555 Calculator

[Michael A. Terrell]

Jim Thompson wrote:

Forget that idiot- he is clearly wrong about that hokus pokus 50% duty
circuit National publishes. I see no practical use for that touchy thing:
View in a fixed-width font such as Courier.

.
.
.
.
.
. Vcc
. |
. RA TH=069*RA*C
. |
. DIS-RB--+ RA*RB RB-2RA
. | TL= ------- * C *Ln( ------ )
. | RA + RB 2RB-RA
. ===
. |
. |
. --- substituting RA*C=1.44*TH
.
.
.
. TL 1.44*RB RB-2RA
. -- = ------- *Ln( ------ )
. TH RA + RB 2RB-RA
.
.
. RA TL 1.44 1-2r
. substituting r= -- => -- = -------*Ln( ---- )
. RB TH 1 + r 2-r
.
.
. TL ~
. D=50% => -- = 1 , makes r=2.3 , RA and RB always in same ratio.
. TH
.
.
. TL
. (--)
. TH - r 4.32
. Then S = ----- *( 1 + ------------)= -3.5
. r 1 + r (1-2r)*(2-r)
.
.
.
.
.
. For the case:
.
.
. Vcc
. |
. RA
. |
. DIS---+
. |
. RB
. |
. ===
. |
. ---
.
. TL RB 1
. where RA<<RB or r->0, -- = ------ = ----
. TH RA + RB 1+r
.
.
. TL
. (--)
. TH - r
. => S = ----- ->0 also
. r 1 + r TL
. (--)
. TH
. Example case of C=0.001u RA=10K RB=720K makes S =-0.014
. r
.
. Looks to be 3.5/0.014=250x better with initial accuracy.
.
.
. The "standard ckt" is patently inferior.

The simple way to ensure a 50% duty cycle is to run it at twice the
desired frequency and shove it through a flip-flop.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Fred Bloggs]

The simple way to ensure a 50% duty cycle is to run it at twice the
desired frequency and shove it through a flip-flop.

That has already been noted- but I was confining it to just the 555.

 

[Jim Thompson]

Jim Thompson wrote:

Forget that idiot- he is clearly wrong about that hokus pokus 50% duty
circuit National publishes. I see no practical use for that touchy thing:
View in a fixed-width font such as Courier.

.
. [snip]
.
. The "standard ckt" is patently inferior.

Ehhh? What was it that _I_ wrote ?

...Jim Thompson

 

[Fred Bloggs]

Jim Thompson wrote:

Forget that idiot- he is clearly wrong about that hokus pokus 50% duty
circuit National publishes. I see no practical use for that touchy thing:
View in a fixed-width font such as Courier.

.
.
[snip]

.
. The "standard ckt" is patently inferior.

Ehhh? What was it that _I_ wrote ?

...Jim Thompson

I know but I was confining it to the 555 alone - there is a middle
ground between good'nuf and absolute exactitude you know.

 

[Roy L. Fuchs]

"Fred Bloggs"

Grow the **** up, PhilTard!

 

[Phil Allison]

"Jim Thompson"

Every time I've completely forgotten Phil,

** Thompson is just as big an asshole as Bloogs.

Plus a gutless libeller as well.




....... Phil

 

[Jamie]

doesn't 50% duty cycle imply that the output
be on the same time frame as it is off ?
i don't see where the rail-to-rail comes
into this?
since the trigger and threshold are at
33% and 66% of Vcc i don't see where the
in ability to effect 50% duty is coming from ?

 

[budgie]

Fsck you are a priceless piece. I used the CIRCUIT once.

 

[budgie]

Vcc is 12V. I'm using it to drive a stepper driver board; L297/L298 for
a constant speed application. The RPM is critical, thus the 1KHZ req.

If the 1kHz is *critical*, what is the tolerance? If it is within +/-5% I
wouldn't even be considering a 555.

 

[Phil Allison]

"budgie"

If the 1kHz is *critical*, what is the tolerance? If it is within +/-5% I
wouldn't even be considering a 555.

** More " budgie droppings ".

They may be small, but they are sure smelly and plentiful.




........ Phil

 

[John Fields]

doesn't 50% duty cycle imply that the output
be on the same time frame as it is off ?
i don't see where the rail-to-rail comes
into this?
since the trigger and threshold are at
33% and 66% of Vcc i don't see where the
in ability to effect 50% duty is coming from ?

---
Vcc
|
C
B Q1
E
|
+--[R]--+
| |
C [C]
B Q2 |
E GND
|
GND

When Q1 switches on, C will charge to 2/3Vcc through R with a source
voltage of Vcc - Vce, but when Q2 turns on it'll discharge to 1/3
Vcc through Q2's Vce(sat), so since Vce(sat) is less than Q1's Vcc -
Vce, it'll take the cap less time to discharge to 1/3 Vcc than it
will to charge to 2/3 Vcc.

 

[martin griffith]

If the 1kHz is *critical*, what is the tolerance? If it is within +/-5% I
wouldn't even be considering a 555.

Well, a lot of capstan motor servos on Studer pro audio tape decks
used 555s for decades. They worked extermely well.
I suppose the designer just new how to use them properly


martin

 

[budgie]

Well, a lot of capstan motor servos on Studer pro audio tape decks
used 555s for decades. They worked extermely well.
I suppose the designer just new how to use them properly

Without knowing the configuration it's a bit diffcult to comment. For all I
know there is tachofeedback, in which event the repeatability factor is largely
removed.

 

[Jasen Betts]

["Followup-To:" header set to sci.electronics.misc.]

doesn't 50% duty cycle imply that the output
be on the same time frame as it is off ?
i don't see where the rail-to-rail comes
into this?
since the trigger and threshold are at
33% and 66% of Vcc i don't see where the
in ability to effect 50% duty is coming from ?

I=V/R

if the output gets closer to the rail one way than it does the other

diifferent charge/discharge currents will result.

 

[JR North]

What youse guys failed to note in your circuit recommendations is that
output freq varies with VCC. Since this application is a 12V marine
elect. sys, variation in system voltage causes changes in motor RPM.
Perhaps useful for something, but not for me. Modified the circuit with
a 8V reg, since a 5V reg drops the output below TTL and the stepper
board doesn't like it.
BTW: this driver runs a stepper to rotate a radar antenna. Thus the RPM
req. Variation in antenna RPM causes the display to lose sync.
Back on track now.

 

[martin griffith]

Without knowing the configuration it's a bit diffcult to comment. For all I
know there is tachofeedback, in which event the repeatability factor is largely
removed.

I must admit I havent look at the typical Studer circuit in long long.
But I was always astounded how good this little chip worked in the
servo


martin

 

[Fred Bloggs]

What youse guys failed to note in your circuit recommendations is that
output freq varies with VCC.

NOT MY RECOMMENDATION, DUMBASS!!!!!!! As usual with you f____g,
know-nothing OPs, you chose the the IDIOT RESPONSE! You have only
yourself to blame.

 

[Bill Bowden]

What youse guys failed to note in your circuit recommendations is that

output freq varies with VCC.

Using the single resistor and cap and CMOS timer, the frequency is
1.44/ 2RC. Where do you see Vcc in that formula?

-Bill

 

[Lostgallifreyan]

Using the single resistor and cap and CMOS timer, the frequency is
1.44/ 2RC. Where do you see Vcc in that formula?

-Bill

I think the umpire just ruled that one out LBW, the batsman standing on
his stumps. Although that would probably be a different Bill Bowden?

 

[JR North]

Back to your meds, Fred. Leave the cap loose.
JR