Using MOSFET in existing circuit

Discussion in 'The Projects Forum' started by tumler, Jul 1, 2012.

  1. tumler

    Thread Starter New Member

    Aug 16, 2010
    Hey guys, long time lurker, first post.

    I've been following a design to make a colour organ, this one specifically : and it works great (I used different op-amps and 2n2222 transistors) when powering a couple LEDs.

    What I would like to do is change the output, so instead of 2 LEDS, I will connect a whole 5m LED strip such as this one for this I will need a bigger power transistor / MOSFET. I decided to give designing in a MOSFET a go as I would like to learn and I thought it would be beneficial in minimising power consumption (compared to a transistor) especially when used on multiple channels.

    To accomodate the MOSFET (I'm using a FQP30N06, threshold volt. ~2v -> 4v) I've made the voltage reach 3v to safely turn on the MOSFET thereby switching on the lights when it reaches the required audio level, I achieved this by modifying the active filter to have a gain of 2.5 and changing the resistor connected to the gate from a 100k to a larger value ~820K.

    After all this, the light is turning on and reacts to sound as expected (I am still testing with only a couple LEDs as I don't have the strip yet) but it is giving a different effect. With the transistor the colour of the LED would go through the spectrum of dim to very bright according to sound level. While the MOSFET is either hard on or hard off, this looks like it's in saturation,

    I'm just hoping someone would be able to help me and point me in the right direction as to how I can achieve a dim to very bright effect using the MOSFET? (correct me if I'm wrong, but I need to get it operating in it's Triode region and how would I go about this? or are MOSFETs not able to give the same effect?)

    Thanks in advance!
  2. shortbus

    AAC Fanatic!

    Sep 30, 2009
    The "threshold voltage" is the turnoff voltage, not the turn on voltage of a mosfet. Threshold voltage is used when designing a mosfet amplifier. Thats why the threshold voltage is hidden farther into the data sheet. The VGS (volts, gate to source) voltage is the turn on voltage, in this mosfet it is ~10V.

    A mosfet is most effectively used as a switch. They heat up when the VGS is not correct. That said, the way to use one for a dimming type circuit is to use a PWM signal on the gate.
    tumler likes this.
  3. tumler

    Thread Starter New Member

    Aug 16, 2010
    Let me see if I get this straight, as I have never quite heard it put that way. The threshold voltage is usually measured when Id = 250 uA. So does that mean any voltage higher than that will have a larger current and be operational (and could still be used or is that a bad way to go about designing circuits) and anything lower will be off?

    The Vgs of ~10v that is given in the datasheet, is that just a recommended spec they took to indicate the Rds at a given point (I'm guessing, indicating one of the lowest values that Rds reaches?)

    Yeah I did know PWM is often used with MOSFETs but I will be not using PWM in this circuit (though I will in a future project) and was just wondering if it was possible without it
    Last edited: Jul 1, 2012
  4. absf

    Senior Member

    Dec 29, 2010
    In that case you could used transistors like TIP31A 3A 70V or darlington like
    TIP120 5A 60V.

    These are linear devices which behave like 2n2222 but with higher current. Only precaution is you need to mount heatsinks or else it'll get too hot.

    tumler likes this.
  5. tumler

    Thread Starter New Member

    Aug 16, 2010
    Yeah that was what my next step was going to be as I have a few TIP122 's on hand. Thanks for the info so far guys.

    Still curious on the MOSFET's, if anyone would be so kind
  6. wayneh


    Sep 9, 2010
    i think you can, in theory, control a MOSFET in the narrow region where it's a (non) linear device. I know this because anyone that's tried to use a MOSFET has probably seen what happens when you fail to turn the gate fully on; dim LED and a warming MOSFET. But the hoops you have to jump through to achieve that level of control on purpose aren't usually worth the effort. There's not as much advantage to using a MOSFET if you need to design around its peculiar characteristics. (It still has the advantage of not needing current on its gate, just voltage.)

    If you want to experiment, you need to find the "sweet spot", the gate voltage range in between dim and almost full brightness, then squeeze your signal into that range. For instance, instead of 0-10V it might be 3-7V.
    Last edited: Jul 3, 2012
    tumler likes this.
  7. tumler

    Thread Starter New Member

    Aug 16, 2010
    Thanks for your replies, they have helped me