NE555 squarewave oscillator not reaching rail voltage

Discussion in 'The Projects Forum' started by hrs, Apr 30, 2016.

  1. hrs

    Thread Starter Member

    Jun 13, 2014
    82
    7
    Hi,

    I'm trying to drive a transformer with an NE555 circuit. The output voltage of the NE555 circuit appears to be lower than expected. The output signal about 58kHz, ~8V p-p and not quite but close to 50% duty cycle. It is said here and there on the web that the maximum output voltage is around 1.5V lower than the rail voltage. Is that in the datasheet(Texas Instruments part)? I couldn't find it.

    Should I doubt the measurement or the part? I bought an old scope some time ago and have no idea when it was last calibrated, it reads the output as slightly less than 8V p-p. The manual of my true RMS DMM specifies RMS voltage only up to 10kHz. At 58kHz it reads the output as 8.5V RMS. I'm guessing that at high frequencies you get the peak voltage instead of the RMS value, or perhaps a nonsense reading.

    What would you expect the p-p output voltage to be?
     
  2. dannyf

    Well-Known Member

    Sep 13, 2015
    1,828
    365
    Yes.
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,153
    3,059
    Agreed, it's in there.

    You could get to the rail by switching a MOSFET with the output of the 555, but you need two transistors and a more elaborate circuit to get the push-pull of your current circuit.
     
  4. DickCappels

    Moderator

    Aug 21, 2008
    2,662
    633
    The output of the NE555 is not intended to swing from Rail-to-Rail.

    I have used the buffer below in two different applications. Notice if the NE555 is removed from the circuit (socket) both transistors turn on which could destroy some things. Still, I haven't had that problem (yet). This application drives a voltage multiplier, the other application was that of driving a tweeter (speaker) through a capacitor. The circuit worked satisfactorily in both cases.

    [​IMG]
     
  5. hrs

    Thread Starter Member

    Jun 13, 2014
    82
    7
    Oh right, the high-level and the low-level output ... But worst case would be 12.5V - 2.5V = 10V so I'm still missing a few volts.

    I'll give your circuit a try Dick, but with a 2N3904/2N3906 pair because that's what I have. But if the low-level output only reaches 2.5V wouldn't that mean the 2NSC2655 never switches off?

    I have some 2N7000 and BS250 too. Do you have an example of a push-pull arrangement Wayneh?
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,153
    3,059
    The 555 output can get close to the negative rail, just not the upper rail.

    Dick's circuit is exactly what I was referring to. It's a little trickier with MOSFETs (you need a high-side drive voltage if you don't use a P-channel).
     
  7. hrs

    Thread Starter Member

    Jun 13, 2014
    82
    7
    All right, I'll give it a shot with BJTs first and see if I get enough drive voltage. If not, I'll consider MOSFETs (BS250 is a P-channel).
    Thanks!
     
  8. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    The output of the 555 depends upon the load.
    Perhaps the transformer is saturating and causing a large load current which is pulling down the 555 output.
    What is the transformer rating?
     
  9. hrs

    Thread Starter Member

    Jun 13, 2014
    82
    7
    It's a DIY transformer using a 22 mm outer diameter toroid. The primary side is 28 turns of 0.5 mm wire, 2 secondaries have 16 turns each. When I look at either side I see a square wave with some ringing so it doesn't look like a lot of high frequency signal is being filtered, but it doesn't look saturated either. Here is what I think a saturated waveform would look like:
    http://sound.westhost.com/xfmr17-3.png
     
  10. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    That waveform looks correct for the current of a saturated transformer, but not necessarily the voltage.
     
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