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I know the anode has a + sign and cathode has negative sign. But are these signs reversed depending on how the diode is connected with the circuit?

enter image description here

In the above pic, in fig 4.2b, why isnt the - sign on the top and + sign on the bottom of the diode, since the cathode is on the top side? Also, what does -v indicate in fig 4.2b?

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  • \$\begingroup\$ Note that Fig 4.2 (b) violates every known datasheet Absolute Maximum for Vr ( if it were an LED) Although we refer to Zeners as the reverse Voltage -V they are often assumed positive as if you understand it is an absolute value as a component \$\endgroup\$ Sep 8, 2018 at 0:25
  • \$\begingroup\$ Think of the diode as a push/pull type door. From the Push side you can go right threw with no resistance(no voltage drop). From the pull side if you push it blocks you. Now with enough force(Breakdown voltage) you will eventually get through. \$\endgroup\$
    – Robert Fay
    Sep 17, 2019 at 20:43
  • \$\begingroup\$ "Also, what does -v indicate in fig 4.2b?" I can't see it. 4.2b? \$\endgroup\$
    – Transistor
    Sep 30, 2021 at 18:00

4 Answers 4

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The + and - signs are not attached the the diodes. They're attached to the voltage readings, 0 V and 10 V.

enter image description here

Figure 1. The voltages across the diodes.

Since the positive supply is at the top the voltage readings across the diodes will be positive on the top side.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 2. Another common way of displaying the voltage is to use an arrow with the tip pointing in the direction of increasing potential.

(Note: I have seen some drawings use the opposite convention where the arrow points in direction of decreasing potential so beware!)

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  • \$\begingroup\$ I wouldn't say that the +/- signs are not attached to the diode. They indicate which of the two terminals of the diode is more positive, i.e. the direction of the voltage across the diode. They don't have any meaning without the diode symbol next to them. \$\endgroup\$
    – Curd
    Sep 30, 2021 at 22:09
  • \$\begingroup\$ @Curd, OP seemed to think they were terminal designations on the diodes and related to the anode and cathode. You're with me. They're indicating the potential gradient direction, not the diode terminals. \$\endgroup\$
    – Transistor
    Sep 30, 2021 at 22:14
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In 4.2b, you have +10V at the top of the circuit, so the cathode must be more positive than the anode. In that case, the diode acts as an open circuit - no current will flow, so there is no voltage drop across the resistor, and 10 volts, with polarity as shown, across the diode.

The + and - signs beside the diode indicate the polarity of the voltage applied to the diode.

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  • \$\begingroup\$ why is anode called the positive terminal then? clearly, in that figure, the anode is at a lower potential right? Also, in fig 4.1b, in the graph, what does -v indicate if the voltage drop across the diode is +v? \$\endgroup\$
    – David
    Sep 7, 2018 at 23:05
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    \$\begingroup\$ "Anode" and "Cathode" are the names of the terminals of the diode - they don't change just because you turn the diode around, or the circuit applies a voltage the "wrong" way. You don't call the front of your car "the back" when you are backing up, do you? \$\endgroup\$ Sep 7, 2018 at 23:11
  • \$\begingroup\$ If positive potential applied to anode (relative to cathode) then diode is forward biased. That's why some people may think anode as positive. Basically anode is where you need to give higher voltage for diode to allow current flow. \$\endgroup\$
    – Curious KP
    Sep 22, 2020 at 4:45
  • \$\begingroup\$ so anode is not called positive that is wrong \$\endgroup\$
    – Curious KP
    Sep 22, 2020 at 4:45
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You're confusing two different uses of the + and - signs. When we talk about \$V_D\$, the diode voltage, we always assume that the anode is + and the cathode is -. So, when the diode is reversed bias we say that the diode voltage is negative. That is consistent with the graph in part (b) of your included figure (From Sedra, I suspect). Likewise, when we talk about the diode current we follow the passive sign convention and say that positive current enters the anode.

In figure 4.2 the marked voltages are just the voltage across two points in the circuit and are not necessarily equal to the diode voltage.

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You wrote

I know the anode has a + sign and cathode has negative sign.

That is, however, wrong.

Provided that the voltage value associated with the +/- signs is positive (like in this example), then

  • the terminal that has the more positive voltage is where the + sign is and
  • the terminal that has the more negative voltage is where the - sign is.

And of course it depends on the whole circuit and its state which terminal (currently) has a more positve voltage than the other.

In both circuits of Fig. 4.2 the upper terminal has a more positive voltage than the lower terminal (although in 4.2a is says the difference is 0V in reality there would be a small difference of least 0.2V..0.6V depending on the type of diode). That's why in both cases the + sign is at the top and the - sign is at the bottom.

The + sign is at the anode and the - sign at the cathode if the diode is forward biased; but that is not always the case.

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  • \$\begingroup\$ You're right, and I see what you're getting at now (I confess I didn't read the whole question, just the answer). \$\endgroup\$
    – TypeIA
    Sep 30, 2021 at 17:53
  • \$\begingroup\$ @Carl I did, and as I said I'm with you now. I apologize for the confusion. I deleted the comment. I'm not sure what more you need. \$\endgroup\$
    – TypeIA
    Sep 30, 2021 at 18:19
  • \$\begingroup\$ It would be nice if the downvoters explained what they consider wrong with this answer. \$\endgroup\$
    – Curd
    Sep 30, 2021 at 19:12
  • \$\begingroup\$ I didn't downvote but the terminals don't have any signs. The signs are on the voltage readings, not the diode. \$\endgroup\$
    – Transistor
    Sep 30, 2021 at 20:12
  • \$\begingroup\$ The terminals do have signs in the schematic (or does the schematic not show "terminals"?! Well if not then it also doesn't show a diode... Just some darker and lighter pixels) . It's obvious that I didn't mean on the real device because I wrote that it may depend on the state of the circuit (i.e. it might change). \$\endgroup\$
    – Curd
    Sep 30, 2021 at 21:51

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