Go Down

Topic: Using mosfet to dim LED (Read 186 times) previous topic - next topic

tjones9163

I know that you can dim an led with just a potentiometer but what is the benefit of also using an n channel MOSFET with the potentiometer to do the led control or motor control etc.

Johnny010

Not entirely sure what you are after...but PWM is quite useful for LED "brightness"?

tjones9163

What is the difference between a variable resistor and a MOSFET if they both are controlled by voltage?

Due_unto

True that a MOSFET is controlled by voltage, but resistors, being passive devices, are not controlled by voltage, but rather is used to control voltage and current.
Do not look into laser with remaining good eye.

tjones9163

so whats the benefit of let's say controlling motor speed with  just a potentiometer or a potentiometer hooked up to the gate pin of a MOSFET

TomGeorge

#5
Aug 13, 2017, 02:43 am Last Edit: Aug 13, 2017, 02:46 am by TomGeorge
Hi,

so whats the benefit of let's say controlling motor speed with  just a potentiometer or a potentiometer hooked up to the gate pin of a MOSFET
In both cases you will be controlling the current through the motor.

How ever if you use just a potentiometer to control current, all the motor current will be going through the pot.
This means that the pot will have to have the power carrying capacity, in Watts, as much if not more than the motors Wattage.

If you use a pot and MOSFET, in linear mode, the MOSFET now needs to be rated at the high power rating, the pot can be small, say 1/4W, as it is only controlling the gate of the MOSFET.
The MOSFET may need a heat-sink however to dissipate any heat.
The MOSFET is more compact and easier to cool than a pot,  so easier to use.

HOWEVER in both cases the efficiency is not high due to the energy being lost or wasted in the pot or MOSFET.

The alternative is to use PWM with the MOSFET to control the motor or LED.
Pulse width modulation is a method of controlling the amount of power delivered to a load without dissipating any wasted power.

A controller, such as an Arduino, outputs pulses to the MOSFET.
These pulses either switch the MOSFET fully ON or Fully OFF.
This switches the motor to the powersupply in pulses.

A short pulse ON time with a long OFF time , is called LOW Duty cycle, and switches the motor in short bursts of current, the inductance of the motor smooths the current to a near DC level.
The level is determined by the length of the ON and OFF pulse, or its duty cycle.
So short or low duty produces a low motor current.
A long  or high duty cycle produces a high motor current.

The advantage is the switching of the MOSFET.
It is either FULLY ON, and has very little resistance, so dissipates very little power.
Or it is FULLY OFF, when it is passing NO current, so dissipates no power.

The MOSFET runs cool and doesn't need heat-sinking like the MOSFET in the above linear example
I hope its a good explanation, others may have a better way of explaining it.

Tom... :)
Everything runs on smoke, let the smoke out, it stops running....

tjones9163

I get it. With the PWM of the Arduino, I am giving the MOSFET only the power it needs, so it doesn't have to dissipate as much unwanted heat. Isn't there also actual code to where I could separately run the pot to the A0 of the Arduino. Then have the MOSFET hooked up to a digital PWM pin?

 I am fairly new to Electronic Engineering and this site, but I am very impressed with the online community and the amount of help that I receive. I will make sure once I learn the material to pass on the knowledge to someone else.
Thanks!!

gpsmikey

I get it. With the PWM of the Arduino, I am giving the MOSFET only the power it needs, so it doesn't have to dissipate as much unwanted heat. Isn't there also actual code to where I could separately run the pot to the A0 of the Arduino. Then have the MOSFET hooked up to a digital PWM pin?
 else.
Thanks!!
Not exactly - the MOSFET (when using PWM) is turned fully on or fully off by the PWM drive - that way, there is either no current through the MOSFET (when off) or the full current through the MOSFET when turned on and the "average" of the on time vs off time determines the effective output power. In either case, there is very little power dissipation in the MOSFET (power = voltage * current) since in one case, there is full voltage across the MOSFET but no current through it and in the other case (turned on), there is full current but almost no voltage across the MOSFET.  This results in very low power dissipation in the MOSFET.  In the case you asked about with a variable resistor determining the voltage to the gate of the MOSFET, you are now operating in the "linear mode" and at 1/2 power for example, you may have 6 volts and several amps (12 watts) for example if you were working with a 12 volt supply.  Another advantage to PWM is that motors in particular do not like to run at low speed very well with a steady supply voltage to them, however, PWM tends to make motors handle low speed better because of the pulses that are part of the PWM.
mikey
-- you can't have too many gadgets or too much disk space !
old engineering saying: 1+1 = 3 for sufficiently large values of 1 or small values of 3

Go Up