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Im having a bit of trouble drawing a circuit with a diode in. I know the current flows from negative to positive, but convention says that current goes from positive to negative.

When drawing a circuit with a diode do you draw the arrow pointing to the positive (matching the actual flow of electrons) or towards the negative terminal (matching conventional current)?

If you follow convention then surely when building circuits from the diagram the diode will not allow current as it is actually facing the incorrect way?

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In normal circuit analysis, we almost never think about which way the electrons are flowing. We nearly always calculate and visualize how the "conventional current" is flowing.

In the case of a diode (to simplify things somewhat---see Steven's answer for some special cases), conventional current flows through the diode from the anode to the cathode; that is, conventional current flows in the direction that the "arrow" of the diode symbol is pointing.

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    \$\begingroup\$ but doesn't that mean when you put the physical diode in the circuit its the wrong way round. Or are all diodes "labelled" backwards? \$\endgroup\$ – Jonathan. May 13 '12 at 16:45
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    \$\begingroup\$ The anode is the anode and the cathode is the cathode. You need to place the diode in the circuit so that (conventional) current will flow from the anode to the cathode. You need to draw your diagram to show how you intend to build the circuit. \$\endgroup\$ – The Photon May 13 '12 at 16:48
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    \$\begingroup\$ @Jonathan. All the components (including diodes) are labelled with regards to the conventional current flow. \$\endgroup\$ – m.Alin May 13 '12 at 16:50
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    \$\begingroup\$ @m.Alin is right; The only case I can think of where knowing the direction of electron/hole flow matters is in the labeling of the pins of a MOSFET: "source" and "drain" refer to the actual carriers in that device, electrons in an n-channel FET and holes in a p-channel FET. And even in that case, all device specs are always given in terms of conventional current. \$\endgroup\$ – The Photon May 13 '12 at 21:54
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    \$\begingroup\$ The diode arrow on a schematic may point in the direction of conventional current but, alas, this consistency doesn't generalize to all markings and components. Electrolytic capacitors sometimes often have arrow pointing toward the negative terminal (which is the shorter of the two). The arrow contains little minuses to remind you that it's electrons. \$\endgroup\$ – Kaz May 13 '12 at 22:04
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The arrow symbol is actually a simplified depiction of a point contact diode.

enter image description here

It happens to point in the direction of conventional current. The "way electrons move" is not important for circuit design and you should not worry or even think about it, it will lead to nothing but confusion.

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The diode arrow points in the direction of conventional current.

Conventional current flows in the opposite direction from the actual flow of electrons.

Digging deeper into the physics, there are actually two possible types of current: electron current and hole current. Electron current is the movement of electrons. Hole current is the movement of an absence of an electron. In your statement of "current flows from negative to positive", you are referring to the flow of electrons. Unless one is dealing with the physics of a semiconductor devices, you typically don't use "hole current".

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Like the others said, we always think of conventional current, flowing from the higher voltage to the lower, and then the arrow points in the current direction.

One exception is the zener diode, which is mounted backwards. If you place a common diode backwards it will block the current, save a small leakage. But if you increase the voltage at a certain point there will be a breakdown where suddenly current starts flowing, and that can be a lot of it. The voltage will not increase much anymore. Every diode will behave like that, and zener diodes are especially made to use this behavior. In their production we can control at what voltage that breakdown will occur.

If you would place a zener diode the normal way, pointing in the direction of conventional current, it would look like a normal diode, also showing the 0.7V voltage drop.

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  • \$\begingroup\$ Second exception: photodiode. Normally used with cathode at higher potential than anode, and conducts in the "reverse" direction when light is applied. \$\endgroup\$ – The Photon May 14 '12 at 4:45
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    \$\begingroup\$ @ThePhoton - Yes, when your nick is ThePhoton it's only natural to think of that one! :-) \$\endgroup\$ – stevenvh May 14 '12 at 5:02
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The anode of a diode is + and the cathode is - to forward bias it and allow current to flow through it. If you make the anode - and the cathode + the diode will not allow current to pass through it.

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