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I have simulated a simple half wave rectifier circuit and plotted the current in the circuit and the voltage across the load resistor of 4 kilo ohms. The diode is an ideal one. The alternating voltage source has a peak voltage of 60V at 50HZ. The blue graph is for the current and the green one is for the voltage drop. I understand that in the positive half cycle, the diode doesn't conduct hence the voltage drop across it is 0V. In the negative half cycle(when the current goes anticlockwise) the diode conducts and the voltage across it is negative. However, I don't get the plot for current in the circuit. Shouldn't it be in the negative y-axis when the diode conducts as current is travelling anticlockwise?

Anay Chadha
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  • Look at this answer https://electronics.stackexchange.com/questions/206055/why-am-i-getting-negative-base-current-in-ltspice-simulation – G36 Sep 11 '22 at 11:53

2 Answers2

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The current measurement through diode is obviously made from anode to cathode direction, so in that notation, current through diode is always positive through forward direction, because a diode will not pass current through in reverse direction.

So it's positive because in which way the diode is in the circuit.

Negative voltage from supply forward biases the diode and current flows in forward direction in the diode.

Justme
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  • If I reverse the diode connection the sign for voltage when the diode is conducting changes. However, the current is still positive. So does current never become negative when a diode is there in the circuit? – Anay Chadha Sep 11 '22 at 11:42
  • If you reverse the diode connection it does not change anything, to the diode itself, it will still conduct forward current when forward biased, no matter if the forward bias direction is left or right. – Justme Sep 11 '22 at 11:54
  • When I ask it for the current in the resistor then it plots the same graph for current in the negative y-axis. What is happening? – Anay Chadha Sep 11 '22 at 12:00
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    @AnayChadha when you ask a simulator to plot the current through a resistor i.e. I(Rx) then what direction does the simulator use as the correct direction for current? How does it know whether you mean flowing from top to bottom or bottom to top? Read what the guys are saying. – Andy aka Sep 11 '22 at 12:07
  • If I am asked to plot a current vs time graph in an exam for a circuit like the one in the post. What convention should be used? – Anay Chadha Sep 11 '22 at 12:35
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    I don't think someone is listening carefully enough. – Andy aka Sep 11 '22 at 13:10
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Be aware that some simulators change the current direction with a change of resistor "direction".
It is the reason for the change of the "resistor" shape ("+" pointer).

See this example of reversing "resistor" R2.

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Antonio51
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