AC signals; Mosfets; Rambling

Discussion in 'General Electronics Chat' started by RedCore, Jul 31, 2011.

  1. RedCore

    Thread Starter New Member

    Jul 31, 2011
    I am currently working on using a frequency generator to switch a mosfet.
    I have been working on getting this to work for quite sometime now, and I feel I am getting closer and closer. I feel I am missing a fundamental part of how AC and Mosfets operate and this is my problem.

    Here is more background on my project. Feel free to skip directly to my questions if you would like.

    As viewed from my oscilloscope, my freq generator is putting out a ~1kHz triag wave with these attributes: Vmin=-4.08 Vmax=3.04 RMS=2.32 Vdc=0
    or a sine wave with Vmin=-2.48 Vmax=2.32 RMS=1.6 Vdc=0
    Please tell me what other attributes compose a wave other than these, if any.

    Originally I was trying to run the mosfet off of the sinewave, I am using a IRF510A. The datasheet lists Vth as 2-4V. Since it was not working I tried using a dc biais to bring up the max a bit. No luck.
    Tried using trig wave. Used many different bias and resister configurations. No luck.

    AC questions:
    What happens when an AC current is biased where its min is above 0v?
    If the signal is Vmin=0v to Vmax=5v. Is this AC? does the potiential of the dc and the potential of the negative swing of the ac cancel each other out leaving only a net movement forward? Not AC but still sine wave?

    What is the proper way to bias a mosfet? If i understand correctly, even if I have a 1v ac signal, I can bias a 3v Vth mosfet at 3v and the the combined signal of 2-4V will switch the mosfet. What am I missing?
    Do I need to take into account any charging of the gate capacitor?

    While writing this, I have thought of some issues that I have overlooked. The threshold of the mosfet is 2-4V but it doesn't pass 1A until 5V. I remember now that mosfets are current controlling devices and that even if I hit the 3V threshold, I will not get the current I am wanting. So my next question is will I always see the waveform on the mosfet? Even if I don't pass or get near the threshold, its possible that I will see the waveform no matter what.
    How does this work if I am using a 20v on the drain of the mosfet? Should I not see this potential also, just no current is flowing?

    I have read alot lately on mosfets but the pieces are just not falling in place yet. I'm sorry for the rambling post.

    Project details:

    Design a circuit using a frequency generator(5v) to control a mosfet switching 20V@1amp into an inductor. Purpose is to determine resonance frequency of the inductor and feed that frequency into it. First step is to design the oscillator circuit. Next obstacle is determining how the inductor will interact with the frequency fed into it. Will the frequency shift? How do I handle this?

    Thank you in advance for any help.
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    Please post a schematic showing what you want to do. Your descriptions of this are unfortunately not too clear.

    The one comment that I would make is that to switch a MOSFET properly, it is not sufficient for the gate drive voltage to just barely reach Vgsth.

    The manufacturers datasheet for the IRF510 shows RDS(on) quoted for VGS = 10V, so 10V drive may be regarded as a minimum for proper switching - a bit more could help.
  3. Audioguru

    AAC Fanatic!

    Dec 20, 2007
    You need to turn on the Mosfet.
    But its input is only the THRESHOLD voltage where it is almost turned OFF with a current of only 0.00025A.

    It is difficult to bias a Mosfet so it is a linear amplifier because its input voltage and its gain have wide ranges.

    If you want the Mosfet to switch on and off then its datasheet shows a gate-source voltage of 10V, not 2V.
  4. RedCore

    Thread Starter New Member

    Jul 31, 2011
    Thanks for the responses.

    Here is the circuit I'm trying for. It is very basic and is missing parts that may be needed in reality.(Caps?)

    I am trying to make the coil ring at its resonance freq. I would like to calculate the frequency using the inductance, and feed this freq in. This may not work as I would like.
    I also may need some way to handle the back EMF from the coil oscillations. or maybe even any interference that may affect the mosfet.(Diode?)

    The two images are of the signal from the frequency generator. One is with no load on the mosfet. The other is with a resistor load. (No inductor, and still getting interference)

    Falstad Applet

    The cap with the switch is there just to see how the circuit reacts with the cap in parallel with the coil.

  5. Audioguru

    AAC Fanatic!

    Dec 20, 2007
    Guess what? A coil by itself does not resonate.
    It needs a capacitor in series or parallel to resonate.

    Stray capacitance of the wiring might add a few pF for a small coil to resonate at a very high frequency.