Can you help me understand N-Channel Mosfets?

Thread Starter

Ezeikiel Rich

Joined Sep 15, 2019
20
Hello, My circuit is working however the voltage drops from 2kV to .2kv when it gets to the load.

I dont understand Mosfets well. Could someone please help me understand why the drop in voltage?

Thank you,

Mosfet IXTL2N450 Data:
fet-IXTL2N450.png fet-draw-1.png
 

Audioguru

Joined Dec 20, 2007
11,251
When the opto feeds the gate with +8.1V then the source goes only as high as about +1V to maybe +3V. No good.

But if you connect the source to ground and when the gate goes to +8.1V then the Mosfet turns on and connects the load to ground. The other end of the load should be connected to the high voltage. Then the load gets the high voltage on it.
 

crutschow

Joined Mar 14, 2008
23,508
R1 should be connected to the MOSFET source terminal, not ground.
As you have it, the source cannot go above about 7V.

The common for the 9V supply must also go the the MOSFET source.

Did you not see my circuit for that?
 

Thread Starter

Ezeikiel Rich

Joined Sep 15, 2019
20
R1 should be connected to the MOSFET source terminal, not ground.
As you have it, the source cannot go above about 7V.

The common for the 9V supply must also go the the MOSFET source.

Did you not see my circuit for that?
Thank you, I tried your circuit several times. I couldnt get it to work. Then i started over from scratch and still couldnt.
With this circuit I made now I put 9v at the gate and 15v on the drain for testing and got 12v at the source and put in a strip of 12v leds and they also got 12v on the strip and it lit up very bright with a current of .15 ampers

So I switched the circuit to high voltage, now with 9v on the gate and 2kV on the drain I am getting 100-200V at the source. and current of 5ma. But I am not getting the 2kv.

I have R1 on the gate to pull the gate down if not the fet stays on.
 

Ylli

Joined Nov 13, 2015
790
The MOSFET requires at least +3.5 volts between the gate and source to turn it on. When your opto conducts and the gate becomes +9 volts, what voltage does the source go to? Now what is the voltage between the gate and the source? Polarity matters.
 

Audioguru

Joined Dec 20, 2007
11,251
You tried +9V at the gate and +15V on the drain for testing and got +12V at the source which is correct. Then if you connected +9V at the gate and + hundreds of volts on the drain for testing then you will also get +12V at the source. Then if you +9V at the gate and +2kV on the drain for testing then you should also get +12V at the source.

With +9V on the gate and +2kV on the drain then if the source is higher than about +5V at 5mA then the Mosfet is defective.
Maybe you had the pins on the Mosfet connected backwards which damaged it?
 

Thread Starter

Ezeikiel Rich

Joined Sep 15, 2019
20
When the opto feeds the gate with +8.1V then the source goes only as high as about +1V to maybe +3V. No good.

But if you connect the source to ground and when the gate goes to +8.1V then the Mosfet turns on and connects the load to ground. The other end of the load should be connected to the high voltage. Then the load gets the high voltage on it.
The operation you are describing is a low-side switch. The way a n-channel mosfet normally operates. With the load connected to Vcc and the ground to the drain.
 

Thread Starter

Ezeikiel Rich

Joined Sep 15, 2019
20
As I stated in needs to go to the MOSFET source, not ground.

Post the exact circuit of mine that you tried.
If it's connected correctly it will work.
crutschow I believe this is the exact same circuit I am using! :) is it not?...

oops wait im not showing the ground from the 9v+ on my circuit... hold on...

crochow.png

This is what I have right now... I corrected it...
fet-draw-1.png

Did I do it right?
 
Last edited:

Thread Starter

Ezeikiel Rich

Joined Sep 15, 2019
20
No.
As I've stated twice, and as my circuit shows, R1 needs to go to the MOSFET source, not GND.
Sorry crutschow, im so slow, you were right. Thank You! It worked great! I was just missing that one. Got my full charge on 2kV to the cap. However a couple issues...
  1. The Source now leaks between 1-2v no biggie.
  2. The gate stays high when i remove the trigger. Do i need to add another resistor to pull it down?
 
Last edited:
Top