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Need a bit of help understanding voltage drops

  1. Nov 2, 2014 #1
    2 Questions... (Iam sort of new so please forgive if what I ask don't make any sense.)

    #1. If I am receiving an 1000v output from an experimental electric device and I only need around 700v what happens to the 300v of electricity that's left after I take only what I need?

    #2.Can someone tell me why would I receive a voltage drop when connecting diodes in series? I was told that you cannot connect diodes in series and that only one of the diodes would work and just use the others as a heat sink but I found when I connect up to 2 identical diodes my output increases(which is helping with the voltage drop in the line) but when I connect the 3rd identical diode voltage output on all 3 diodes drop!

    Any help would be greatly appreciated!
  2. jcsd
  3. Nov 2, 2014 #2
    Typically, you don't have the option to only take 700 volts; you get the whole 1000 v if you take any at all. If that means that your device is smoked, well, so be it.
  4. Nov 2, 2014 #3

    Simon Bridge

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    Wecome to PF;
    #1. depends on the nature of the device ... if you have a constant voltage PSU that supplies 1000V, and you connect a device that only needs 700V, then the device will probably get hot and maybe break. It's like throwing something from a 1000ft cliff which can only withstand a 700ft drop. If the PSU supplies more voltage than you need, then you will have to provide another channel for some of the energy to get used up - usually an extra resistor.

    #2. for this one I'd need a circuit diagram: it does not make a lot of sense.
    Diodes drop voltage across them for the same reason wires do: because they have resistance.
    ... the "volt-drop" of a component usually refers to the potential difference between the ends.

    The voltage drop for diodes in series, which conduct, should work like resistors - when they don't conduct, then it would be like insulators.
    I don't know what you mean by "output" - there are lots of things diodes could have as an input and lots of arrangements that could be considered "output". I cannot see what you have done: you have to tell me.
    ... a voltage is not a property of a particular place in a circuit - it is a comparison between two places. A diode has only one output, so it is not possible to have a voltage at the output. You can place the positive terminal of the voltmeter to the output of the diode and get a reading labelled "voltage", but that will make no sense unless we also know where the negative terminal is connected.
  5. Nov 4, 2014 #4
    @Simone Bridge, Thank you for the reply Simone I understand if what I am explaining doesn't make much sense and my apologies for that and if I sound crazy trying to explain a bit more but our instructor ask for everyone to come up with an explanation as to why non frictional electrical devices burn out over time and my theory was because of either a design flaw or over voltage. I thought if someone is feeding a circuit power that there will always be extra electricity that is not used, I believe that maybe that electricity can sometimes bottle up throughout the circuitry and cause damage and perhaps even flow backwards(which people say is impossible to flow backwards).
    Now this also goes into question #2 I asked because the device were working on will use this extra energy to keep itself running! You say "The voltage drop for diodes in series, which conduct, should work like resistors - when they don't conduct, then it would be like insulators." Can you explain that a bit more? Why wouldnt they conduct? Also when I say output Iam referring to the voltage I receive from the diodes using a volt meter. I will draft a diagram for you to see and post it as soon as I can. Thank you in advance for your help!
  6. Nov 4, 2014 #5

    Simon Bridge

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    You are looking in the wrong place.
    In a simple circuit of just a battery and a light-bulb, the components eventually wear out.
    If the battery is very long-life, the bulb may burn out.
    Usually you think of machines wearing out because they have moving parts - do electric circuits have moving parts?

    ... A diode is a device designed to conduct only on one direction ... so, when they are reverse biased, they do not conduct because that is just how they are made.

    You need to say how you are using the voltmeter to measure voltage ... where do you put the positive terminal, and where do you put the negative terminal, to get your reading? A diagram would be a great help - thanks.
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