I think maybe my question is somewhat too basic for this site.

I am trying to understand the three main functions of a transistor and was confused on the diode function. The book show the N-type P-type interface with a barrier potential.

Although I followed the logic for how the diode controls the direction of current flow the diagrams showed the battery being placed in reversed direction to demonstrate how the diode reversed the flow. This bothers me because I read the flow of current in a typical battery is from anode to cathode or from positive to negative to begin with.

If this is true then no matter which way the diode is hooked up in the circuit would the current not flow in only one direction to begin with by definition? I think maybe I am obviously missing the main point here.

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    \$\begingroup\$ There's no such thing as "too basic." Sometimes the best questions are about the basics. I'm just glad your question is actually on topic! \$\endgroup\$
    – JYelton
    Nov 29, 2022 at 21:44
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    \$\begingroup\$ Please post an image or two, so we can be sure of what the diagram is trying to convey. \$\endgroup\$
    – Aaron
    Nov 29, 2022 at 21:58
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    \$\begingroup\$ If you think the diode is reversing the direction of flow then you've misunderstood. It allows current to flow one way and prevents it flowing the other. \$\endgroup\$
    – Finbarr
    Nov 29, 2022 at 22:01
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    \$\begingroup\$ @JYelton, actually I agree with the OP, there definitely is a too basic for the site. Questions on fundamentals of electronics or other established areas are much better served by the swathes of pre-written texts and tutorials freely available on the internet, already well-presented with diagrams and honed text. That serves the learner much better than us 'having a go' at rewriting a short version of it again. The site's great for certain things while other things are better on other sites. \$\endgroup\$
    – TonyM
    Nov 29, 2022 at 22:08
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    \$\begingroup\$ The diode's like a water pipe with a one-way valve in it. Have a search on the internet for 'diode blocks current basics', you'll find tons of well-written webpages already covering this (such as this one). \$\endgroup\$
    – TonyM
    Nov 29, 2022 at 22:21

1 Answer 1


With the DC current the single diode can control if the current flow or not. In the study example it can be just an illustration of the diode working. In real cases the single diode can block DC current if the source polarity is reversed, avoiding damage to the device.

But with 4 diodes we can create a device called "diode bridge". A diode bridge can give the fixed polarity at its output regardless of the polarity at its input.


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