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I am struggling to understand why I can't get a full voltage range when I have the diode the correct way round. If you check this link:

https://electronics.stackexchange.com/q/266201/

If you look at the answer I have set my circuit up like circuit 2. I have a rheostat and have put the diode in parallel and I am using a 3V battery. When the diode is the wrong way round I get a range of 0-3V when I slide the rheostat. When it is the correct way round I only get 0-1V. When it is at 1V, the p.d. across the battery is still 3V. Where is this missing 2V? If I have connected the diode in parallel surely it must get the full voltage range (which it does in reverse bias but not forward)?

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    $\begingroup$ Which way around do you consider correct and incorrect? Please include a schematic in your question so we don't have to click over to another site to understand what you're asking. $\endgroup$ – The Photon Feb 21 '18 at 15:47
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Based on you description when the diode is in forward bias you are only getting 1 volt across the diode because the threshold voltage has been reached. Due to the non-linear characteristics of a diode once the threshold voltage is reached (in your case 1 volt) the diode is essentially a short.

When the diode is in reverse bias you see the full voltage range because a diode in reverse bias is essentially a variable capacitor. The depletion region of the device changes and as the voltage is increased the electric field in the depletion region also increases.

It may be useful to look up a diode IV curve and also look into the band diagram of a p-n junction diode.

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