Driving a PNP with a 555
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From: pawihte on 28 Dec 2009 07:42 Bob wrote: > On Dec 23, 9:50 am, "pawihte" <pawi...(a)fake.invalid> wrote: >> Jamie wrote: >>> pawihte wrote: >> >>>> I want to use a classic 555 timer IC to drive the base of a >>>> PNP >>>> transistor through a resistor, the emitter of the transistor >>>> being tied to the 555's Vcc. The 555 datasheet gives a graph >>>> for >>>> the high-state output voltage vs. sourcing current, but not >>>> when >>>> the load is tied to Vcc. >> >>>> Vcc >>>> -------------------------- >>>> | | >>>> .|. | >>>> | | | >>>> | | | >>>> '-' | >>>> | | >>>> 555 out ___ | |< >>>> ------------|___|------| >>>> |\ >>>> | >>>> | >> >>>> What I'm concerned about is: Is there a possibility that the >>>> high-state output of the 555 drops low enough below Vcc to >>>> partially turn on the PNP transistor? I could increase the >>>> turn-on threshold of the transistor with diodes, an LED or a >>>> resistive voltage divider, but I'd like to avoid that if >>>> it's >>>> not >>>> needed. >> >>> No. >>> The output of a 555 is not low due to a low side pulling on >>> it >>> there >>> for, you should not see biasing effects being generated from >>> some low >>> side source of the 555. >>> THe output of a 555 on the high side is a emitter, so what >>> you have >>> there, using that pull up R, will actually bring the base to >>> the VCC >>> when the 555 is in the high state.. >> >> That's what I thought, but I thought I'd better check with you >> guys in case there was something I missed. >> >>> At least it works for me that way. > > I hate to be a spoil sport, but with total component cost > coming in at > under a buck, why don't you just breadboard it up and see what > happens? There's always that of course, and the cost factor is certainly trivial. But working things out on paper and thinking through the whys, hows and ifs have their own reward. I'll bet this thread has prompted more than one reader to channel their thoughts in a direction they never took before. I'm not against empirical techniques and I employ them at times, but they do have limitations. For instance, a circuit may work with a specific set of parts under one particular environment, but may become unsatisfactory with a slight change of one or more parameters. Thanks for your interest. |