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MOSFET circuit powers on wrong state... help!

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xxplorer

New Member
help! i built the circuit below, and it works, except that each time i apply power to the circuit, it starts out with the LOAD side ON, instead of off.

i need it to toggle a 12v lamp on and off, but i need it to come up each time with the lamp off.

i tried moving the diode, etc., on the left side (the 10K resistor and capacitor) to the other side of the transistor, but that did not work.

how can i modify this to start with the LOAD (a 200ma relay) in the off position???

thanks

**broken link removed**
 

Chippie

Member
According to Mr Bowden:

"The combination (10K, 10uF and diode) on the left side of the schematic insures the circuit powers on with the MOSFET turned off and the NPN transistor conducting. These components can be omitted if the initial power-on condition is not a concern. In this initial state (MOSFET off), the voltage at the gate of the MOSFET will be near zero and the voltage on the 1uF capacitor connected to the switches will also be near zero"

So I guess there must be something wrong in the way you have constructed yours.......
 

bogdanfirst

New Member
you might have placed the cap in reverse...it happent to me.
but you are using a 4A MOSFET to power a relay.?why dont you connect the load directly....? or is it from the mains...?
 

xxplorer

New Member
chippie-

i really paid attention and built it exactly to spec...

i am not sure if the diode i placed is reverse. on the diode, one end has the black cylindrical marking. i assume that corresponds to the black triangle on the schematic...

bogdan --

the need for the relay is simply because i need a single positive output to light a lamp on the instrument cluster. its already grounded, and all i need to do is supply a positive signal. i am also using the smaller mosfet....

which cap are you referring to? i checked all the markings, and have tried reversing the caps.

help!

thanks for replying
 

bogdanfirst

New Member
i am reffering to the 10uF cap especially, but you should check the 1uF one too. it could happen that you put it in reverse. the diode should have a white line( if it is black) or a black line(if it is metalic or transparent red) on one end. that end corresponds to the line in front of the arrow at the schematic, not the triangle. so i assume that you put it in reverse so that is why it is not functionig well.
 

xxplorer

New Member
YES!

bogdanfirst --

the diode was backwards.. you are absolutely correct!!

IT WORKS!

Thank-you for the explanation. I am not e-literate enough to figure the polarity of the diodes, etc... I am suprised there is no industry standard marking amost all the different components that are polarized.....

thank-you again! htnak-you too chippie (you were also right on!)!

:D :D
 
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