4

I've designed this circuit using Multism 10.1 :

This circuit is part of clock generator for 8088 uP:

Why does the wave of ch1 of Oscilloscope which represent the voltage across the capacitor take this form?

What is the relationship between that and the delay that is caused by the capacitor? Click here

Community
  • 1
ammar
  • 619
  • 1
  • 11
  • 29

1 Answers1

8

When you charge the capacitor the 100k resistor limits the current so the voltage on the capacitor is:

$$v = V \left( 1 - \text{exp} \left( - \frac{t}{C \cdot R}\right) \right)$$

Where V is size of the input square wave and R is 100k.

The discharge current goes mainly through D1 and not through the 100k resistor. So the current is not limited by the resistor value and the discharge is much faster.

If you turn the diode around you should see a fast charge and slow discharge.

Warren Hill
  • 4,840
  • 20
  • 32
  • 2
    This is very true but if you tried it by just switching off the power supply it might make you think it isn't working because the discharge path relies on a low impedance to ground when the power supply is disconnected. – Andy aka Dec 10 '13 at 15:23
  • Thanks. But what is the relationship between that and the delay that is caused by the capacitor? http://electronics.stackexchange.com/questions/92686/what-is-the-function-of-the-capacitor-and-the-diode-here/ – ammar Dec 10 '13 at 15:56
  • And what do you mean by saying "turn the diode around you"? – ammar Dec 10 '13 at 15:57
  • 1
    So, the portion of the delay caused by the capacitor does not change. It is the same in both directions. The portion of delay caused by the resistor, however, does. When the current goes "against" the diode (when the cathode voltage is higher), the diode acts like an open circuit. So the RC constant uses the resistor value. When the current is going "in the direction" of the diode (that is, when the anode voltage is higher than the cathode), the diode acts like a small resistance (close to a short circuit for your purposes). So, you use the R value of the diode for the RC constant. – scld Dec 10 '13 at 16:07
  • 1
    @ammarx The rate at which the capacitor charges or discharges is the amount of current flowing through it. When the input voltage is switched high this current is limited by the 100k resistor and as it charges the voltage across the resistor falls so less current flows and the capacitor charges more slowly. When the input voltage switches low initially the current is only limited by the diode and so the cap discharges rapidly. Once the voltage falls to about 0.6 volts however the diode stops conducting and all the current has to go through the resistor hence the slow discharge tail. – Warren Hill Dec 11 '13 at 13:15