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We have the circuit in the figure.I have to find the currents in the diodes and in the resistances R1 and R2. The diodes are real and Vd=0.7 V

Awesome.

Here is the original figure

enter image description here

I have drawn the circuit and I have replaced the diodes with the voltages

enter image description here

I apply the KVL in the first loop and I have -20+ 0.7+0.7 - I2R2=0 so I2 here is negative...what am I doing wrong?

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  • \$\begingroup\$ what are the diodes? are the arrows supposed to be the diodes? \$\endgroup\$ – Bruce Aug 24 '15 at 10:34
  • \$\begingroup\$ Are "awesome" diodes also invisible? \$\endgroup\$ – Andy aka Aug 24 '15 at 10:37
  • \$\begingroup\$ I have replaced the diodes in the figure with their voltages of 0.7 Volts \$\endgroup\$ – Xizi Aug 24 '15 at 10:38
  • \$\begingroup\$ The arrows show the direction of the currents \$\endgroup\$ – Xizi Aug 24 '15 at 10:39
  • \$\begingroup\$ I added the original figure \$\endgroup\$ – Xizi Aug 24 '15 at 10:41
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Assuming all the diodes take 0.7V.

VR1 is parallel over D2 = 0.7V

VR2 = 20V- 2* 0.7V = 18.6V

With these values known you can calculate the currents

enter image description here

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  • \$\begingroup\$ I don't understand why is VR1 0.7 Volts? \$\endgroup\$ – Xizi Aug 27 '15 at 20:26
  • \$\begingroup\$ over the diode there can be no more than 0.7V of voltage the resistor is connected parallel over the max voltage of the diode 0.7V so the voltage will be the same \$\endgroup\$ – Bruce Aug 28 '15 at 0:28
  • \$\begingroup\$ I understand.I want to know,what Vi interval should I study to know when the diodes are on or off? \$\endgroup\$ – Xizi Aug 28 '15 at 10:36
  • \$\begingroup\$ If there a voltage below the forward voltage (0.7V) then that voltage will also be over the diode If the voltage rises above 0.7V there will only be 0.7V over the diode. A diode is on (closed switch) if there is a voltage of 0.7V across the diode. if it's less than 0.7V or a negative voltage the diode blocks current. \$\endgroup\$ – Bruce Aug 28 '15 at 10:55
  • \$\begingroup\$ Does 20 Volt voltage influence that? \$\endgroup\$ – Xizi Aug 29 '15 at 10:45
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In the direction of the current I2 is selected as you have drawn, It just should be negative. Your 20 volt battery (which is much higher than 0.7) drives all the currents clockwise.

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