555 timer math check

K

Keith M

Guest
Can someone please take a look at this and check my 555 timer math?

http://www.techtravels.org/555math.gif

I've got Ra = 6800 ohms, Rb = 6800 ohms + 0-10k ohms, and C = 60 pf.

When the 10k pot is 0, then the frequency is ~1.18mhz.
When the 10k pot is 10k, then the frequency is ~594khz.

This look correct?

Thanks

Keith
 
On Fri, 02 Oct 2009 21:40:12 -0700, Keith M wrote:

Can someone please take a look at this and check my 555 timer math?

http://www.techtravels.org/555math.gif

I've got Ra = 6800 ohms, Rb = 6800 ohms + 0-10k ohms, and C = 60 pf.

When the 10k pot is 0, then the frequency is ~1.18mhz. When the 10k pot
is 10k, then the frequency is ~594khz.

This look correct?

Thanks

Keith
Click to expand...
Well...

That's an awfully high frequency for a 555. A new one may actually go
that fast, but my experience has been that you really can't trust them to
be reliable in frequency much above 100kHz or so (truthfully I can't
remember just how high they can be made to go reliably -- but it ain't
1MHz!).

--
www.wescottdesign.com
 
On 2009-10-03, Keith M <kmongm@gmail.com> wrote:
Can someone please take a look at this and check my 555 timer math?

http://www.techtravels.org/555math.gif

I've got Ra = 6800 ohms, Rb = 6800 ohms + 0-10k ohms, and C = 60 pf.

When the 10k pot is 0, then the frequency is ~1.18mhz.
When the 10k pot is 10k, then the frequency is ~594khz.

This look correct?
Click to expand...
1.18 megahertz is pushing the envelope. last time I looked the datasheet
said the max was 1Mhz.

if you want one and a bit megahertz a 555 can probably manage that
but you're pushing it, don't expect component values from the
calculator to work, build a prototype, start with C=0, R=1K (resistor
pin 3 to pins 2 and 6)

if you want 1.18 megahertz it'd be easier to use a resonator or
crystal stabilised oscilator.

if you actually want 1.18 millihertz look at the CD4066
 
On Fri, 2 Oct 2009, Keith M wrote:

Can someone please take a look at this and check my 555 timer math?

http://www.techtravels.org/555math.gif

I've got Ra = 6800 ohms, Rb = 6800 ohms + 0-10k ohms, and C = 60 pf.

When the 10k pot is 0, then the frequency is ~1.18mhz.
When the 10k pot is 10k, then the frequency is ~594khz.

You never use a 555 up there. It's beyond stretching the capability of
Click to expand...
the device, and even if it works, it will work badly. It's designed
for low frequency use, in the audio range or lower, not radio frequency.

There are other issues. That 60pf is very small, extraneous capacitance
will start to affect that and cause the frequency to shift.

You need to say what you are trying to do, so a proper solution can be
had. If you don't need a variable frequency, one can get a crystal and
make a crystal oscillator (though often then it make sense to use a higher
frequency crystal and divide it down, which may work better for your plans
since then you can get some multiple fixed frequencies.

A coil and capacitor oscillator is the traditional method, and at 1MHz or
so is really easy to build (unlike at higher frequencies where things
get finicky).

A 555 is not the solution for everything. Once its limits are reached,
you need to look to other things.

Michael
 
On Oct 3, 5:30 am, Jasen Betts <ja...@xnet.co.nz> wrote:

1.18 megahertz is pushing the envelope. last time I looked the datasheet
said the max was 1Mhz.

if you want one and a bit megahertz a 555 can probably manage that
but you're pushing it, don't expect component values from the
calculator to work, build a prototype, start with C=0, R=1K (resistor
pin 3 to pins 2 and 6)
Click to expand...
Jason & Tim,

Thanks for the reply. I almost added a link to the datasheet for this
reason.

http://focus.ti.com/lit/ds/symlink/tlc555.pdf

"The TLC555 is a monolithic timing circuit
fabricated using the TI LinCMOS process. The
timer is fully compatible with CMOS, TTL, and
MOS logic and operates at frequencies up to
2 MHz."

and from the "operating characteristics", fmax Maximum frequency in
astable mode is Minimum 1.2mhz, typical 2.1mhz.

Jason: Are you saying that the datasheet formulas break down and are
no longer accurate once you get above a certain frequency?

This isn't a circuit I'm designing, but someone else's creation that
I'm looking at. I wasn't sure if, based on the schematic, I was
selecting the right Ra and Rb for the formulas.(aka plugging in the
right values from the schematic to calc freq) And that C was 60pf
because 1/Ct = 1/C1 + 1/C2 when in series.

I'm actually not building this circuit either, but I would think the
nominal frequency should be 500khz, or a little faster. It would
never be used up at the 1mhz level anyways. This is one of the
reasons why I posted, the minimum appears to be 594khz which is a
little higher than I would have guessed.

Thanks

Keith
 
On Sat, 03 Oct 2009 08:05:56 -0700, Keith M wrote:

On Oct 3, 5:30 am, Jasen Betts <ja...@xnet.co.nz> wrote:

1.18 megahertz is pushing the envelope. last time I looked the
datasheet said the max was 1Mhz.

if you want one and a bit megahertz a 555 can probably manage that but
you're pushing it, don't expect component values from the calculator to
work, build a prototype, start with C=0, R=1K (resistor pin 3 to pins 2
and 6)

Jason & Tim,

Thanks for the reply. I almost added a link to the datasheet for this
reason.

http://focus.ti.com/lit/ds/symlink/tlc555.pdf

"The TLC555 is a monolithic timing circuit fabricated using the TI
LinCMOS process. The timer is fully compatible with CMOS, TTL, and MOS
logic and operates at frequencies up to 2 MHz."

and from the "operating characteristics", fmax Maximum frequency in
astable mode is Minimum 1.2mhz, typical 2.1mhz.
Click to expand...
Read that as "it stops working at 2.1MHz", not "you can trust it at
2.09999MHz". 555's get weird long before they reach their maximum
frequency. Basically, between propagation delay and stray capacitance
the ON and OFF times are a bit indeterminate. As long as you're swamping
this indeterminacy out with your own long on and off times you're fine --
but when you try to go to high frequencies you get all the weirdness.

Jason: Are you saying that the datasheet formulas break down and are no
longer accurate once you get above a certain frequency?

This isn't a circuit I'm designing, but someone else's creation that I'm
looking at. I wasn't sure if, based on the schematic, I was selecting
the right Ra and Rb for the formulas.(aka plugging in the right values
from the schematic to calc freq) And that C was 60pf because 1/Ct =
1/C1 + 1/C2 when in series.

I'm actually not building this circuit either, but I would think the
nominal frequency should be 500khz, or a little faster. It would never
be used up at the 1mhz level anyways. This is one of the reasons why I
posted, the minimum appears to be 594khz which is a little higher than I
would have guessed.

"The formulas shown above do not allow for any propagation delay times
Click to expand...
from the TRIG and THRES inputs to DISCH. These delay times add directly
to the period and create differences between calculated and actual values
that increase with frequency."

IOW, things get shaky at high frequencies.

--
www.wescottdesign.com
 
On Sat, 03 Oct 2009 12:41:35 -0500, Tim Wescott <tim@seemywebsite.com>
wrote:

"The formulas shown above do not allow for any propagation delay times
from the TRIG and THRES inputs to DISCH. These delay times add directly
to the period and create differences between calculated and actual values
that increase with frequency."

IOW, things get shaky at high frequencies.
Click to expand...
You can easily get 1MHz and fifty percent duty from a CMOS 555. I
wouldn't say it's the best most reliable way to go about it but if
it's just for a one off or experiments why not?

If you go to TI's site you can get an excel calculator for the TLC555.
This will let you see what you can obtain with the values you have on
hand.

Get it here.

http://focus.ti.com/docs/toolsw/folders/print/tlc555calc.html

I did up a tlc555 on a 0.5 by 0.75 single sided PCB a while ago for
testing FET's and other GP uses.My PCB has a itty bitty driver MIC at
the timers output. I'm using trimmers and a diode and a 1% PPS film
0603 cap 100pf.

Heres a screen capture of the calculator for 1.18MHz.

http://i33.tinypic.com/2z9bfja.png

Here are the actual waveforms DSO capture.

CH1 is the driver output,CH2 is pin3 of the TLC555.

http://i36.tinypic.com/otdqj6.jpg

You may need a buffer on the output of the 555 depending on what your
driving.
 
On Oct 3, 4:27 pm, Hammy <s...@spam.com> wrote:

You can easily get 1MHz and fifty percent duty from a CMOS 555. I
wouldn't say it's the best most reliable way to go about it but if
it's just for a one off or experiments why not?
Click to expand...
Hammy: Thanks for the reply and the links, definitely useful.

Now going back to my original question, when using calculators or the
formulas directly,

am I applying the correct values for Ra, Rb, and C given the posted
schematic?

The generic formula is frequency = 1.443 / ( (Ra+2Rb)C)

Thanks

Keith
 
On Sat, 03 Oct 2009 00:07:11 -0500, Tim Wescott <tim@seemywebsite.com>
wrote:

On Fri, 02 Oct 2009 21:40:12 -0700, Keith M wrote:

Can someone please take a look at this and check my 555 timer math?

http://www.techtravels.org/555math.gif

I've got Ra = 6800 ohms, Rb = 6800 ohms + 0-10k ohms, and C = 60 pf.

When the 10k pot is 0, then the frequency is ~1.18mhz. When the 10k pot
is 10k, then the frequency is ~594khz.

This look correct?

Thanks

Keith

Well...

That's an awfully high frequency for a 555. A new one may actually go
that fast, but my experience has been that you really can't trust them to
be reliable in frequency much above 100kHz or so (truthfully I can't
remember just how high they can be made to go reliably -- but it ain't
1MHz!).
Click to expand...
Hi Keith . I kill file all Google Groups post to eliminate spam. This
is in response to your post done through google groups. Do yourself a
favour and use a real news provider maybe your ISP offers it for free.


It's been a while since I went over the 555 data sheet in detail but…

The reason you are seeing discrepancies in your frequency for your
calculated values were as already mentioned by Tim propagation delays.
Other causes will be your external component tolerances (initial and
drift with temperature). Rise and fall times of your output. The
variations in timers the trigger and threshold won't be exactly to
data sheet specs it will vary. External parasitic's mainly capacitive.

If you read the TLC555 data-sheet they show how to account for
propagation delays to a certain extent.

For example using TI's excel 555 calculator they show for a 1.196MHz
frequency at 48% duty cycle R1 should be 1.192k and R2 should be
3.318k with C at 100pf. In the actual circuit my measured values are
R1 is 1.75k and R2 is 2.4k. I know my cap is 99.8 pf I measured it.
The rise time into just my probe 77nS fall time is 43.5nS.

Using a 555 isn't for precise repeatable results. There fine if you're
just building a couple of something that you can hand tune the values.
But you definitely don't want to use it for building something in
large quantity where you want consistent behaviour.

For consistent calculable behaviour you would use a micro with a
crystal or as already mentioned a discrete crystal oscillator and a
divide by n IC.

Here is another page with 555 equations.

http://www.csgnetwork.com/ne555timer2calc.html
 
On Mon, 05 Oct 2009 14:08:47 -0400, Hammy <spam@spam.com> wrote:

On Sat, 03 Oct 2009 00:07:11 -0500, Tim Wescott <tim@seemywebsite.com
wrote:

On Fri, 02 Oct 2009 21:40:12 -0700, Keith M wrote:

Can someone please take a look at this and check my 555 timer math?

http://www.techtravels.org/555math.gif

I've got Ra = 6800 ohms, Rb = 6800 ohms + 0-10k ohms, and C = 60 pf.

When the 10k pot is 0, then the frequency is ~1.18mhz. When the 10k pot
is 10k, then the frequency is ~594khz.

This look correct?

Thanks

Keith
Click to expand...

If it isnt already clear You are using the right equation.

The higher in frequency you go though the less accurate it will be
(this was already mentioned as well). This is because all the delays
and the error contributed by component tolerances and parasitic
capacitance make up a larger percentage of the overall period.


To improve your accuracy you would have to also take into account
propagation delays as well as component tolerances. Which isn't really
worth the effort just use the calculators as a starting point and use
trimmers to fine tune it.
 
Hammy, Tim, everyone,

Thanks for the responses.

Verizon, whose FIOS service I subscribe to, discontinued usenet news
access about a week ago.

I used to subscribe to Giganews, but I don't use newsgroups any longer
enough to justify the price.

Thanks

Keith
 
On Wed, 07 Oct 2009 20:03:18 -0700, Keith M wrote:

Hammy, Tim, everyone,

Thanks for the responses.

Verizon, whose FIOS service I subscribe to, discontinued usenet news
access about a week ago.

I used to subscribe to Giganews, but I don't use newsgroups any longer
enough to justify the price.

Eternal-september.org is free.
Click to expand...

I wonder how they pay for it?

Cheers!
Rich
 
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