series parallel resoanance

Discussion in 'General Electronics Chat' started by arthur92710, Sep 10, 2007.

  1. arthur92710

    Thread Starter Active Member

    Jun 25, 2007
    307
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    I have a 555 out put connected to a coil and i get the frequency i need.
    But now i need the 555 to power more coils. How should i connect the coils to keep the same frequency. In parallel or Series?

    The 555 will not directly power the coils. It will switch a Mosfet and that will provide the power for the coils.

    The 555 will have to work about 24/7. Can it?
     
  2. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    If you are talking about the 555 timer timebase, it only depends on those two resistors and capacitors that it uses to define the pulse width and period. So, the timebase will remain constant, independently of the number of coils or the fact that they are in series or in parallel.

    It should hold. Of course it is advisable that the chip comes from a proper manufacturer, since that there are lots of fake 555 chips around. Also, don't forget to protect the mosfet from the back emf generated by the coil by using protective diodes.
     
  3. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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    Can you tell us your ultimate application for this circuit? Sometimes it helps the discussion along to know the intended application of the circuit under discussion.

    hgmjr
     
  4. arthur92710

    Thread Starter Active Member

    Jun 25, 2007
    307
    1
    Yes i can tell you, but why tell when you can show? A pictures worth a thousand words.

    I took a copper wire and coiled it around 8 scerws. then when i turn it on the led on the secondary coil lights up. a.k.a IT WORKS!
    But the 555 gets hot in like a few seconds. Because this is a prototype i am not using a fet yet.
     
  5. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    Looks like you have the MOSFET on the High Side. The symbol shows an N-CH type. You should switch to a P-CH or put the N-CH on the low side with the source connected to GND.
     
  6. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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  7. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    Indeed, a picture worths a thousand words.

    The 555 gets hot and may get fried as well. It can source or sink up to 200mA. More and it may be destroyed. You really should consider using the FET, even if it is only for testing the circuit.

    Also, I noticed that the protection diode is wrong. It won't do nothing there, since it is the FET that will receive the back emf generated by the coils. So, it should be connected in paralel with the coil and backwards (it should not be used in forward biasing, since it will short circuit, destroying itself and maybe destroying the FET in the process). A 1N4001 diode will serve.
     
  8. arthur92710

    Thread Starter Active Member

    Jun 25, 2007
    307
    1
    Oh ok. so some in parralel and some in series.

    about the diode: do i need a reverse diode on every coil or just... somewhere else?
     
  9. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    You will need only one diode for all or the coil association. The objective here is to short the back emf, so it will biase the diode instead of damaging the FET.
     
  10. arthur92710

    Thread Starter Active Member

    Jun 25, 2007
    307
    1
    ok i changed the diagram. Now i have 4 diodes. D1-D4.
    Which diodes do i keep and which do i remove?

    ok does any one know where i can get a free sample of an FET? but not Fairchildsemi, some how I exceeded the limit this week by not ordering anything.
     
  11. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    Onsemi will provide samples for a small processing fee but you can put several items in the same request.
     
  12. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    They are all wrong. Just put one diode backwards, and in parallel with the coil association.
     
  13. arthur92710

    Thread Starter Active Member

    Jun 25, 2007
    307
    1
    :confused: All wrong.... X-X
    What do you mean by in parallel with the coil association? Can you show me a diagram?

    BTW: yup the back emf will kill your 555's. Second one today-.- but its ok i ordered a 10 sample 555 from some site and they sent me 20 of them:eek:!!

    *Fairchild wont send me samples those bastards* I have also ordered some fets all negative channel and high Amps. 10, 18, 30 and 80 amps, 10 of each :D wating for them to come. They should be here tomorrow. *Fairchild wont send me samples those bastards*
     
  14. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    Here it is. I cannot be more explicit than that.
    View attachment 1219

    Back emf will kill sensitive components that are in parallel with the coil, or components conected to the coil. If you had used a FET, it would kill your FET instead. That is why a diode in reverse is used, to give an alternate path for the current generated by the coil. The diode won't be damaged because it is a low resistance device and as so, it will cause the back emf to be very small. Remember that a coil can act as a constant current generator. Using Ohm's law, you will see that the back emf generated will be inversely proportional to the "load" resistance.
     
  15. arthur92710

    Thread Starter Active Member

    Jun 25, 2007
    307
    1
    So thats what you meant. I'll never forget it. so I fixed it in my diagram and now im trying to get some high current fets.
     
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