 W.
But 0.01 A will not turn the motor.

I do understand that aspect.
Its just that i DO want it to be able to run at very low power and low 
voltage too. I want the TEG to run the Fan succesfully even at very low 
power outputs from the TEG. Thats the most important thing in my little 
project.

x3oo wrote:

> Lets say the Fan is deisgned for 5V and 200mA: 1Watts
>
> I am expecting the fan to run at lower Voltages too.

Why do you expect this? There is no rational reason to expect something 
like this.

> I still dont know
> the minimum Voltage to start the fan.

Doesn't matter at all. For an useful BLDC driver the only target is to 
reach the designated rotation speed. It allways uses the whole available 
voltage, but controls the effective current (and by the way indirectly 
the the power consumption and finally the rotation speed) by PWM.

> I want to convert the voltage efficiently for low power scenarios too.

There is nothing to convert. The fan will use allways exactly the power 
required to reach and keep the designated rotation speed. Exactly this 
is, what an BLDC driver does.

But, there is a problem for very low rotation speeds. For those cases 
most BLDC drivers cannot work, because regulation feedback by EMK isn't 
large enough to perform a proper speed regulation. For such cases you 
need fans with extra sensors for rotation speed measurement and BLDC 
drivers with support for such sensors.

x3oo wrote:
> Thats the most important thing in my little project.

I understand. Then you have to use/select a fan which runs at the same 
or a lower power which the output power of the TEG.

The fan cannot use more power than the source can provide. This would be 
in conflict with physics.

x3oo wrote:
> Lets say the Fan is deisgned for 5V and 200mA: 1Watts
> Lets say the TEG is producing 0.2W. How do i design the DC-DC Converter
> to do that

just like an over unity engine, a perpetuum mobile.

> Are there converter cuircits ready?

 No, you're the first one, run to patents office.

> lets say the TEG is providing 0,2 volts. at 0.05W.

You need 25 of them. Connect them in series to boost the voltage.

MaWin wrote:
> x3oo wrote:
>> Lets say the Fan is deisgned for 5V and 200mA: 1Watts
>> Lets say the TEG is producing 0.2W. How do i design the DC-DC Converter
>> to do that
>
> just like an over unity engine, a perpetuum mobile.
>
>> Are there converter cuircits ready?
>
>  No, you're the first one, run to patents office.
>
>> lets say the TEG is providing 0,2 volts. at 0.05W.
>
> You need 25 of them. Connect them in series to boost the voltage.

Sry there is a misunderstanding. A small Fan for 5V will run at 4V at a 
slower speed. I dont want to break physics, i just want to be able to 
run the fan at low power output too, (at slower speed)

-gb- wrote:
> x3oo wrote:
>> Thats the most important thing in my little project.
>
> I understand. Then you have to use/select a fan which runs at the same
> or a lower power which the output power of the TEG.
>
> The fan cannot use more power than the source can provide. This would be
> in conflict with physics.

sry, misunderstanding. the fan ist designed for 0.5 W

if the TEG only provides 0.25 W at that moment i want to be able to run 
the fan at a slower speed at 0.25W

The TEG only provides very low voltage at 0.25W, thats why i think i 
need a boostconverter.

x3oo wrote:
> the fan ist designed for 0.5 W

What fan model? Look at the datasheet if the fan is able to run at lower 
voltage and with less power.

x3oo wrote:
> The TEG only provides very low voltage at 0.25W

What is very low in numbers?
The power, here 0.25 W is always the product of current and voltage.

c-hater wrote:
> For such cases you need fans with extra sensors for rotation speed
> measurement and BLDC drivers with support for such sensors.

The fan is most likely build like a cheap PC fan, with a 4 pin IC with 
integrated hall sensor to controll the motor and little more in it.

you need a very special converter. It has to be able to run (and start) 
from a very low voltage. it also has to implement some kind of MPP. A 
normal boost will cause the input voltage to collapse due to its current 
consumption which exceeds the current the TEG can provide.

you could try to find some energy harvesting Chip. there should be some 
breakout boards available, maybe you can adapt one of those.

to get the fan running, you will definitely need an energy storage, a 
big capacitor should be enough. the problem is to get it to start 
running, which requires a lot more current (and voltage) than to simply 
keep it running.

sorry for the multiple posts, i got a spam warning and couldnt figure 
out why:
Your Post Seems To Contain Spam: "C i a l i s"

probelm seems to be the combination of spec-ialised, avai-able, onl-ine 
and ener-gy or something like that