Driving a PNP with a 555
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From: Jamie on 22 Dec 2009 20:33 pawihte wrote: > Tim Wescott wrote: > >>On Wed, 23 Dec 2009 01:05:52 +0530, 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. >> >>If it's a CMOS 555 then the output will drive to the rail, near >>enough. >> >>If it's a bipolar 555 then chances are the output is a >>totem-pole, >>which (if I remember correctly) won't drive _to_ the +V rail at >>all >>vigorously, but get in the way of a pullup at all. In fact (if >>I >>remember correctly) this was one way of interfacing bipolar >>parts to >>CMOS, if you didn't mind a bit of a speed hit. So check. >> >>Since your bias network provides that pull-up, you're probably >>fine. >>To really drive things fast you may want a resistor from the >>pin to >>+V, before the base current-limit resistor. > > > Thanks. It's bipolar and, according to the datasheet, the output > is a totem-pole NPN-NPN push-pull. I want to drive an IR LED at > 38kHz at about 250mA peak with a 20% duty factor. Do you think > it's OK as is? > > Hmm. that's a large IR LED? are you sure about the current demand? The 555 will do 200 mA on its on.. That in it self can drive 4..5 average LED's -- "I'd rather have a bottle in front of me than a frontal lobotomy"
From: Jamie on 22 Dec 2009 20:38 Jim Thompson wrote: > On Wed, 23 Dec 2009 01:31:42 +0530, "pawihte" <pawihte(a)fake.invalid> > wrote: > > >>Tim Wescott wrote: >> >>>On Wed, 23 Dec 2009 01:05:52 +0530, 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. >>> >>>If it's a CMOS 555 then the output will drive to the rail, near >>>enough. >>> >>>If it's a bipolar 555 then chances are the output is a >>>totem-pole, >>>which (if I remember correctly) won't drive _to_ the +V rail at >>>all >>>vigorously, but get in the way of a pullup at all. In fact (if >>>I >>>remember correctly) this was one way of interfacing bipolar >>>parts to >>>CMOS, if you didn't mind a bit of a speed hit. So check. >>> >>>Since your bias network provides that pull-up, you're probably >>>fine. >>>To really drive things fast you may want a resistor from the >>>pin to >>>+V, before the base current-limit resistor. >> >>Thanks. It's bipolar and, according to the datasheet, the output >>is a totem-pole NPN-NPN push-pull. I want to drive an IR LED at >>38kHz at about 250mA peak with a 20% duty factor. Do you think >>it's OK as is? >> > > > Refresh my memory, what's the sink current of a bipolar 555? > > You can't connect one end of LED to plus rail? > > ...Jim Thompson Its 200 mA
From: pawihte on 23 Dec 2009 01:54 Jon Kirwan wrote: > On Tue, 22 Dec 2009 13:23:44 -0700, Jim Thompson > <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com/Snicker> wrote: > >> On Wed, 23 Dec 2009 01:31:42 +0530, "pawihte" >> <pawihte(a)fake.invalid> >> wrote: >> >>> Tim Wescott wrote: >>>> On Wed, 23 Dec 2009 01:05:52 +0530, 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. >>>> >>>> If it's a CMOS 555 then the output will drive to the rail, >>>> near >>>> enough. >>>> >>>> If it's a bipolar 555 then chances are the output is a >>>> totem-pole, >>>> which (if I remember correctly) won't drive _to_ the +V rail >>>> at >>>> all >>>> vigorously, but get in the way of a pullup at all. In fact >>>> (if >>>> I >>>> remember correctly) this was one way of interfacing bipolar >>>> parts to >>>> CMOS, if you didn't mind a bit of a speed hit. So check. >>>> >>>> Since your bias network provides that pull-up, you're >>>> probably >>>> fine. >>>> To really drive things fast you may want a resistor from the >>>> pin to >>>> +V, before the base current-limit resistor. >>> >>> Thanks. It's bipolar and, according to the datasheet, the >>> output >>> is a totem-pole NPN-NPN push-pull. I want to drive an IR LED >>> at >>> 38kHz at about 250mA peak with a 20% duty factor. Do you >>> think >>> it's OK as is? >>> >> >> Refresh my memory, what's the sink current of a bipolar 555? > > Datasheet says sink and source is up to 200mA. But the curves > only go > up to 100mA on this sheet and the spec doesn't provide a > guarantee > output voltage when sinking 200mA -- only at 100mA and only at > Vcc of > 15V, which is 2.5V. The curves show a nice 2V output at 100mA > at Vcc > of 5V, but that's probably typical and not a guarantee and it's > almost > twice that shown for 100mA at Vcc of 15V (which is closer to 1V > on the > graph.) > >> You can't connect one end of LED to plus rail? > > Not at 250mA. The darned thing has no guarantee low-out even > at 200mA > and the guarantee at 100mA is 2.5V! Hauling it to 250mA might > work > but even if Vcc minus that drop is adequate, I'd start to worry > about > package dissipation. The packages vary from 100C/W to 200C/W, > roughly, and the junction temp should not exceed 150C. So 1/2 > watt > would be about the max I'd want to mess around considering. At > up to > 2.5V drop and 250mA... that's already exceeded. > > I think the OP is definitely right to use a discrete. > I considered all those factors before but didn't include the details to keep my post from becoming too lengthy. Thanks for the confirmation.
From: pawihte on 23 Dec 2009 02:01 Jamie wrote: > pawihte wrote: > >> Tim Wescott wrote: >> >>> On Wed, 23 Dec 2009 01:05:52 +0530, 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. >>> >>> If it's a CMOS 555 then the output will drive to the rail, >>> near >>> enough. >>> >>> If it's a bipolar 555 then chances are the output is a >>> totem-pole, >>> which (if I remember correctly) won't drive _to_ the +V rail >>> at >>> all >>> vigorously, but get in the way of a pullup at all. In fact >>> (if >>> I >>> remember correctly) this was one way of interfacing bipolar >>> parts to >>> CMOS, if you didn't mind a bit of a speed hit. So check. >>> >>> Since your bias network provides that pull-up, you're >>> probably >>> fine. >>> To really drive things fast you may want a resistor from the >>> pin to >>> +V, before the base current-limit resistor. >> >> >> Thanks. It's bipolar and, according to the datasheet, the >> output >> is a totem-pole NPN-NPN push-pull. I want to drive an IR LED >> at >> 38kHz at about 250mA peak with a 20% duty factor. Do you think >> it's OK as is? >> >> > Hmm. that's a large IR LED? are you sure about the current > demand? > > The 555 will do 200 mA on its on.. That in it self can drive > 4..5 > average LED's I need to project the beam a considerable distance. Hence the high peak current. It's also why I want to have the option of increasing the peak current above 250mA. Most 5mm IR LEDs I've seen are rated for 1A peak, 100mA continuous.
From: pawihte on 23 Dec 2009 12:39
Spehro Pefhany wrote: > On Wed, 23 Dec 2009 12:31:18 +0530, "pawihte" > <pawihte(a)fake.invalid> > wrote: > >> Jamie wrote: >>> pawihte wrote: >>> >>>> Tim Wescott wrote: >>>> >>>>> On Wed, 23 Dec 2009 01:05:52 +0530, 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. >>>>> >>>>> If it's a CMOS 555 then the output will drive to the rail, >>>>> near >>>>> enough. >>>>> >>>>> If it's a bipolar 555 then chances are the output is a >>>>> totem-pole, >>>>> which (if I remember correctly) won't drive _to_ the +V >>>>> rail >>>>> at >>>>> all >>>>> vigorously, but get in the way of a pullup at all. In fact >>>>> (if >>>>> I >>>>> remember correctly) this was one way of interfacing bipolar >>>>> parts to >>>>> CMOS, if you didn't mind a bit of a speed hit. So check. >>>>> >>>>> Since your bias network provides that pull-up, you're >>>>> probably >>>>> fine. >>>>> To really drive things fast you may want a resistor from >>>>> the >>>>> pin to >>>>> +V, before the base current-limit resistor. >>>> >>>> >>>> Thanks. It's bipolar and, according to the datasheet, the >>>> output >>>> is a totem-pole NPN-NPN push-pull. I want to drive an IR LED >>>> at >>>> 38kHz at about 250mA peak with a 20% duty factor. Do you >>>> think >>>> it's OK as is? >>>> >>>> >>> Hmm. that's a large IR LED? are you sure about the current >>> demand? >>> >>> The 555 will do 200 mA on its on.. That in it self can drive >>> 4..5 >>> average LED's >> >> I need to project the beam a considerable distance. Hence the >> high peak current. It's also why I want to have the option of >> increasing the peak current above 250mA. Most 5mm IR LEDs I've >> seen are rated for 1A peak, 100mA continuous. > > Well, possibly the most elegant circuit would be a CMOS 555 > driving a > P-channel MOSFET gate directly, assuming a reasonable supply > voltage > so as not to be too close to the maximum Vgs. > Technically, yes, it's an elegant solution. But P-channel MOSFETs are not easily available here and the object was to minimize component count. Otherwise the 12V power supply is high enough to allow inserting a zener diode or an LED in series with the base resistor to raise the turn-on threshold. > Eg. http://www.diodes.com/datasheets/ds31125.pdf > http://www.nxp.com/documents/data_sheet/ICM7555.pdf > > Total cost about 30 cents in 1K, even from Digikey, and it > could > switch as much as a couple of amperes with 20% duty cycle, and > the 555 > won't need any extra bits to get 20% on-time. > > Of course if it's a school project you may have to use the > parts in > the parts crib, eh? Cost is not a primary concern here. I think I'll just reduce the b-e shunt resistor from 1k to 220 ohms. That will steal 3-3.5mA from the base drive, but still leave about 20mA which should be enough. |