[simpits-tech] RC Timer delay

[Heidner, Troy W]

Stig,

You've got the time constant formula correct.  The units for the RC time
constant are seconds.  i.e. if you used a 1 microfarad capacitor and a 1
megaohm resistor, the value for one time constant would be:
0.000001 F  x  1,000,000 ohm =  1 second.

The total charge/discharge time for this circuit is equal to approximately
five time constants.  It actually tapers off on a logarithmic curve down to
zero over an infinite period of time, but 5 time constants is generally
accepted to being "close enough" to zero.  I don't have the curve values off
the top of my head, but (as you have the circuit drawn and as you stated),
the output should be roughly equal to the strobe voltage in the first
instant.  After the period of one time constant (as calculated above) the
output voltage would have dropped to approximately 37% of the input voltage.
After two time constants it would be down to approximately 16% of the input
voltage, etc...  Like I said it is generally accepted that after five time
constants have passed it will have dropped to zero for all practical
purposes.

There are at least two more questions to be answered here though.  When you
say how long does it take for the output to go "low", that depends on what
voltage value your circuit is expecting as a "low".  Do you mean complete
zero?  Are you using TTL devices?  Then "low" would mean about 0.3 volts.
It will also depend on your input "strobe" voltage as well.  For example if
you were using TTL devices and you had an input "strobe" of 5 volts then
after the first time constant you would only have decreased in voltage to
1.85 volts (5V x 37%).  Not enough to be a "low".  But if you were using TTL
devices and your input "strobe" was 1 volt then after one time constant you
would be down to 0.37 volts (1V x 37%).  Almost a "low".

Secondly there is one other SERIOUS consideration here.  The calculations
that we have made above are for the theoretical circuit that you have drawn
for us (which means one with nothing connected to the output).  Once you
connect another device to the "output" of your circuit you will change the
parameters and thus the timing values.  So it depends very much what you are
going to connect your "output" to.  If you connect the output to a device
with a very high input resistance or impedance then the values that we
calculated here are probably going to be alright.  But if you connect it to
a medium or low impedance type device then you are going to radically change
your timing values.  If you don't know the input resistance or impedance for
the device you are wanting to connect your output to it will be impossible
to calculate the timing value.

In that case it might be possible to arrive at the correct value through a
little trial and error.

I hope that any of this helped.  If you have any more questions, just post
them and I'll try to help if I can!

Troy



> -----Original Message-----
> From: Stig Joergensen [mailto:stig.joergensen@clearsky.dk]
> Sent: Thursday, February 21, 2002 3:54 PM
> To: simpits-tech@simpits.org
> Subject: [simpits-tech] RC Timer delay
> 
> 
> Hi Gang,
> 
> I need some help here. I need to calculate the time for RC (resistor
> -Capitor) delay circut. I know the formula is t = r*c, but 
> what are the
> units?
> 
> more specific, consider the following :
> 
> strobe----|
>         __|__
>          cap
>         _____
>           |
>           |----- out
>           |
>          | |
>          |r|
>          |_|
>           |
> gnd-------|
> 
> 
> when stobe goes high, out will go high almost at the same 
> time, but what
> should the walues (r and c) be for out to go low, say 5 ms 
> after strobe
> drops to low again?
> 
> /Stig
> 
> 
> 
> 
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