# IRFZ34N doesn´t switch on when a NE555 timer pulse is applied simulate this circuit – Schematic created using CircuitLab

I have the above circuit in my breadboard. The problem in a nutshell is that when the NE555 timer is switching at 50kHz in astable mode the IRFZ34N seems not to be able to follow the circuit switching. Instead, it gives me the following signal:

The signal in yellow is probing mosfet drain and the the pink one is the 555 output.

My thoughts on why it´s not working have to do with gate-source capacitance not being able to discharge completely for another cycle, or maybe some stray inductance causing the voltage to become negative. Anyway I´m not sure these are the problems and do not know how to solve this issue either.

I have not added any pull down resistor once I think the 555 timer already pulls the output to ground when off, not leaving the output floating

EDIT: I got back to the bench and probed the drain voltage again, unfortunately the pictures I´ve taken are over 2MB which means I cannot upload them. Anyway I´m gonna explain what I visualized:

The same 555 timer switching, but now the resistor R1(drain resistor) has been changed a couple of times and I could see it plays a really importante role in the mosfet switching speed. The greater it was, the slower the drain voltage would reach 15V as the gate signal had been switched to 0V.

I´ve made some tests with $R_1=33k\small{\Omega}$, $R_1=2.2k\small{\Omega}$ and $R_1=33\small{\Omega}$.

When $R_1=33k\small{\Omega}$, as the 555 pulse reached $0V$ the drain voltage reached about $10V$ within 2 microseconds.

Similarly, the outcome for $R_1=2.2k\small{\Omega}$ was $800ns$ to reach $15V$ and about $200ns$ for $R_1=33\small{\Omega}$.

From this data, what role does the drain resistor play in this switching? How does it affect the mosfet rise and fall time?

• Nothing wrong with your circuit at this speed. What does the gate look like? At 33 ohms gate resistance I'm surprised your 555 output is so clean. It almost looks like the source is floating. – John Birckhead Dec 4 '18 at 21:41
• @John I've forgotten to take a picture of the gate. My bad because I don't remember the look of it either. Tomorrow I'll be back to the bench and I'll take a picture to update here. – Iron Maiden Dec 4 '18 at 22:49