Driving a PNP with a 555
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From: pawihte on 22 Dec 2009 14:35 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: Tim Wescott on 22 Dec 2009 14:42 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. -- www.wescottdesign.com
From: Jim Thompson on 22 Dec 2009 14:47 On Wed, 23 Dec 2009 01:05:52 +0530, "pawihte" <pawihte(a)fake.invalid> 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. > Bipolar 555 or CMOS? CMOS, no problem. Bipolar (assuming 5V supply), use 750 Ohms base-to-emitter, 1.5K base-to-555-output. ...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 15:01 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?
From: pawihte on 22 Dec 2009 15:15
Jim Thompson wrote: > On Wed, 23 Dec 2009 01:05:52 +0530, "pawihte" > <pawihte(a)fake.invalid> > 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. >> > > Bipolar 555 or CMOS? CMOS, no problem. Bipolar (assuming 5V > supply), > use 750 Ohms base-to-emitter, 1.5K base-to-555-output. > Thanks. It's a bipolar 555, quasi-complementary NPN-NPN output with a 12V supply. I want the transistor to drive some IR LEDs in series at about 250mA peak, 0.2 duty. I thought I'd use 470 ohms as the base drive resistor and perhaps 1K base-emitter. Do you think that's not enough to ensure that the transistor will be off when it should be? |