Yes, sorry the arrangement I had was connecting the Source to the +12V, drain to the LED positive (or the scope+load right now), and gate to pin 3. (...) Am I reading right that I incorrectly wired my MOSFET?

Yes, you incorrectly wired it. The LEDs would be on all the time, fed by the forward-biased substrate diode in the MOSFET.

 

Vgs must be 10V to turn the device on fully. Notice the substantial drain current (7.5 Amperes).

Noticed that as I hook up the load, my output voltage dropped- I'm guessing this is due to the required Vgs.

Wiring the MOSFET to switch the ground will fix this as well, correct?

 

Yes that should help.

 

Yes, sorry the arrangement I had was connecting the Source to the +12V, drain to the LED positive (or the scope+load right now), and gate to pin 3.
Am I reading right that I incorrectly wired my MOSFET? that is, I wired the Drain to my load and the Source to +12V,.

Right. The body diode of the MOSFET is conducting the +12V from the source to the drain (output)!

 

Hmm... well this one is interesting.
After wiring the MOSFET to switch the ground, I've hooked it back up to the load and scope, to see this:
It would seem I'm getting a pwm signal through, but my voltage isn't dropping to 0 between pulses (note the offset on the trace)

 

The drain voltage won't drop to 0V. If the FET RdsOn is 100mΩ and your LED current is 2.5A (30W LED string) it won't drop below ~ 0.25V.

 

The drain voltage won't drop to 0V. If the FET RdsOn is 100mΩ and your LED current is 2.5A (30W LED string) it won't drop below ~ 0.25V.

Is there a formula for this?

It's staying above 10v, but it's currently only hooked to a 50 ohm load.

Thanks

 

You should be getting within a gnat's whisker of +12V on the drain when the FET is off, and just a few tenths of a volt above ground when it's on. Anything else, and you've either got a dead FET or you've connected it wrong. This really, REALLY should be simple...

 

Is there a formula for this?

Yes. It's called "Ohm's Law".

 

Yes. It's called "Ohm's Law".

Yeah, I'm familiar with that one.

Why then is a formerly functional FET only dropping to 10v between gate cycles When hooked to a 50 ohm load?

 

Your circuit is quite strange, the oscillator is fast, but the Monostable is very slow, and if you use the N type mosfet then you should connect the load to the D pin and 12V, and the S pin connected to Ground, otherwise it will hard to work properly, because the Vgs needs 10V to turn it on, if the load needs 12V then you have to offering a 10V+12V=22V, and it should be separate from 12V, then the mosfet can be work properly.

If you need a pwm circuit, check this and you just choosing the PWM function.

 

Your circuit is quite strange, the oscillator is fast, but the Monostable is very slow, and if you use the N type mosfet then you should connect the load to the D pin and 12V, and the S pin connected to Ground, otherwise it will hard to work properly, because the Vgs needs 10V to turn it on, if the load needs 12V then you have to offering a 10V+12V=22V then the mosfet can be work properly.

If you need a pwm circuit, check this and you just choosing the PWM function.

Yeah, it was the best I could piece together from reading wiki articles.

The other contributors in this thread have suggested that I use the MOSFET to switch the ground for the Vgs reason mentioned

I'm starting to think I may have miscalculated the monostable circuits timing

But, all goofiness aside I could Have this running if the mosfet would turn all the way off.

I'm going to double check my scope probe leads to make sure I haven't screwed that up, and been looking at the wrong thing.