MOSFET Switch help

I have the switch on the left working just fine.



What I would like to do is for my light to share a common ground.

Will the configuration on the right work just as well for me? Any issues I need to be aware?

The LED is going to be one of those 12V LED bulbs. The Mosfet is an FQU13N06 N channel logic level mosfet.

 

The circuit on the left needs a "logic level" Mosfet.
The circuit on the right WILL NOT WORK because when the gate of the Mosfet goes to +5V then the source pin that drives the LED barely gets any voltage.

 

Yeah. Big problem. The mosfet is going to act like an emitter follower so the output voltage will never be 12 volts. In addition, the gate of the mosfet must be a few volts higher than 12 volts to get most of the 12 volts to the LED.

 

Thanks

Is there any way I can have a logic level mosfet switch and share a common ground with the light?

Would a P channel configuration work?

 

Use a mosfet for the transistor on the left.
I recommend this circuit so often that I keep a copy on my desktop.

 

Sorry I am confused where is the load? And that would be a P channel mosfet?

Why would I need a transistor and a mosfet if I have logic level mosfets?

 

FQU13N06 is not a logic level device.

 

I left out the L From the datasheet FQD13N06L / FQU13N06L 60V LOGIC N-Channel MOSFET

 

What is your PIC supply voltage? What is the Fwd voltage of your LED? The current?

 

The pic pin is 5V. The voltage to be switched is 12V. In the actual project it will be an 2 12V LED bulbs in parallel containing it's own current limiting device. Each bulb has 60 leds and is rated at 3W.

 

I would also like to know how to search for logic level mosfets on mouser. I asked this question before and was told search for "logic level" . Well that does not work. For example NDP6020P is a logic level P Channel FET yet does not come up on mouser when "logic Level" is searched.


Edit:

A log of searching I found this:

http://forum.allaboutcircuits.com/showthread.php?t=27262

from Ron.

So I need a mosfet with a vgs of 4.5V and a low rds(on) is there anything else I need to know other than the parameters on the voltage and current I plan to switch?

Edit again:

Rats It looks like mouser does not have a search for vgs on mosfets. How on earth are you supposed to find logic level mosfets?

 

Here is what I want to switch on and off. I was wrong they are actually 3.5W.

http://dx.com/p/gu5-3-3-5w-6500k-420-lumen-60-led-white-light-bulb-12v-131888

 

OK. I changed the symbols to mosfets and labeled the load terminals.

 

What are the values of the resistors? Again why do I need two mosfets?

 

You can use an NMOS and a PMOS, an NPN and a PMOS, an NMOS and a PNP, or an NPN and a PNP. The first option is the most efficient, and you only need the two transistors and one resistor.

The attachment shows how I would do it.

This all assumes that you aren't going to be doing any high-speed switching.

 

It's ok if you don't want to use the circuit I posted.

 

Gee thanks. I guess it is too much trouble to answer my question.

 

I might want to do PWM at some point. How would that change things?

Also can you please tell me why two mosfets are needed? And can you recommend 2? I would assume the one driven by the mcu would be a general purpose mosfet?

I think I have seen p channel switches with one mosfet for mcus. Or am I wrong?


Edit: Oops I see you made a recommendation for the first mosfet. Thanks


Edit: I do have some SPP18P06Ps on hand but I don't have any GP MMOS but I do have some GP NPNs . Can the SPP18P06P be used for that second PMOS?

 

What would your max PWM frequency be? PWM requires faster rise and fall times than quasi-static operation. You would probably need a more complex driver, or at least waste more current in the first transistor.

The reason you need two transistors is because you want a high side driver that can switch 12V, and you have a 5V control signal. You can't use the circuit you posted because, with a MOSFET, Vgs has to be several volts before the device begins to turn on. When your gate is at 5V, the source will be around 2-3V. If it tries to go higher, it turns off. You can't turn on a 12V LED lamp with 2 or 3 volts.
You can't use a single PMOS because the source has to be at +12V, and Vgs has to be zero volts to turn it off. This means your gate voltage has to go to +12V. Your PIC only goes to +5V.

 

Not sure right now what my PWM would be. I'd need to check my source code and see what freq I am currently running to see what pwm freq is possible.

Thanks for the explanation. But I am still a little confused with my NMOS, I am currently switching 12V off and on with 5V on the gate. But I didn't measure the voltage. Are you saying I would not have 12V on my source (in my right most figure)?

Sorry for being so dense.