555 driving PNP transistor?

Discussion in 'The Projects Forum' started by imbaine13, Nov 3, 2013.

  1. imbaine13

    Thread Starter Member

    Oct 6, 2013
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    Is this possible? I saw some article somewhere on the internet about connecting a 1KΩ resistor from the power rail to the base of the PNP (output of the 555)? Could someone please help me understand how this works?
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Sure it's possible. The 555 can both source or sink current. When sinking, it would turn on the PNP in your arrangement.
     
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  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Here is a PNP 555 current booster
     
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  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Here is the way I would do it.
     
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  5. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    To me, OP is connecting a resistor from VCC to pin3 of 555. Acts as a pullup so the output swing is more more uniform, allows closer 50% duty cycle.
     
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  6. SgtWookie

    Expert

    Jul 17, 2007
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    MikeML, the 555 timer included with LTSpice is an "idealized" model which is convenient for speed of simulation, but unfortunately doesn't emulate the output of either a bjt 555 nor a CMOS 555; although if you ignore the source/sink capability (or lack thereof) of a real CMOS timer, it comes closer to that.

    A component model (of the real bjt 555 timer) has a Darlington follower in its' output (which is what Bernard is alluding to), and thus won't rise within ~1.3V of Vcc without some assistance. You'd really need a base return (pullup) resistor on your PNP's base, otherwise it may turn off pretty slowly. This is what our OP was asking about.

    However, take another look at your circuit; you've used a 2N2907 with a 100 Ohm base resistor to drive a load that will require 1.2 Amperes - I think you meant to use a less demanding load, as the 2N2907's tongue will be hanging out above ~450mA collector current.

    Tracecom, you've used a 2N3906 in an emitter follower configuration, and the part number shows you're using a bjt 555 timer. Were you aware that the 2N3906 is designed for up to 100mA of collector current, whereas the output of the bjt 555 timer is rated for sourcing or sinking up to 200mA?
     
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  7. tracecom

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    Apr 16, 2010
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    Yes, I do realize that, from a current capability standpoint, the circuit doesn't make much sense. However, the OP seemed to want to know how to use a 555 with a PNP transistor, and the 2N3906 was the first one I found in my parts tray.

    Thanks.

    ETA: I tried MikeML's circuit and could not get it to drive an LED.
     
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  8. imbaine13

    Thread Starter Member

    Oct 6, 2013
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  9. iimagine

    Active Member

    Dec 20, 2010
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    Another way of doing this is adding two diodes (or an LED) in series with the bias resistor to increases the forward voltage required for the PNP transistor to about 2.1 volts so that the 1.7 volt difference from supply to the output, is not enough to turn on the transistor. Doing it this way would produce sharper turn off edge
     
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