Dear all,

I'm struggling to understand the concept of the mosfet. Buy looking at the datasheet, what actually we are looking for to sync with our design. i try to use mosfet to turn the LED on by using all those parameters as shown in the diagram below. How is that possible. Fyi, i'm using IRFZ44N and the block diagram as well might be wrong. Thanks

 

The source pin of your Mosfet is the output to your LED.
But its gate must be 10V higher than the source for it to fully turn on. You do not have +12V or higher pulses for the gate.

The IRFZ44 needs 10V pulses at its gate, not just 4V. A logic-level Mosfet needs 4.5V. Then the source pin is connected to 0V and the LED and its current-limiting resistor is connected between the drain pin and +10V.

 

Look at the Drain Current vs. Drain to Source Voltage characteristic in the IRFZ44 datasheet (Fig.1). The LED voltage drop is about 1 to 2V, depending on the LED type, power, etc. Therefore there will be about 8 to 9V between the MOSFET drain and source. Draw a vertical line at 8.5V and you will see that the line intersects the characteristics at 4.5V gate-source if the LED takes 10mA, 5V if the LED needs 20mA, and so on.

So, this is the gate-source voltage the MOSFET needs to turn on the LED. Add to this voltage the voltage drop on the LED and you will come up with the minimum gate level in order for the LED to function. At 100ma the transistor needs 8V gate-source voltage. That is why Audioguru said in his response that you need about 10V pulses in the transistor gate. These pulse are referred to ground.

Do your calculation based on the LED specs.

 

hmmmmmmmmmmmm I've been connecting the output to Source and I assumed i was wrong because of this youtube video

http://www.youtube.com/watch?v=Te5YYVZiOKs

he says the out goes to Drain (out to the LED) (he's wrong) i should complain.

He got the Source and Drain backtofront i spent 30 minutes trying to work out why it would not work ..

Skip to 2 Minutes 12 Seconds to see his Circuit Diagram it's confusing, I prefer Gate (to switch on) Drain (Provide power for source) Source (provide power to application)

Which is the correct way?

 

In his video, the IRF510, an N-channel enhanced power MOSFET, is sinking current from the lamp via the MOSFETs' drain; and the source is connected to ground. He's controlling the gate with 5v, and that's what I have a problem with.

The IRF510 needs 10v to be fully turned on. Other than that, it's OK.

Except that an IRF510 wouldn't really be adequate for an automotive headlamp.

 

I should add here that this is not the ideal way of driving an LED. The current through the LED is controlled by either the pulses amplitude or by their duty-cycle. Small variations in the gate can create large current variations in LED and you might go over its maximum acceptable current.

You need a current source. The transistor needs to be in a negative feedback loop so that it is easily controlled with a voltage generator.

 

The Mosfet does not need negative feedback since it is used as an on-off switch.
The current-limiting resistor in series with the LED creates a current source for it.
The circuit shown in the video is correct except the gate voltage should be 0V to 10V.

 

If there is a resistor in series with the LED, yes it works. I just did not notice one.

 

Thanks all. manage to work it out. really appreciate.