funny waveform from driving an IR LED

Thread Starter

bug13

Joined Feb 13, 2012
1,792
Hi team

Just wondering what this waveform mean? I am expecting a pretty good square wave. What's the problem and how can I fix this?
Datasheet for IR led (link).

The input waveform is pretty good. I am probing between the resistor and MOSFET. And there is only one mosfet active at a time.


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Sensacell

Joined Jun 19, 2012
2,544
Nothing to fix, it's normal.


The LED's look like an open circuit when they are not conducting.
When the MOSFET switches off and the LED's stop conducting, the drain effectively becomes a floating node.

The residual charge just leaks away, that's the sloping section of the waveform.

It's irrelevant because the current in the LED shuts off crisply, your circuit works perfectly.

Put a 1K resistor across the LED's as an experiment, you will then see the "clean square wave" you expect.
 

BobaMosfet

Joined Jul 1, 2009
943
Hi team

Just wondering what this waveform mean? I am expecting a pretty good square wave. What's the problem and how can I fix this?
Datasheet for IR led (link).

The input waveform is pretty good. I am probing between the resistor and MOSFET. And there is only one mosfet active at a time.


View attachment 196769
View attachment 196770
Welcome young Jedi. Capacitance. Understand the value of this in FETs. Learn it well you will, or fall prey the Dark Side. Voltage runs to and fro, through all living circuits. Understanding when it ebbs and flows, and how the force of it reacts, finding purpose is the way of the Jedi!!!
 

Delta prime

Joined Nov 15, 2019
126
All you guys and gals are silly. But if I ever have a question I know where to go and that's no joke. I salute each and everyone of you.Exceptional!
 

Reloadron

Joined Jan 15, 2015
4,917
This is the dual MOSFET you are using. As mentioned if you want a nice clean wave less that trailing sedge just place some 10K resistors from the Gate pins to Ground. You can use 1K as mentioned, I like 10K but anyway as covered, what you see is actually normal and what would be expected.

I loved the birds, That is so cool. :)

Ron
 
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