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Output voltage after using mosfet

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zerostar

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Output voltage drop after using mosfet

Hi,

I am making a little smoke device for the train,
and using e-cig atomizer and AVR in project.

Since the heater require 0.6A to run, mosfet IRF3205 is added for a switch and
prevent burning the AVR.



The power supply is 3.7V li-ion battery.

When Heater directly collect to power source, it performs very well.
* V= 3.6V I= 0.67A

However, after adding a mosfet ,
it only received :

V= 1.9V I= 0.35A

It drop in a half of value..
How can I increase the output voltage even using mosfet??
 
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883918_10152558916575018_1196313046_o.jpg

Sorry for the size, but it just a simple circuit only
 
What is Vgs of the Mosfet used. What MOSFET did you use? There are logic level Mosfets which you probably should be using if your not.

Without knowing what you have one can look into the crystal ball and suggest: 1) Vgs of you MOSFET is too high
Solutions might be: 1) Changing the MOSFET or 2) Upping the power supply.
 
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Here https://www.electro-tech-online.com/custompdfs/2013/03/lt1157.pdf is a part that can help you.

The spec of 2-4 V would mean that's the range of Vgs for that part. So, for a worst case turn on, Vgs would have to be above 4V. Your power supply is only 3.7, so in that case it would not turn on at all. But, if you happened to have a Vgs(t) part that happened to be 2V, there would be a good chance it would work.

Read some of the results of this search: https://bit.ly/13zBglI
 
In reading your other thread on exactly the same subject, it seems you have mA and A confused. Your true current is probably in amperes.

Second, use a logic level mosfet that will turn on fully with the voltage you have.

Third, I think the threads need to be merged.

John
 
I cannot believe that the current in your heater is only 0.67mA. That is almost NOTHING.
Maybe the current is actually 0.67A? (one thousand times higher!)

The threshold voltage of your IRF3205 Mosfet is when it is almost turned OFF with a current though it of only 0.25mA. Its datasheet shows its fully turned on gate-source voltage is 10V.

A "logic-level" Mosfet is fully turned on when its gate-source voltage is 4.5V or more which your 3.6V battery will not provide. Some Mosfets are more sensitive than the rating and will work. Mosfets AT THE RATING will not work.
 
A "logic-level" Mosfet is fully turned on when its gate-source voltage is 4.5V or more which your 3.6V battery will not provide. Some Mosfets are more sensitive than the rating and will work. Mosfets AT THE RATING will not work.

While it is true that logic mosfets are usually rated at Vgs of 4.5V or 5V, many are also rated at much lower Vgs voltages. DigiKey lists 486 pages of logic-level, N-channel mosfets. A large percentage of them are rated at Vgs values of 2.5V to 4V.

The Toshiba SSM3K123TU is one of many such devices. It is rated at 4.2A continuous with a Vgs of 2.5V.

https://www.digikey.com/product-detail/en/SSM3K123TU(T5L,T)/SSM3K123TU(T5LT)TR-ND/2257763

From the datasheet:
Ron = 66 mΩ (max) (@VGS = 1.5 V)
Ron = 43 mΩ (max) (@VGS = 1.8 V)
Ron = 32 mΩ (max) (@VGS = 2.5 V)
Ron = 28 mΩ (max) (@VGS = 4.0 V)

Moreover, at low currents, the mosfet does not need to be turned on completely to work quite well.

John
 
But Zerostar is using Mosfets that are 35 years older!
Digikey is in America and Canada.
 
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A BJT/Darlington might be easier for the OP to source? It would also be sure to work at 3.7V.
 
I cannot believe that the current in your heater is only 0.67mA. That is almost NOTHING.
Maybe the current is actually 0.67A? (one thousand times higher!)

The threshold voltage of your IRF3205 Mosfet is when it is almost turned OFF with a current though it of only 0.25mA. Its datasheet shows its fully turned on gate-source voltage is 10V.

A "logic-level" Mosfet is fully turned on when its gate-source voltage is 4.5V or more which your 3.6V battery will not provide. Some Mosfets are more sensitive than the rating and will work. Mosfets AT THE RATING will not work.

lol...It is 0.67A, the heater is the component in e cigarette , the atomizer
 
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