# Diodes block voltage?

I understand the basics of a diode, a diode allows current to flow in only one direction, but how? By blocking any opposing voltages source so that no current can flow in opposition?

And of course, a diode has its limit. If there is a voltage in opposition that is higher to what the diode can handle it allows current to flow in opposition. What about the current that is allowed to flow through the diode is it reduced? For example: Current is supplied by a power supply it flows through a diode in forward direction, if the current was 10A before passing the diode will it still be 10A throughout the diode? If the opposing voltage existed does it reduce that 10A?

And how can I relate the forward bias to this?

What about the current that is allowed to flow through the diode is it reduced? For example: Current is supplied by a power supply it flows through a diode in forward direction, if the current was 10A before passing the diode will it still be 10A throughout the diode? If the opposing voltage existed does it reduce that 10A?

It really isn't clear what you're asking but really, all you have to do is to look at the IV curve for a diode.

What is an IV curve? It gives you the current through the diode versus the voltage across the diode. Qualitatively, it looks like this:

Now remember, either the voltage across the diode is positive or negative. If the voltage is positive, there is forward current through the diode.

If the voltage is negative, there is a small reverse current until the breakdown voltage and then there is a large reverse breakdown current.

That's really all there is to it. If there is a 10A forward current through, there is a certain positive (forward) voltage across period.

a diode allows current to flow in only one direction, but how?

There is an enormous amount of material, from the beginner to advance level, on the web describing the operation of the PN junction. What specifically do you not understand? Your question, as is, is too broad. Study the operation of the PN junction and then, if it isn't quite clear, ask a specific question.

• In terms of losses I guess the 0.7V drop is that, I assumed some losses across a diode, and I guess the 10A would be somewhat reduced across by a small value. – Pupil Oct 27 '14 at 22:29
• @Key, if there is 10A entering the diode, there is 10A leaving the diode and there is 10A through the diode. – Alfred Centauri Oct 27 '14 at 22:32
• What about voltage drop across the diode? Applied-0.7V(depending on diode type)? – Pupil Oct 28 '14 at 4:03
• @Key, your reasoning isn't clear to me. Try using the schematic editor to draw the circuit you're asking about. – Alfred Centauri Oct 28 '14 at 11:19

There is a wonderful article on PN junctions (which diodes are) in this article titled "How transistors REALLY work": http://amasci.com/amateur/trshort.html

If you're asking how a diode blocks current physically, you should definitely have a look at that.

Objectively, it might not be in the most modern and organised way, but the content is amazingly sound, the best I've seen so far. The longer article (there is a link in that short article) has diagrams.

See it as a channel with water flowing. The water-level is the voltage, the water-flow is the current. Unless the channel is split-up the water-flow / current will be the same on all parts of the channel, no matter the barriers.
Also when the channels are split up and joined together the flow will be the same before the spit and after the join. The water level will be the same on all connected parts but will be higher on one side of a barrier then on the other.

• A barrier we call resistor.
• A water reservoir we call a capacitor.
• The water speed we call inductance, with a high inductance it is difficult to suddenly stop the water.
• A diode is a vent opening if the the water-level is higher on one side then the other, but keeps closed if the opposite applies.

Back to your question. Yes the diode is blocking the water-flow if the level on the other side is higher, so water levels remain what they were. No current is flowing and the voltages act like there is just an open connection. If you blow the vent by applying a very high water level, it can get stuck, never opening again or can permanently open up. (or something in between but that will mostly not last very long.) It is not possible for the water-flow to be higher before then after the (blown) vent, so the current will be the same before and after the diode. If you increase the water-level on one side (or decrease the difference in water levels) the current will be reduced.

The forward bias is a bit more complex. The vent is not perfect, it needs a difference in water-levels before opening up (most diodes around 0.7V). But before opening, there is already a little bit of water leaking, a small current called the bias current.