Anticipated Circuit Output?

Discussion in 'The Projects Forum' started by Glitchmobile, May 8, 2010.

  1. Glitchmobile

    Thread Starter New Member

    May 8, 2010
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    Hi everyone, newbie here in many ways...

    I've attached a circuit drawing that uses a 555 chip, and was hoping someone might
    know what kind of output signal to expect at pin 3?
    ( I don't get a typical square wave, however don't know what kind of signal to expect).

    The circuit output is to be connected to a mosfet's gate (not shown in drawing).
     
  2. ELECTRONERD

    Senior Member

    May 26, 2009
    1,146
    16
    In the AAC searchbar, type "555" and you get many results. In many of the numerous posts that pertain to the 555 timer, you'll also find a link to "Bill's Index" and several other links of his. I would recommend that you seek those links out within this forum.

    Austin
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The output will be a rectangular wave with its duty-cycle adjusted by the pot making Pulse-Width-Modulation. PWM is used to control the brightness of a lamp or LED, the amount of heat from an electric heater and the speed of a DC motor.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Since pin 1 of the 555 timer is not connected to ground, the circuit won't work properly.
     
  5. Glitchmobile

    Thread Starter New Member

    May 8, 2010
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    Thank you for the insight SgtWookie. I feel this is probably why my circuit was not operating correctly. When I put a scope to the circuit, all I got was a straight line, no square wave.
    Thanks.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You are welcome.

    I'm attaching a similar circuit, but it has needed parts that aren't in your schematic, some unnecessary parts have been removed, and the load on pin 7 has been decreased due to the value of R1 being increased to 1.5k.

    There should always be a minimum of 100 Ohms per volt of Vcc between pin 7 and Vcc. If Vcc=12v, R1 should be at least 1.2k Ohms. If pin 7 has to sink more than 10mA, power dissipation in the timer will become excessive.

    If you are going to drive a MOSFET gate with the PWM output, you will need to use a resistor between the 555 and the gate. I suggest using a resistor of between 10 and 62 Ohms, placed as close to the gate of the MOSFET as you can. If you omit the resistor, the "C" of the gate and the "L" of the wiring will cause the gate to "ring" when the charge level is suddenly changed. This will cause high power dissipation in the MOSFET.
     
  7. Glitchmobile

    Thread Starter New Member

    May 8, 2010
    10
    0
    Thanks for taking the time to elucidate the intricacies of the chip and accompanying parts, plus posting a similar circuit for further clarity. You're especially perceptive, in figuring that my objective is to control a MOSFET, spot-on.
     
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