Driving a PNP with a 555
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From: mook johnson on 22 Dec 2009 15:22 "pawihte" <pawihte(a)fake.invalid> wrote in message news:hgr8jd$96a$1(a)news.eternal-september.org... > 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? Should be fine for speed. Your pass transistor will dissipate ~800mW if you accidently leave it on 100%. Diode probably wouldn't like it either.
From: Jim Thompson on 22 Dec 2009 15:23 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 -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Help save the environment! Please dispose of socialism properly!
From: pawihte on 22 Dec 2009 15:37 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? > 200mA source or sink. > You can't connect one end of LED to plus rail? > I considered that and 200mA might be good enough. But I'd rather have the option of using a higher LED current or additional LED-resistor combos in parallel.
From: Tim Williams on 22 Dec 2009 15:12 This works fine for me: http://webpages.charter.net/dawill/Images/High%20Voltage%20Supply.gif The MJE18008 switches in under a microsecond. I forget if I boosted the base resistors at all -- 470 or even 220 ohms would be better on the 2N4403. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms "pawihte" <pawihte(a)fake.invalid> wrote in message news:hgr72q$uko$1(a)news.eternal-september.org... >I want to use a classic 555 timer IC to drive the base of a PNP transistor >through a resistor, the emitter of the transistor being tied to the 555's >Vcc. The 555 datasheet gives a graph for the high-state output voltage vs. >sourcing current, but not when the load is tied to Vcc. > > Vcc > -------------------------- > | | > .|. | > | | | > | | | > '-' | > | | > 555 out ___ | |< > ------------|___|------| > |\ > | > | > > What I'm concerned about is: Is there a possibility that the high-state > output of the 555 drops low enough below Vcc to partially turn on the PNP > transistor? I could increase the turn-on threshold of the transistor with > diodes, an LED or a resistive voltage divider, but I'd like to avoid that > if it's not needed. >
From: pawihte on 22 Dec 2009 15:47
Tim Williams wrote: > This works fine for me: > http://webpages.charter.net/dawill/Images/High%20Voltage%20Supply.gif > > The MJE18008 switches in under a microsecond. I forget if I > boosted > the base resistors at all -- 470 or even 220 ohms would be > better on > the 2N4403. > Tim > > > "pawihte" <pawihte(a)fake.invalid> wrote in message > news:hgr72q$uko$1(a)news.eternal-september.org... >> 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. Thanks for your interest. As mentioned in my opening post, the object is to avoid unnecessary component clutter. |