i need help in this cicuit 555 timer with nmos transistor

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andrew132

Joined Feb 2, 2017
96
I know how 555 timer work but i want to know when nmos transistor work
All i know is if vg=0v nmos is off
If vg >vs nmos is on
But here the gate voltage its between 0v and -vcc ans vs=0v its

IMG_4202.JPG
 
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bertus

Joined Apr 5, 2008
21,588
Hello,

The schematic shows a BF245, wich is a junction fet.
That is NOT of at Vgs=0.
The JFET is pinched off by the negative voltage on the gate.
See the datasheet for more info.

Bertus
 

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crutschow

Joined Mar 14, 2008
27,895
The schematic shows Q1 as a BF245 N-JFET, not a MOSFET.
That transistor is a depletion-mode device that requires a negative gate-source voltage to turn off.
upload_2017-2-2_12-44-57.png
The 555 is powered by -12V, so the output goes between 0V (Q1 on) and -12V (Q1 off).
 

bertus

Joined Apr 5, 2008
21,588
Hello,

The 555 is powered by the negative voltage.
When the 555 has a high output (it is reletive at 0 Volts) the JFET is conducting.
When the 555 has low output (it is relative at -12 Volts) the JFET is off.

Bertus
 

hp1729

Joined Nov 23, 2015
2,304
1 more question the jfet p channel is on when vgd=0v and off when vgd>0v is that correct ?
In your case the transistor turns off as gate voltage goes negative with respect to the source.
There is a fair description of JFETs in the education section of this forum.
The first understanding you expressed would be correct for an N-MOSFET, enhancement mode device.
 
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