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

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?

Version 4 SHEET 1 916 680 WIRE 336 32 -112 32 WIRE 416 32 336 32 WIRE 656 32 416 32 WIRE 416 128 416 32 WIRE 48 160 -16 160 WIRE 336 160 336 32 WIRE 336 160 272 160 WIRE 48 224 16 224 WIRE 416 224 416 208 WIRE 416 224 272 224 WIRE 656 240 656 32 WIRE 416 256 416 224 WIRE 48 288 -48 288 WIRE 336 288 272 288 WIRE -112 352 -112 32 WIRE 48 352 -112 352 WIRE 304 352 272 352 WIRE 336 368 336 288 WIRE 416 368 416 336 WIRE 416 368 336 368 WIRE 16 416 16 224 WIRE 336 416 336 368 WIRE 336 416 16 416 WIRE -16 464 -16 160 WIRE 416 464 416 432 WIRE 416 464 -16 464 WIRE 656 464 656 320 WIRE 656 464 416 464 WIRE 656 528 656 464 FLAG 656 528 0 SYMBOL Misc\\\\NE555 160 256 R0 SYMATTR InstName U1 SYMBOL voltage 656 224 R0 WINDOW 123 0 0 Left 0 WINDOW 39 24 132 Left 0 SYMATTR SpiceLine Rser=0 SYMATTR InstName V2 SYMATTR Value 12 SYMBOL res 400 240 R0 SYMATTR InstName R2 SYMATTR Value 1e6 SYMBOL cap 400 368 R0 SYMATTR InstName C1 SYMATTR Value 1e-7 SYMBOL res 400 112 R0 SYMATTR InstName R1 SYMATTR Value 1e7 TEXT -386 536 Left 0 !.tran 0 2 0

-- 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.

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

Bye. Jasen