[simpits-tech] RC Timer delay

[Heidner, Troy W]

Stig,

No, I meant small.  But only in terms of keeping your timing value accurate.
You're right that a smaller value will draw more current from your strobe
(and/or output), but if you don't know the input impedance of the load that
you are trying to drive you will end up radically skewing the timing values
you have calculated.

Essentially, whatever you connect the output up to ends up being in parallel
with R1.  That will lower the total resistance and impact your time constant
(RxC) calculation.  If you choose a small value for R1 then the impact the
load or "output" will have on the circuit will be minimized.  It might be
easiest to show with an example.  I will assume that the load that you are
connecting to your "output" line is a TTL input of some kind.  TTL inputs
have generally (and very loosely) about a 1K ohm input impedance.  We'll use
that for our example.  We'll arbitrarily use 1 microfarad for the
capacitance value.

If you chose a large value for R1, say 1 megaohm, then when connected to the
1 kilohm TTL input the total resistance for your time constant calculation
becomes this:  figuring parallel resistance  1/(1/1000 + 1/1000000) = 999
ohms.  Your 1,000,000 ohm resistance just got chopped down by 1000 times.
If you had based your time constant calculations on the 1 megaohm value
(1megaohm x 1 microfarad) you would have gotten a value of 1 second.  When
you hook it up to the output your resistance value changes, then you would
have a value (999 ohms x 1 microfarad) = 999 microseconds instead of one
second!

If you used a small value for R1, say 500 ohms, the calculations would work
something like this.  Computed time constant (500 ohms x 1 microfarad) = 500
microseconds.  When you hook up the load, the resistance would become
1/(1/1000 + 1/500) = 333 ohms.  Which would change the time constant value
to (333 ohms x 1 microfarad) = 333 microseconds.

I hope you see my point.  By using a low R1 value the effect of the load
changing the total resistance is minimized.  That also means that the time
constant value is changed the least also.  You're correct about the current
draw though.  There are practical limits as to how small a value that you
can use for R1 without drawing too much current or causing you to use an
enormous capacitor.  I was just saying that using the smallest you can get
away with would be best.

Obviously the VERY best thing would be to know the input impedance of your
load.  Then you could include it in your calculations and know exactly what
you are dealing with.

I'm very sorry, if this post was needlessly long and or boring.  Just trying
to be helpful.

Troy



> -----Original Message-----
> From: Stig Joergensen [mailto:stig.joergensen@clearsky.dk]
> Sent: Thursday, February 21, 2002 6:28 PM
> To: simpits-tech@simpits.org
> Subject: RE: [simpits-tech] RC Timer delay
> 
> 
> Troy,
> 
> you did mean R1 as high as possible (less resistant, more current
> (amps)) right?
> 
> /Stig
> 
> > -----Original Message-----
> > From: Heidner, Troy W [mailto:theidner@ku.edu]
> > Sent: 22. februar 2002 00:20
> > To: 'simpits-tech@simpits.org'
> > Subject: RE: [simpits-tech] RC Timer delay
> > 
> > 
> > Stig,
> > 
> > Yeah, that should work.  Especially if you try to keep your 
> > R1 value small
> > as I mentioned in my second post.
> > 
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