question on Time Delay circuit from Art of Electronics

Thread Starter

tash13

Joined Oct 1, 2021
2
delay-circuit.pngdelay-1.png

I have few questions on above excerpt from the book:

On the highlighted section, author mentions about circuit loading. I know circuit loading can be an issue with voltage divider, how is that a concern here?

Another question, Does the capacitor has to be fully charged before it starts discharging? what happens when the capacitor is fully charged and circuit is still connected to a voltage source?
 

ericgibbs

Joined Jan 29, 2010
14,189
On the highlighted section, author mentions about circuit loading. I know circuit loading can be an issue with voltage divider, how is that a concern here?
Hi tash,
Consider the source impedance was say 15K, then the TC of the circuit would be doubled.
The Low output impedance of the 1st buffer adds little to the delay period.
E
 

ericgibbs

Joined Jan 29, 2010
14,189
Another question, Does the capacitor has to be fully charged before it starts discharging? what happens when the capacitor is fully charged and circuit is still connected to a voltage source?
Hi tash.
The circuit is a DC edge time delay, so if the input stays High so will the Output after the TC delay period has been exceeded.
When the input goes low, the low impedance of the 1st stage will quickly discharge the cap.
E
 

dl324

Joined Mar 30, 2015
13,532
On the highlighted section, author mentions about circuit loading. I know circuit loading can be an issue with voltage divider, how is that a concern here?
Since most signal sources aren't ideal, they have a non-zero output impedance. That will act like a voltage divider.

The buffer has a high input impedance and a low output impedance.
Another question, Does the capacitor has to be fully charged before it starts discharging?
No, but the R and C values would be chosen to give the cap time to charge and discharge enough to give the desired edge delays.
what happens when the capacitor is fully charged and circuit is still connected to a voltage source?
It stops charging. The book said to assume that the threshold voltage was Vcc/2. Once the input gets to that voltage, the buffer output is at Vcc.
 
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