Driving a PNP with a 555
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From: pawihte on 23 Dec 2009 12:47 Tim Williams wrote: > "pawihte" <pawihte(a)fake.invalid> wrote in message > news:hgrc5p$1ld$1(a)news.eternal-september.org... >> Your 555 output also sources the base drive for the 2N4401 on >> the >> high state. If it still goes high enough to turn the 2N4403 >> off, my >> application should have a better margin. But just to be sure, >> did >> you ever observe the swings with a scope? > > I think it was saturating at 2-3 Vbe's. I could go check. > > The important part is getting the B-E resistors small enough so > the > transistor is certainly on or off. Which actually, with 1k and > 1k, it > should only be turning off with less than 1.2V (if it's 1.8V, > the PNP > might never fully turn off!). Hmm, I should probably change > those > resistor values then. > I'm also thinking along the same lines. I'd originally intended to use 470 ohms for drive and 1k as the b-e shunt. Reducing the shunt to 220 ohms will give me better peace of mind. It will divert 3-3.5mA from the base drive, but the remaining 20mA should still be enough to saturate the transistor. If not, I could halve each resistor and still stay well within the 555's output capability.
From: pawihte on 23 Dec 2009 12:50 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.
From: Hammy on 23 Dec 2009 13:18 On Tue, 22 Dec 2009 17:46:33 -0600, John Fields <jfields(a)austininstruments.com> wrote: >On Tue, 22 Dec 2009 21:49:59 GMT, nico(a)puntnl.niks (Nico Coesel) wrote: > >>"pawihte" <pawihte(a)fake.invalid> wrote: >> >>>Jim Thompson wrote: > >>>> Swap phase and use an NPN booster? Then you have the advantage >>>> of a >>>> true "OFF" state. >>>> >>> >>>The 555 wouldn't go below 50% duty in the opposite phase. At >>>least not with the basic astable circuit. I haven't investigated >>>to see if it's possible to change that with some manipulation. >> >>AFAIK the 555 can do less than 50% duty cycle. > >--- >Not without external help. > >From Signetics' 1979 applications manual: > >news:5dm2j51e8fcob9e8ha0skll5gjdrgkd7a9(a)4ax.com > >JF For people who cant get binaries you can also get it here. Full pdf is available at the top of the page right side. http://www.williamson-labs.com/555_apps.htm Other 555 stuff on the main page as well. http://www.williamson-labs.com/480_555.htm
From: Joel Koltner on 23 Dec 2009 13:27 So what's the history of the LM122? Was it marketed as an improvement to the 555?
From: Bob on 23 Dec 2009 15:43
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? |