NE555 problem

149

01:49:14

by SkeLe ¤¤
Inside the 555. While I was testing my ne555 model I tried to get the 50% Duty Cyle in the simplest way that is to connect the output to the external RC, i compared it with the wave generated by the ec model and I've found that the ec model has the problem that generate an uncorrect duty cycle and also frequency if used in this way. Please fix the problem. REMIND that to make my model working you have to open the switch that is in parallel to the capacitor. Don't touch the other switch.

published 10 years ago

hurz

10 years ago

"Please fix the problem"?????? Check first the 555 internals before talking about problems! The EC model fits in this case better then your simple thing!

SkeLe

10 years ago

Yeah the internal real circuit is a bit more complicated (using transistors) but why the dc isn't the 50% in the ec model?

hurz

10 years ago

Because the original 555 (and any bipolar versions) does not have a rail2rail output stage driver and loosing some 100 millivolt for high output level. Low fits better. Take a newer 555 version in CMOS and they would do a better 50% duty cycle.

SkeLe

10 years ago

I'm not talking about the output voltage that the high level is lower than the alimentation but only about the dc and frequency. Another strange think is that if you connect a capacitor to the control pin the dc change

hurz

10 years ago

The charge and discharge point is internally defined as 1/3 and 2/3 of power supply voltage. If High out is not 3/3 while Low is 0/3 you can't get a nice 50% duty cycle! So output voltage levels are important.

SkeLe

10 years ago

Yes you're right but why if you use a capacitor of a certain value between the ctrl pin and ground the dc will fix to 50%?

hurz

10 years ago

I don't see this effect with the ctrl pin. The cap is for the simulation not needed its only important in reality were noise from extern should be blocked to not modulate 555 e.g. phasenoise

SkeLe

10 years ago

It was a mine error because I tryed to put an high value capacitor in the ctrl pin and the effect at 10ms/s was that the capacitor charge changed the results for a while (till the capacitor is fully charged)

SkeLe

10 years ago

A "solution" could also be to put a resistor between ctrl and ground

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