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Small signal to 5V output amp

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by camerart, Feb 24, 2011.

  1. camerart

    camerart Active Member

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    With no WIRE connected.

    At PIN3 on 555 (disconnected from rest of the circuit). Nice square wave. Both FETS cool.

    At PIN3 on 555 (Connected to the rest of the circuit). Distorted and small wave, PFET too hot to touch. NFET cool. Output Wave similar to PIN3.
     

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  2. ronv

    ronv Well-Known Member Most Helpful Member

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    Sure nice you have a scope. Let's try this so we don't burn things up.

    Lets disconnect the 2 drains from each other and add a resistor ( 1K ) between them instead. That should limit the current.
    It sounds like one of two things:
    Either the N Channel is always on or
    One of the speed up diodes is backwards. The way it is designed the FETs should turn on slow (thru the gate resistors) and turn off fast (Thru the diodes across the gate resistors). If that doesn't happen the current will be very high during switching.
     
  3. camerart

    camerart Active Member

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    I have access to Scopes, and other electronic stuff once a week, and they gave me a mini scope last week, brilliant.

    Will there be any current with NO WIRE connected?

    I am also wondering about the Diodes.

    Does your simulator tell you when there are conflicts or errors? It didn't spot the GND error.
     
  4. dave

    Dave New Member

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  5. ronv

    ronv Well-Known Member Most Helpful Member

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    Diodes

     

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  6. camerart

    camerart Active Member

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    ok, thank you.

    I see on the second picture you have turned the middle Diode around, is this the now correct way? I'll try it anyway, so watch out for smoke!!
     
  7. ronv

    ronv Well-Known Member Most Helpful Member

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    NOoooooo. Hope you added the 1K first. That was just to show the the "shoot thru" current went up to 80 amps.
     
  8. camerart

    camerart Active Member

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    Ah, so that's why it got hot.

    At one time I took the FETs out and there was a square wave at both Gates, but not with them in.
     
  9. ronv

    ronv Well-Known Member Most Helpful Member

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    So lets go back to post #102?
     
  10. camerart

    camerart Active Member

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    OK. I'll add the resistor between the 2 Drains. And turn each Diode round one at a time.
     
  11. ronv

    ronv Well-Known Member Most Helpful Member

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    The diodes should be as in the original schematic. The resistor will just keep things cool.

    What FET did you use for the P Channel?
     
  12. ronv

    ronv Well-Known Member Most Helpful Member

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    Here is how the diodes work:

    The gates of the FETs have capacitance, so by adding the resistors to the gates it slows down how fast they turn on. Then by putting the diode across the resistor it allows them to turn off quickly (Think of the diode shorting out the resistor in one direction). So for the NFET the positive going signal will be slowed down but the negative going will be fast thru the diode. The opposite is true of the P Channel (slow on the negative, fast on the positive). Without this setup when the 555 output changes state their will be a short for a small period of time when both FETs are on.
     
  13. camerart

    camerart Active Member

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    p = ntp2955

    n = stp36nf06
     
  14. camerart

    camerart Active Member

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    Ok, thanks.

    I'll need to read it a few times to perhaps understand it. It's a bit advanced for me.

    Can you answer a question?
    If one end of the WIRE is connected to GND, and the other end to the FETs output, why is there a need for an N FET? I would have thought that when the PFET is ON, it would pull the other end of the WIRE up to 12V, then when it was off it would snap back to 0V-GND
     
  15. camerart

    camerart Active Member

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    If I connect a 1K resistor between the 2 Drains, which side of the resistor does the output Capacitor need to be?
     
  16. ronv

    ronv Well-Known Member Most Helpful Member

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    Driver

    I like your idea, but it has 2 problems:

    The current thru the short wire will be to high.
    The current thru the clamp diode (D1 in the circuit below) is to high.

    If you can make the short wire twice as long it would work!

    I would use the N Channel FET, it is better (higher current rating)

    The problem with the other circuit I gave you is I used the values for the N Channel FET I gave you instead of the one you had which is slower to turn off.
     

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  17. camerart

    camerart Active Member

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    I can just use this Amp for the long WIRE, and for most other WIREs I can use my less strong Amps.

    I have been playing with the circuit + long WIRE, including only the PFET plus its Resistors and Diodes, but without the NFET +R+Ds. The PFET stayed cool, and there still wasn't enough signal. Do you think it was because of the PFET and Diode being not strong enough?

    I used the PFET in the belief that the WIRE would be connected to 0V instead of 12V, and be safer, but now I don't think it matters.

    Can you choose a different Diode as the one on the PNG is surface mount.

    So does the FET current and Diode current change the WIRE signal strength? or is it the resistance of the WIRE that draws the Current, which makes the signal stronger?

    I understand that FETs can be paralleled, so I presume I can simply stick one on the side of the FET in the circuit.
     
  18. ronv

    ronv Well-Known Member Most Helpful Member

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    Cool

    It's hard to say without seeing the circuit.

    It doesn't matter, but if you use the NFET circuit and need more power it will be easier to modify.

    I think a SB540, 50 or 60 is fairly common.

    The FET and Diode just need to be capable of driving the maximum current which is determined by the resistance and inductance of the wire. We can increase the current by raising the voltage if we need to.

    If you use the N Channel it should be ok with 1.
     
  19. camerart

    camerart Active Member

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    It's hard to say without seeing the circuit.

    It's your circuit #75 minus M2,R3,C1,D2.

    It doesn't matter, but if you use the NFET circuit and need more power it will be easier to modify.

    Will do.


    I think a SB540, 50 or 60 is fairly common.

    Found them.

    The FET and Diode just need to be capable of driving the maximum current which is determined by the resistance and inductance of the wire. We can increase the current by raising the voltage if we need to.

    OK.

    If you use the N Channel it should be ok with 1.[/QUOTE]

    Ok.

    (P.S How do I get the shaded quote as your message?)
     
    Last edited: Apr 12, 2011
  20. ronv

    ronv Well-Known Member Most Helpful Member

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    Wire Driver

     
  21. camerart

    camerart Active Member

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    OK, I try an NFET circuit. Thanks.
     

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