# minimum 555 duty cycle

• posted

does anyone know what the smallest duty cycle is that you can get out of a 555 astable multivibrator? I see the equations for getting under 50% with a diode in parallel with a resistor, but I'm not sure what the minimum pulse width is

much thanks

• posted

Well, first, there are a lot of different chips that go under the heading of "555" -- so you need to do some data-sheet diving.

Second, it's not a question of the smallest duty cycle that _I_ can get out of _a_ 555 -- it's a question of the smallest duty cycle that _you_ can get out of _your_ 555, in your circuit.

I assume by your mention of a diode that you're looking for short high- time pulses and long low ones? You may be best served by making R2 small and using an inverter on the output rather than using a diode.

Look at the specification for the maximum current that the discharge pin can handle. If you're just looking for a short off time (i.e., you're willing to have a short low-going pulse), then size R2 such that your discharge pin current isn't exceeded, look at the capacitance you've chosen for C1, choose the biggest value for R1 that you can, and be happy with the result.

If you want to do the "diode across R2" trick, then be aware that the high current that you need through R1 for quick capacitor charging will be on all the time -- when it's not flowing into the capacitor, it'll be flowing into the discharge pin.

An alternate astable circuit with a 555 is to choose a CMOS version whose output goes to the rails. Then you can ignore the discharge pin and the usual two resistors involving it, and just run a resistor back to the output for a 50% duty cycle. Or, you can run a resistor and diode in series, with that mess in parallel with a single resistor, to get a different duty cycle. Again, the duty cycle you can achieve depends on the maximum current on one end, and the minimum you trust on the other.

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Tim Wescott
Control system and signal processing consulting```
• posted

astable multivibrator? I see the equations for getting under 50% with a diode in parallel with a resistor, but I'm not sure what the minimum pulse width is

If you use the discharge pin you can make that cycle quite short. I don't have any numbers... refer to the data sheet and try it. ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |```
• posted

astable multivibrator? I see the equations for getting under 50% with a diode in parallel with a resistor, but I&#39;m not sure what the minimum pulse width is

thanks for the feedback, this answers my question

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