mosfet gate

Discussion in 'General Electronics Chat' started by Metalfan1185, Dec 2, 2008.

  1. Metalfan1185

    Thread Starter Active Member

    Sep 12, 2008
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    Ok, if i use a 555 to drive a MOSFET instead of a transistor, do i still need the resistor on the base (or gate in the MOSFET case)? also If i connect the load to be switched on the ground side, I would imagine that i can wire the more positive pin (mind fart, it's either source or drain) straight to positive? i don think i need a resistor there.

    Just double checkin, havent played with MOSFETs in a while

    last question, is there any advantage to switching on the High side vs the Low side? the MOSFET collection I have are made by phillips and they are "Enhancement Mode" N Channel MOSFETs


    Thanks fellas!
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    If you will use high or low side drive depends on your load. For a N-channel MOS its easier to use the high side drive (source on ground and load between drain and positive supply voltage.
    It will work if you drive it directly form the 555 provided the 555 output is high enough to drive the MOS in full conduction (10V for most MOS). If you use a transistor to drive the gate you will achieve faster switching and reduce heating of the MOS. Also, you need to include a 100 ohm resistor in series with the gate as to limit the current when charging/discharging the gate capacitance.
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    It's usual to place a series resistor in the drive to the gate to prevent oscillation. A resistor with a values around 100 ohms is probably going to be good.

    To put a FET into conduction, it is necessary to keep the gate at least 10 volts above the source. If the source is grounded, it's no problem. If the load is between source and ground, then the source voltage will rise as the FET conducts, and the device will come out of full conduction. You need a high side driver with a charge pump to maintain enough voltage to saty in conduction
     
  4. Metalfan1185

    Thread Starter Active Member

    Sep 12, 2008
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    Thanks, i didn't know that, So if i used a 555 Astable setup with the power on the output of a 7812, the output would be 2/3 the voltage of the ower of the 555, so we would have 4 volts from pin 3, so i would need pin 3 to a resistor (idk what value, maybe a 1k?), then the base of a say...2n2222 or something, connect the collector to + and use a resistor off of the emitter to ground. Then connect the Gate of the mosfet through a 100 ohm resistor to the emitter before the load reisitor and that should bring the voltage up there so the Mosfet can fully conduct for the bigger load...(btw, the drain for the MOSFET is the same + off of the 7812 for the 555)

    is it bad to drive a mosfet without full conduction? (like, use a variable resistor on the gate and change the output like a dimmer kinda thing?)
     
  5. AchMED

    Active Member

    Aug 5, 2008
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    This is how a regulator (voltage or current)that uses a mosfet as a pass devices works. It is not bad as long as your mosfet can handle the power dissipation.

    For a dimmer function use a programmable current source.

    Forgot to add you can buy dedicated IC's that perform the function you seem to be after current source with dimming capability. I dont recall a specific part number I Usually build my own, but if you search Digikey for current sources or led drivers you should find a couple hundred.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    If you're powering an astable 555 from 12VDC, the output (pin 3) will toggle between nearly 0v and around 10.8v. You could drive a MOSFET gate directly using a 100 Ohm resistor at moderate frequencies.

    Running MOSFETs in the linear region (gate from about 2v to 8v) will result in the MOSFET heating up due to power dissipation.
     
  7. Metalfan1185

    Thread Starter Active Member

    Sep 12, 2008
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    ok, thanks very much, but why is it 10.8V? i was told it was 2/3 of the +V, so wouldn't it be around 8V?

    and by moderate you mean not too fast or not too slow? im lookin at about 1HZ to 15HZ for the frequency

    sorry to bother the hell out of you guys with this lol
     
  8. mik3

    Senior Member

    Feb 4, 2008
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  9. Metalfan1185

    Thread Starter Active Member

    Sep 12, 2008
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    I see where that 10.8V is from now, ok,



    it says the output is about 1.7 Volts less than +V, so in my case it would be close to 10.3V, like sgtwookie said.

    and the "2/3 Voltage thing" is the peak Voltage that the cap charges to before it is triggered. I must have gotten them confused.

    Thank You very much for this, it's a great article
     
  10. mik3

    Senior Member

    Feb 4, 2008
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    If you are patient and search the internet you can find lot of information. But be careful because not all you find on the internet is correct.
     
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