Hello,

I have about 3.x volt signal.
I need feed it to mosfeet n-chanell, but problem is it won't like to open at 3.xv.

I have 12V voltage rail, so i can feed it to mosfeet, but i need some kind of transistor acting like switch.

I used before NPN transistors, and i connect it between my load and collector.
But here, that trick won't work... i try resistor ( nchanell gate) and colelctor, but it work in reverse, when transistor is off n-channell got voltage, when transistor is on, n-channel is off. That is easy to understand, because we get all current into transistor. I check voltage on emitter, and it's almost same as base voltage.


Do You have any circuit for using transistor as switch for n-channel mosfeet ?

Regards

 

Post a diagram of what you are trying to do.

 

Here You go!

At 5V input to NE555, i can't drive that n-mosfeet.

I know i should use logic level, but i don't have any right now.
That's the reason i use non logic level.

Regards

 

With a supply voltage of only +5V then the output high from an ordinary 555 is +3.7V.
Even many "logic-level" mosfets will barely turn on.

So why not power the 555 from your +12V so its output high is +10.7V?
Then ANY mosfet will turn on properly.

 

With a supply voltage of only +5V then the output high from an ordinary 555 is +3.7V.
Even many "logic-level" mosfets will barely turn on.

So why not power the 555 from your +12V so its output high is +10.7V?
Then ANY mosfet will turn on properly.

You have right, i just start from idea using LMxx5 to allow circuit work from many voltages 12-24v, but now i realize, i just need use 9V stabilizer and should work fine... i don't know why i stick to that 5V, just stupid...

But, anyway i found a solution for my problem!.
I tested it on breadboard and work well !

I put a optocoupler, and zener diode ( i need this to run safe on 24V ), to prevent sending this voltage to gate ( i name rail 12V but it can be 12V - 24V ) .

 

The photo-transistor in the opto-coupler is an emitter-follower fed from the 8.6V zener diode so its maximum output voltage is only 7.9V which might not be high enough to fully turn on the Mosfet.

 

The photo-transistor in the opto-coupler is an emitter-follower fed from the 8.6V zener diode so its maximum output voltage is only 7.9V which might not be high enough to fully turn on the Mosfet.

That's a common misconception. The phototransistor has no connection to its base. The electrons generated by photons impinging on the base-collector junction can drive the transistor into saturation in common emitter or common collector mode. See the attachment.
Yes, I realize this is a simulation. This app note supports my viewpoint.
I encourage others to check this with hardware.

 

You have it right Ron H. A phototransistor can go into full saturation in either a common emitter or common collector configuration, makes no difference.

 

It is amazing to learn that a photo-transistor emitter-follower can saturate. Thanks for letting me know about it.

 

It is amazing to learn that a photo-transistor emitter-follower can saturate. Thanks for letting me know about it.

I know. For years, I thought Vce would be about 0.7V.