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

[Mr. J D]

I have seen different site using 556 set up as one timer in astable
controlling a second timer in monostable mode. Other sites show PWM by
using only one 555 timer set up in astable mode. I would really rather
use only one 555 timer, but what is the upside to using a setup with a
556 timer?

 

[BobG]

I have seen different site using 556 set up as one timer in astable
controlling a second timer in monostable mode. Other sites show PWM by
using only one 555 timer set up in astable mode. I would really rather
use only one 555 timer, but what is the upside to using a setup with a
556 timer?

==========================================================
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? <1%
to >99%? or 10% to 90%?

 

[James Thompson]

==========================================================
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? <1%
to >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.

 

[Rich Grise]

I have seen different site using 556 set up as one timer in astable
controlling a second timer in monostable mode. Other sites show PWM by
using only one 555 timer set up in astable mode. I would really rather use
only one 555 timer, but what is the upside to using a setup with a 556
timer?

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

 

[John Fields]

I have seen different site using 556 set up as one timer in astable
controlling a second timer in monostable mode. Other sites show PWM by
using only one 555 timer set up in astable mode. I would really rather
use only one 555 timer, but what is the upside to using a setup with a
556 timer?

---
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.

 

[Mark Fortune]

---
Using a single 555 in astable mode will only allow duty cycles of


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

 

[John Fields]

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.

---
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

 

[Mark Fortune]

On Wed, 16 Aug 2006 00:26:54 +0100, Mark Fortune

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.

Version 4
SHEET 1 916 680
WIRE 336 32 -112 32
WIRE 416 32 336 32
WIRE 656 32 416 32

------8<--------------------8<----------------

What file format is that? i'm assuming its spice (although i've never
used it myself) but would like to try it (if it's free) since everyone
else seems to use it. where can I get a copy for *nix?

Mark

 

[jasen]

What file format is that? i'm assuming its spice (although i've never
used it myself) but would like to try it (if it's free) since everyone
else seems to use it. where can I get a copy for *nix?

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