Driving a PNP with a 555
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From: Nico Coesel on 22 Dec 2009 16:49 "pawihte" <pawihte(a)fake.invalid> wrote: >Jim Thompson wrote: >> On Wed, 23 Dec 2009 02:07:15 +0530, "pawihte" >> <pawihte(a)fake.invalid> >> wrote: >> >>> 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. >>> >> >> 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. -- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... "If it doesn't fit, use a bigger hammer!" --------------------------------------------------------------
From: Spehro Pefhany on 23 Dec 2009 11:14 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. 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?
From: ian field on 23 Dec 2009 11:14 "pawihte" <pawihte(a)fake.invalid> wrote in message news:hgrcm2$5kn$1(a)news.eternal-september.org... > Jim Thompson wrote: >> On Wed, 23 Dec 2009 02:07:15 +0530, "pawihte" <pawihte(a)fake.invalid> >> wrote: >> >>> 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. >>> >> >> 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. > String a trimpot between 2 diodes (facing opposite ways) wiper to pins 2 & 6 (IIRC) and the common of the 2 diodes to pin 7. With the pot at one extreme the cap will charge faster than it discharges, swing the pot the other way and the cap will discharge faster than it charges. That will give you a duty cycle of not far short of 0 to 100%.
From: John Fields on 22 Dec 2009 17:57 On Wed, 23 Dec 2009 02:54:11 +0530, "pawihte" <pawihte(a)fake.invalid> wrote: >Jim Thompson wrote: >> Do you desire variable or fixed duty cycle? >> > >Fixed. There's no precise requirement for the duty cycle but it >should be around 20%. If you don't already have it, download LTspice IV, free, from: http://www.linear.com/designtools/software/ and run this: --- Version 4 SHEET 1 948 748 WIRE 800 16 -128 16 WIRE 800 32 800 16 WIRE 800 144 800 112 WIRE -128 192 -128 16 WIRE 224 192 -128 192 WIRE 544 192 448 192 WIRE -128 256 -128 192 WIRE -80 256 -128 256 WIRE 32 256 0 256 WIRE 224 256 32 256 WIRE 512 256 448 256 WIRE 800 272 800 208 WIRE 32 320 32 256 WIRE 64 320 32 320 WIRE 176 320 144 320 WIRE 224 320 176 320 WIRE 608 320 448 320 WIRE 736 320 688 320 WIRE 480 384 448 384 WIRE 32 416 32 320 WIRE 80 416 32 416 WIRE 176 416 176 320 WIRE 176 416 144 416 WIRE 176 480 176 416 WIRE 512 480 512 256 WIRE 512 480 176 480 WIRE -128 512 -128 256 WIRE 480 512 480 384 WIRE 480 512 -128 512 WIRE -128 528 -128 512 WIRE 176 544 176 480 WIRE -128 624 -128 608 WIRE 176 624 176 608 WIRE 176 624 -128 624 WIRE 544 624 544 192 WIRE 544 624 176 624 WIRE 800 624 800 368 WIRE 800 624 544 624 WIRE -128 688 -128 624 FLAG -128 688 0 SYMBOL Misc\\NE555 336 288 M0 SYMATTR InstName U1 SYMBOL voltage -128 512 M0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 15 SYMBOL res 48 304 M90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 58 VTop 0 SYMATTR InstName R1 SYMATTR Value 30K SYMBOL res -96 240 M90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R2 SYMATTR Value 6.2K SYMBOL cap 192 544 M0 WINDOW 0 -33 32 Left 0 WINDOW 3 -39 58 Left 0 SYMATTR InstName C1 SYMATTR Value 1E-9 SYMBOL diode 80 400 M90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName D1 SYMATTR Value 1N4148 SYMBOL res 704 304 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R3 SYMATTR Value 1000 SYMBOL npn 736 272 R0 SYMATTR InstName Q1 SYMATTR Value 2N4401 SYMBOL res 784 16 R0 SYMATTR InstName R4 SYMATTR Value 100 SYMBOL LED 784 144 R0 SYMATTR InstName D3 SYMATTR Value QTLP690C TEXT 0 656 Right 0 !.tran .0002 uic JF
From: Tim Williams on 22 Dec 2009 18:44
"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. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms |