4046 VCO driving IRFP 450 MOSFET?

Discussion in 'General Electronics Chat' started by electronice123, Oct 20, 2008.

  1. electronice123

    Thread Starter Senior Member

    Oct 10, 2008
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    I have a 4046 VCO that produces a 50% duty cycle, 0-10kHz square wave. I am wanting to use the output at pin 4 of the 4046 to drive an IRFP 450 MOSFET.

    My question is: If I want the MOSFET current to be adjustable from about 10ma to 2A, what would be required?

    Do I need to use an amp and a pot at the output of pin 4, or would I just need a pot? The source voltage is 10V.

    How can I do this?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    The 4060 may be able to drive the FET, but only as a switch. FET's like that are not easy to use in the linear region. A transistor is much more the thing for controlled current.

    Can you post up a schematic of your circuit?
     
  3. electronice123

    Thread Starter Senior Member

    Oct 10, 2008
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    It's basically just the VCo from a 4046 that produces a square wave output.

    The transistor will be used to drive the primary of a transformer, I want to be able to adjust the current flowing through the primary by adjusting the gate voltage amplitude with a pot, I just don't know what value of resistor or pot I would need. As I said before I want the current to be adjustable from about 10ma-2A.
     
    Last edited: Oct 22, 2008
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    You would need a resistor in series with the gate, but a FET is not going to give you that kind of current control. A transistor is the thing to use. I was about to recommend a 2N3054, but they aren't available anymore.

    What kind of control circuit do you have in mind to control the current?
     
  5. mik3

    Senior Member

    Feb 4, 2008
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    You can use PWM to control the current through the coil. Its very efficiently than by controlling it linearly with a transistor.

    What exactly are you going to do with the transformer?
     
  6. electronice123

    Thread Starter Senior Member

    Oct 10, 2008
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    The transformer produces high voltage for tesla expirements.
     
    Last edited: Oct 22, 2008
  7. mik3

    Senior Member

    Feb 4, 2008
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    If you dont know you to handle very high voltages i suggest you to stop experimenting. This is dangerous.
     
  8. electronice123

    Thread Starter Senior Member

    Oct 10, 2008
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    I know enough to be safe with it. I have studied and worked with electricity as an aircraft mechanic. But when it comes to designing circuits I have no idea where to start.

    The voltages I am expecting from the small transformer I have built are going to be about 1-2kV.

    The current will be limited to 1ma in the circuit.
     
    Last edited: Oct 22, 2008
  9. beenthere

    Retired Moderator

    Apr 20, 2004
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    One usual characteristic of Tesla coils is that they get driven a radio frequencies, not audio. If you want the full effects, a drive frequency is the hundreds of KHz is more common.
     
  10. electronice123

    Thread Starter Senior Member

    Oct 10, 2008
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    I am just expermienting with various LC circuit combinations and audio frequencies for now.

    I think I need a BJT?
     
    Last edited: Oct 21, 2008
  11. electronice123

    Thread Starter Senior Member

    Oct 10, 2008
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    I am not sure if this will do what I am expecting so I posted it above. I am expecting it to gate (greater off time) every 6 pulses continuously. I forgot to draw the 10V and ground to the 4069, but you get the idea.
    Please let me know if you see anything wrong with the circuit, I would be glad to hear it! thanks

    This circuit was recommended to me, I think I still need a flip flop though as mentioned to me before.

    I also don't think the frequencies will be the same as what is coming out of the 4046 because of the far right AND gate, I am thinking it needs to be an exclusive OR gate?
     
    Last edited: Oct 22, 2008
  12. beenthere

    Retired Moderator

    Apr 20, 2004
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    Sorry, I got behind this thread a bit. Considering the voltage and projected current, there really isn't a good reason why using a FET is not ok. The linear region is more abuupt, so fine control may be interesting, but as proof-of-principle should indicate what you want to see.

    I might pass the square wave onto a potentiometer of about105K resistance with the bottom to ground through a 4.7 K resistor.. Take the controlled signal to the gate through a small resistance of 47 ohms. The reason for the resistor under the pot is that conduction won't start until the gate sees about 2 volts, and there is no reason to have a lot of dead area on the control pot.

    Now I need to look at the schematic. Can you give some idea of what range of pulse counts you would like to pass?
     
  13. electronice123

    Thread Starter Senior Member

    Oct 10, 2008
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    Awesome, thanks for the help, I'll try adding in the pot and resistors, do you think I might need an amplifier before the pot as well?

    Sorry for all the questions, I'm just really wanting to make this circuit work>>>

    I want the circuit to operate the at around 3-5kHz with a 50% duty cycle, and a longer off time every 6 pulses or so. I might later add more to the circuit to have several ranges of gating like 6-12-18-30. But for now I want is simple so I can understand it better!

    The thing I wonder most is if I need the far right AND gate to be an exclusive or gate instead???
     
  14. beenthere

    Retired Moderator

    Apr 20, 2004
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    A buffer like an op amp couldn't hurt.

    Your circuit doesn't quite work. I drew something up that should let you use all 7 bits out of the 4024. When you get to the count, the whole thing freezes until you reset it with the SPDT toggle switch.
     
  15. electronice123

    Thread Starter Senior Member

    Oct 10, 2008
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    Thanks again for the help and the schematic.
    So when the circuit gets to the count where it is gated, it just stops the circuit?
    Is there any way the circuit can be kept constant without having to manually reset it?
    What about splitting the first gate from the VCo and inputting that to pin 4 on the 4013?

    Sorry again for all the questions? Is there an easy way to designing circuits, and any good books on such a thing?

    I'm just getting into this and I can tell I have much to learn...

    THANKS AGAIN!
     
    Last edited: Oct 21, 2008
  16. beenthere

    Retired Moderator

    Apr 20, 2004
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    Do you want a set number of pulses to pass, or to wait a set period between pulses? If you want to run without interruption, just run the VCO signal around the gates. Maybe use another switch for gated-continuous?
     
  17. electronice123

    Thread Starter Senior Member

    Oct 10, 2008
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    After much study and thought I found the circuit I need

    I think this circuit will do what I am looking for and have a very wide range of gating.

    It uses two identical 4046 vco's and the outpus go two a NAND gate, which the output goes to another NAND gate, the gated signal depends on the seperate frequencies.

    I am going to add a buffer to the gated signal, Also I will add the potentiomater and resistor as you said to control the transistor!

    THANKS everyone for the help, it is greatly appreciated!!!!
     
    Last edited: Oct 22, 2008
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