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switching with mosfet - not working

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ratan.1185

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mosfet working

Dear all ,

I was using following circuit for my switching of LEDs but it was not switching . But when I put 1M ohm resistance between Gate and ource of Irf 840 .
It was working properly .
Can anybody pls Explain it.
 

Re: mosfet working

from the diagram it seems that the leds switch on once.
but afterwards it is in that state

is it ?

srizbf
3rdjuly2010
 

The only way it can be made to work is if the LEDs are on when the opto-isolator LED is OFF. Then you will have to carefully select R2 and R3 to avoid blowing up the MOSFET by exceeding its maximum gate voltage and also not blowing up the opto-isolator. You will need to drive the opto LED pretty hard to get the MOSFET to turn off, depending on the values of R2 and R3. That could result in a low value being needed for R4 which may be a problem for the micro to drive.

Keith
 

what are the values of R2 and R3 ?
using 110V or anything else ?

srizbf
3rdjuly2010
 

Dear srizbf,

My R2=100k
R3=2k
R4=470E
and I am using 230V DC as input.
 

Assuming you still have a 110V zener then I don't see how it can turn the MOSFET on. You will only get 2V on the gate and the MOSFET threshold is 2V to 4V. Increase R3 to at least 4.7k.

Keith.
 

if you use 110v use R3 as suggested by keth1200rs.

even then the circuit is not proper in switching LED.
you have to modify.

srizbf
6thjuly2010
 

keith1200rs said:
Assuming you still have a 110V zener then I don't see how it can turn the MOSFET on. You will only get 2V on the gate and the MOSFET threshold is 2V to 4V. Increase R3 to at least 4.7k.

Keith.
Click to expand...

Keith , actually I am not using 110V as showed in my schematic but I am using 230 as it is so I am getting Near about 4 to 5 Volt .

I just want to know why It is working when I am using 1 Mohm Resisstor betweeen Gate and Source reminal of IRF 840
 

As the main problem of the original circuit, the MOSFET will never turn off, because there's no means to pull down the gate voltage to zero. Actually, the motivation for the circuit design is completey unclear to me.

What I would do: Use a zener diode to generate a voltag level within the MOSFET and photo transistor ratings, e.g. 10 to 15 V with a suitable series resistor and a sufficient low pull-down resistor between gate and source, at least 100 k, or less, depending on the intended switching speed. With 1 MOhm pull-down, you get switching speeds in the ms range.
 

FvM said:
, depending on the intended switching speed. With 1 MOhm pull-down, you get switching speeds in the ms range.
Click to expand...
Dear FvM,
Thats what I wanted to know how does the pull down Resistor help in switching of mosfet? How does the value affect the switching speed ?
 

The resistor value has a time constant with the gate capacitance of the MOSFET used. The IRF840 has around 1.2nF. So with 1M CR=1.2ms.

Keith.
 

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