The difference between using one 555 timer and a 556 timer for PWM control

========================================================== Maybe a wider PWM range or more stable frequency can be obtained with the separate trigger timer? What range of duty cycle do you need? 99%? or 10% to 90%?

Without seeing the circuit in question, my guess would be to have a timed set of pulses then a rest and another set of pulses. Maybe to fast charge a battery so there is a cool down period ? Tell the link the the circuit in question.

The one with the dual timer gives you "pulse frequency modulation" or PFM, while the single timer gives you "pulse-width modulation", or PWM. One is a stream of equal pulses at a varying frequency, and the other is a stream of variable-width pulses at a constant frequency.

Both ways have advantages and disadvantages - it depends on the application.

Good Luck! Rich

--
Using a single 555 in astable mode will only allow duty cycles of
>50%, at any frequency, while using an astable to determine rep rate
and a one-shot to determine the pulse width will allow you to get
from very close to 0% to very close to 100% duty cycle.

With a diode between pins 7 and 6 you can obtain duty cycles < 50% as it only uses one resistor to charge instead of two.

Mark

--- Damn! I forgot about the "magic diode"! Good catch.

But, it goes in parallel with R2 and with the anode on pin 7, no?

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-- John Fields Professional Circuit Designer

That is correct. And for people like me who always have to google to remember which one is the anode i'll draw a little zoomed in picture

IC1 - 555

| 7 6 2 | ----------------------- | | | \\/\\-+-/\\/\\/-+---+-||-+ R1 | R2 | C1| +-->|---+ 0v D1 etc.

------8

nix is easy linear technology give away ltspice for windows.

*nix is harder berkely spice is free by works from a command-line... theoretically it can be coupled with oregano (an X-based gui for drawing circuits) but last time I tried I couldn't figure it all out.

Apparently ltspice works well under wine. I haven't tried installing wine yet.

Bye. Jasen