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Timer 555 one-shot trouble
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Hello.
I?m trying to test some ciruits with several models of the tlc555 timer(i.e. the models here in the database or provided with LTSpice), but am not able to run it in monostable (one-shot) mode like in this description : C1 (and so the threshold voltage)charges up to Vcc and nothing switches. The output only followes the trigger signal, provided by a voltage controlled switch. Each model i tried behaves the same. Astable circuits do operate well. The crackpoint seems to be the connection between discharge and threshold pins ? Yours Reinhard |
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Reinhard, I don't have a clue how to fix an ltspice problem, if it is, but I can tell you I could never get that circuit to work, either.
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What I did was to add a trigger circuit so the 555 would be reset and retriggered into delay mode within a few microseconds of turn-on, thus ensuring it would work correctly. Dunno if that helps; I can dig the circuit out if you need me to. best, charlie -----Original Message-----
From: "Reinhard" <compfox@...> Sent: Wednesday, November 18, 2009 8:44am To: LTspice@... Subject: [LTspice] Timer 555 one-shot trouble Hello. I?m trying to test some ciruits with several models of the tlc555 timer(i.e. the models here in the database or provided with LTSpice), but am not able to run it in monostable (one-shot) mode like in this description : [] C1 (and so the threshold voltage)charges up to Vcc and nothing switches. The output only followes the trigger signal, provided by a voltage controlled switch. Each model i tried behaves the same. Astable circuits do operate well. The crackpoint seems to be the connection between discharge and threshold pins ? Yours Reinhard [Non-text portions of this message have been removed] |
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The trigger pulse width must be shorter than the desired one-shot duration. Otherwise, it forces the output flip-flop to stay set. Thus, the output will appear to follow the trigger input, even though the timing (threshold) capacitor has charged to a voltage greater than 2/3 Vcc. A common practice is to place a capacitor in series with the trigger pin. This provides a short negative pulse to the trigger input and allows the trigger input to return to a high level regarless of the length of the trigger pulse. Depending on the 555 timer model, you may need a resistor pull-up?on the trigger pin side of the cap.
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? ?? - Philip --- On Wed, 11/18/09, synchronousmosfet@... <synchronousmosfet@...> wrote:
From: synchronousmosfet@... <synchronousmosfet@...> Subject: RE: [LTspice] Timer 555 one-shot trouble To: LTspice@... Date: Wednesday, November 18, 2009, 3:42 PM ? Reinhard, I don't have a clue how to fix an ltspice problem, if it is, but I can tell you I could never get that circuit to work, either. What I did was to add a trigger circuit so the 555 would be reset and retriggered into delay mode within a few microseconds of turn-on, thus ensuring it would work correctly. Dunno if that helps; I can dig the circuit out if you need me to. best, charlie -----Original Message----- From: "Reinhard" <compfox@yahoo. de> Sent: Wednesday, November 18, 2009 8:44am To: LTspice@yahoogroups .com Subject: [LTspice] Timer 555 one-shot trouble Hello. I?m trying to test some ciruits with several models of the tlc555 timer(i.e. the models here in the database or provided with LTSpice), but am not able to run it in monostable (one-shot) mode like in this description : [ ik-kompendium. de/sites/ slt/0310121. htm] ik-kompendium. de/sites/ slt/0310121. htm C1 (and so the threshold voltage)charges up to Vcc and nothing switches. The output only followes the trigger signal, provided by a voltage controlled switch. Each model i tried behaves the same. Astable circuits do operate well. The crackpoint seems to be the connection between discharge and threshold pins ? Yours Reinhard |
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Reinhard,
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? I refer you to page 7 of the data sheet for the NE/SA/SE555/SE555C from Philips Semiconductor () ? "Trigger Pulse Width Requirements and Time Delays ? Due to the nature of the trigger circuitry, the timer will trigger on the negative going edge of the input pulse. For the device to time out properly, it is necessary that the trigger voltage level be returned to some voltage greater than one third of the supply before the time out period. This can be achieved by making either the trigger pulse sufficiently short or by AC coupling into the trigger. By AC coupling the trigger, see Figure 6, a short negative going pulse is achieved when the trigger signal goes to ground. AC coupling is most frequently used in conjunction with a switch or a signal that goes to ground which initiates the timing cycle. Should the trigger be held low, without AC coupling, for a longer duration than the timing cycle the output will remain in a high state for the duration of the low trigger signal, without regard to the threshold comparator state." ? ? Regards, ? ?? Philip --- On Thu, 11/19/09, Reinhard <compfox@...> wrote:
From: Reinhard <compfox@...> Subject: [LTspice] Re: Timer 555 one-shot trouble To: LTspice@... Date: Thursday, November 19, 2009, 10:49 AM ? Yeah, that helped. But without the capacitor the simulation behaves quite unnormal. And this doesn`t match the real hardware. There the capacitor is not necessary. Therefore circuits developed with these models are not transposable into reality. Thanks Reinhard [Non-text portions of this message have been removed] |
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Hi all,
Yesterday I found the following website. There are models of LTC556/LTC555 for different simulators. maybe it will be helpful to check that models are working well. I have some experiences with LTC556 spice models what I verified on the real circuitry. The simulation is not real in some cases. I found some problematic points in the subcircuit, what I modified and partly solved that discrepancies between measurements and simulation. ... But I cannot swear, those modification are universal for all solutions. Please, check, how good are the following models on the bottom of this page: Regards, Peter |
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What versions of SPICE did you use to test the simulations of the SPICE models supplied by TI ? Did you use LTspice, Pspice or Tina TI? May I assume you meant TLC556/TLC555 and not LTC556/LTC555?
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Howard Peter Kapas wrote: Hi all, |
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Hi Howard,
You're right. The first Models of 555/556 was from the LTspice "Masc." folder. The second attempt was the transistor realization from LTspice's /examples/Educational NE555.asc, following Mike's advise. Behavioral and transistor model gave me the same (wrong) results for small signal inputs. I investigated on the website and I found a few different realization of these subcircuits, but they had the same problems. Correctly the problem was with low voltage behavior of Trigger and Threshold inputs. I tried to make PWM circuitry which was perfect in simulation in range of 0 to Vcc, but on the bench the physical realization felt under 1V ov input voltage. I asked for samples from IC vendors (ST, TI). On both these imputes behaved differently, they lost linearity under 1V at Vcc=5V. Regards, Peter |
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