Signal Generator Considerations

Discussion in 'Off-Topic' started by gammablot, Sep 12, 2010.

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  1. gammablot

    Thread Starter New Member

    Sep 12, 2010
    Hello all - I'm glad to be here, and thought I might open a
    question about the following circuit, designed to emit pulses
    of varying width for energizing various things.

    Currently my level of experience is that of an enthusiast,
    learning things at a steady pace as I delve into this new
    realm of electronic devices.

    The question I'd like to pose has to do with the capacitor
    C1 shown next to the LM555 chip. The parts list indicates
    that it should be 47nF, although doesn't mention anything
    about what voltage it should be, confusing me to no end!

  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    It isn't really an important consideration - which is why it is not specified.

    Just be sure that its working voltage is higher than your power supply voltage. I other words don't ever be tempted to use a capacitor with a working voltage of 6.3V in a 12 volt circuit.(mostly concerns electrolytic and other polarized types). Small value caps in this, non-power supply type circuit are usually non polarized and have higher working voltages. Ceramic disc caps often have a WV of 50 to 1000. I'd suggest using one with a 50 to 100 volt working limit in this case, but use whatever you have available that will satisfy the circuit requirements.
  3. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    Good rule of thumb is double you supply voltage.
  4. SgtWookie


    Jul 17, 2007
    It's a very poorly designed circuit.

    The 200mA source/sink capability of the 555 timer is wasted, and the LM393 comparator has a very small 6mA sink and 0mA source capability, leading to very slow charge/discharge times for the MOSFET.

    555 timers can only take a Vcc of 15v to 18v, depending upon the exact manufacturer and part number. However, the author has chosen a power MOSFET that has a Vdss of 400v. Why? The gate drive circuit is already crippled, and so they use a MOSFET with Vdss ratings 22 times higher than the main component could ever survive? What's the point, except increasing the gate charge time?

    Boy, I'm really starting to sound like AudioGuru here. :p

    Sorry, but that is a really poorly thought-out circuit.

    Why don't you tell us what kind of power you want to use to supply the circuit with (voltage and current, or battery type), and what your load is, and we'll come up with something much better.
  5. gammablot

    Thread Starter New Member

    Sep 12, 2010
    Oh no - I was hoping otherwise! Thanks for your responses on my
    capacitor voltage information deficiency.

    The source of power for the circuit will be a 12v lead acid,
    while my intended purpose for it will be to signal a power transistor
    at an adjustable rate, its emitter connected to the supply from
    the same battery.

    The circuit layout above is more of a power signal generator I'd
    wager, lots of amps going through. Here's another I've been
    considering -

  6. gammablot

    Thread Starter New Member

    Sep 12, 2010
    Here's an overview of how it will be built. Eventually I'll
    see about increasing the voltage...

    Last edited: Sep 13, 2010
  7. beenthere

    Retired Moderator

    Apr 20, 2004
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