In 555 timer astable circuit with constant current source, capacitor is fully discharging and giving 0-2/3Vcc instead of 1/3Vcc -2/3Vcc.What could be the possible reason?
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I can't say for sure. Try a 10 ohm or 22 ohm, if that is not enough then just increase it until the bottom trigger voltage seems right. Keep in mind even a 99:1 duty cycle for a sawtooth generator is considered good. Using just a crude 555 you may have to settle for slightly worse than that.Thanks,I was also suspecting the same reason,but was not sure.
During design, it gives perfectly fine result in simulations.
I avoided resistor because I wanted to have a perfect saw tooth voltage waveform,as it gives slightly sloppy discharging. anyway, what minimum value of resistor would do to not have sloppy discharge but 1.7 V pk-pk ?
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I don't buy that. The typical monostable application circuit in the datasheet doesn't have a current limiting resistor.Because the discharge pin (which is an open collector pulldown) is directly connected to the cap.
When discharge switches on you get a very high current discharge pulse that will fully discharge the cap. It also risks damaging the 555 timer.
You need to add a resistor in series with pin 7, so the discharge takes some time, and gives time for the internal comparator to sense the cap voltage is under 0.3Vcc and end the discharge cycle.
Yeah, but I was thinking of a BIG cap, like 1 Farad.Big cap., small cap, makes no difference. The discharge current is limited by the effrective series resistance of the discharge transistor which is sized in order to match that resistance to the maximum current it can handle
Exactly. even with modest duty cycles, high frequency astables will run at lower than predicted frequencies, due to the delay of the Schmitt trigger and to stray and device capacitances.Yeah point taken, I was probably extravagant with the claim that the 555 would be damaged not having a series resistor on the pin.
As for the cap voltage going too low (below 1/3 Vcc), I have seen similar things with no series resistor on pin7 especially when the \ slope is very fast and there may be external capacitances (from a breadboard etc). The internal comparator of the 555 is a bit slow to switch, and the discharge transistor on pin7 is a bit slow to turn off. You really need a bit of a delay there from a series resistor if you want a reliable 1/3 Vcc lower trigger point.
if you know this, then Please suggest possible reasonI don't buy that. The typical monostable application circuit in the datasheet doesn't have a current limiting resistor.
by Jake Hertz
by Jake Hertz
by Jake Hertz
by Jake Hertz