Exceeding 555 spec

Discussion in 'The Projects Forum' started by tindel, Jun 9, 2013.

  1. tindel

    Thread Starter Active Member

    Sep 16, 2012
    568
    193
    I've have a pulse delay circuit that I use with my oscilloscope (an older Tek564) so that I can trigger the scope just before doing something with my circuitry. While looking at my design again today to make some mods, I noticed that the TI NE555 spec sheet has an example circuit that violates it's own spec (Figure 22 - Sequential Timer Circuit). http://www.ti.com/litv/pdf/slfs022h

    The spec says that the trigger input voltage shall not exceed the input voltage. I pulled out my simulation and noticed that AC coupling the output pin of one 555 to the trigger pin of a second 555 chip places two times the supply voltage on the trigger pin. Which makes perfect sense if you consider that the fully charged cap is being boosted up above the rail.

    What really blew me away is that the NE555 spec actually shows this circuit in it's app notes! Does it really not matter? I went down to the lab to test the circuit myself, and sure enough - it's going 4V above the rail. I attribute the fact that it didn't go to 5V due to the ESR of the crappy electrolytic I use on this circuit. The scope plot shows 1V/div - AC coupled. The midpoint is at 5Vdc. Sorry the picture quality isn't real good.

    I have run this circuit for many, many hours and have never had a single failure. Am I missing something? I know this is a pretty common use of the part why isn't it spec'd to do the job?

    Another thing I noticed about the circuit that I don't really care for is that C5 isn't current limited... theoretically you'll get unlimited current through the discharge pin to ground for a brief amount of time, assuming the switch is more/less perfect. I'm guessing the discharge pin is current limited somehow, either actively or just with a resistor. Seems like information about this is missing from the datasheet.

    Also note that I'm not using a 2n2222 - I know it can't handle those currents and powers, I just put it in there as reference for those that read this to understand my circuit.
     
  2. tindel

    Thread Starter Active Member

    Sep 16, 2012
    568
    193
    I did find it ingenious, however, that the boosted voltage actually puts current back onto the power rail, conserving some energy.
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,056
    Interesting find, good catch. It could be about the small amount of energy that the 0.001µF capacitor allows to pass. The higher voltage is not per se a problem, but would be if directly connected long enough to the trigger pin. Or maybe the circuit in Fig. 22 would actually begin to have problems if Vcc were taken to the higher reaches of the Vcc spec (18V). None of the example circuits use a Vcc greater than 15V. So maybe if Vcc=10, a trigger signal up to 14V could be tolerated whereas 17V and 21V would not.
     
  4. OoglieBooglie

    New Member

    Jun 3, 2013
    21
    2
    Ooh! Ooh! I've got it!

    See enclosed picture.

    If nothing else, I've entertained you for a minute. :)
     
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