I was wondering if i could regulate the Voltage of my LEDs with the mosfet instead of a resistor directly before the LED.
I know it's a basic question but i'm relatively new to the game...which doesn't mean i havent used the search function
Basically my current setup is this:
I've got a my arduino controlling a HP LED ( 3.5V 350mA )
Main power comes from a 5V source, with a 4.7 Ohm resistor to reduce the Voltage for my LED. The resistor gets kind of hot, since it has to burn 0.5 W.
For a red LED it's even worse, where the resistor burns 1 W.
My question is, is it a good idea to reduce the voltage by not fully opening the MOSFET?
Trying to drive high power leds (350ma and greater) by using series current limiting resistors is a very inefficient and unstable method. If you look at any of the commercial lamps using leds of this size and higher you will find that they drive the leds with constant current drivers, some of which can be pwm modulated to allow for variable brightness.
Here is an example of a constant current driver suitable for a 350ma LED:
I know about LED drivers, but they are quite expensive, i could buy 2 LEDs for the price of one driver...
What happens with the Mosfet when i regulate the voltage through it, is it the same as using a resistor?
Trying to drive high power leds (350ma and greater) by using series current limiting resistors is a very inefficient and unstable method.
That includes if you are also using a pwm controlled mosfet. By the way a IRF510 is not a logic level mosfet (it needs +10vdc gate voltage for full on condition) and as such cannot be wired directly to a arduino +5vdc output pin.
Yes, not fully saturating a MOSFET is the same as using a resistor; the excess energy will be converted to heat. For larger LEDs, a switching circuit really is better than a current sink.
retrolefty:
By the way a IRF510 is not a logic level mosfet (it needs +10vdc gate voltage for full on condition) and as such cannot be wired directly to a arduino +5vdc output pin.
I'm also trying to pwm control a motor which requires around 1A and I tried IRF510 as mosfet. First I tried a circuit exactly like this (except for IRF510 instead of RFP30N06LE). It works, but the motor won't spin when arduino signal is below +4vdc.
As you were pointing out, this mosfet can only source around 0.5A with less then +5vdc. What should I use? I believe I need a transistor in between, in order to trigger the mosfet with +12vdc, see attached image (question mark)
So I tried with 2N7000, it works but still I'm getting low current, what should I use instead?
What should I use? I believe I need a transistor in between, in order to trigger the mosfet with +12vdc, see attached image
No that circuit is wrong.
You need a resistor in the base of the transistor.
That arrangement of the transistor is called an emitter follower and can not get more than 4.2V onto the gate making it even worse at driving a FET than having nothing.
You need a common emitter circuit to produce the required gate voltage.
retrolefty: Here is an example of a constant current driver suitable for a 350ma LED:
Note the 93% efficiency rating, that's a lot less wasted energy/heat then using a resistor.
This is confusing the matter. The driver cited - despite the inaccurate description on its page - is not what would be considered "PWM" as such, it is a switchmode power converter.
In respect of the original question, using a transistor or FET to regulate current is neither more nor less efficient than using a resistor and the same power is simply dissipated in the control device, but is a preferred means of regulating the current using a constant-current circuit.
A switchmode converter can be designed to provide dimming but again, while the one cited provides an "enable" control which would be most appropriate for "on-off" control, whether this could usefully be employed as PWM is extremely dubious - there would likely be a problem with flickering as the PWM and chopper frequencies interacted.
This is confusing the matter. The driver cited - despite the inaccurate description on its page - is not what would be considered "PWM" as such, it is a switchmode power converter.
That's funny as I would consider that switchmode power converters utilize pwm to control it's output voltage or current, depending on configuration.
Of course they do in effect, but the algorithm is different.
In a switchmode converter, power is switched on until the specified current is reached, then switched off until the voltage regresses to the set threshold; not the same as executing a pre-determined duty cycle.
Paul__B:
This is confusing the matter. The driver cited - despite the inaccurate description on its page - is not what would be considered "PWM" as such, it is a switchmode power converter.
It is a switching driver but the control signal for dimming is also PWM. Or putting that another way, the particular driver chip, the MBI6651, is switching at 40-1000KHz but the control signal for dimming is 100-1000Hz.
