MOSFET Drain to Source issue

Thread Starter


Joined Jul 2, 2021
Im trying to pulse an LED on for 200 nanoseconds. Im using the DS1040z One shot pulse generator to send a pulse width of 200 nanoseconds to the gate of a MOSFET. Using an Oscilloscope, I was able to get that 200 nanosecond pulse width. But when I probe from drain to source, I get this dampening or dissipative oscillating signal at the Drain to source. Im not too sure why or whats causing this to happen. Also, my 200 nanosecond pulse is not a pretty square wave. What can I add to the circuit to improve the shape of the square wave?


Joined Feb 24, 2006
Since the gate of a MOSFET looks like a non-linear capacitor, it seems unlikely that you can achieve the square shape you are looking for. A voltage source driving a MOSFET gate will have the usual RC time constant behavior, but with the capacitance changing as a function of the voltage. An alternative strategy is to use a push-pull current source.


Joined Jan 30, 2016
Have you got a resistor between gate and pulse source? What voltage pulse are you giving it? Please post a schematic and part numbers


Joined Oct 7, 2019
The DS1040z was never designed to drive a MOSFET. Depending on the turn on voltage for the MOSFET, the DS1040z is probably only supplying 2mA to the Gate. Also most MOSFETs do not like 0-5V logic levels.
I don't know what MOSFET and what current & voltage and what LED. There is not much I do know.