isolating a voltage from its source

Discussion in 'General Electronics Chat' started by cheddy, Nov 13, 2007.

  1. cheddy

    Thread Starter Active Member

    Oct 19, 2007
    87
    0
    I am familiar with the basics of a voltage divider network from what I have learned in my readings. I wonder if there is a practical application for such a circuit to be used dynamically. What I mean by dynamically is that the potential difference across a segment of the voltage divider network would be independent of the other segments or does the voltage divider network have to be specifically designed for the circuit it's going to be used for.

    An example would be a 15V source connected to a series of three resistors of equal resistance. In such a configuration there would be 5V across each resistor giving you access to a choice of 5V, 10V or 15V. But if you were add anything to the circuit it would effect the overall resistance and therefore the voltage across all the resistors would change meaning you would have to go back and change a resistor to balance it all out again.

    Is this the only possible way to use a voltage divider network?
     
  2. Salgat

    Active Member

    Dec 23, 2006
    215
    1
    In a voltage divider circuit, the ratio of voltages across the resistors stays the same, but the amount of voltage dropped across each resistor varies with source voltage. To keep a constant voltage your best bet is the use of devices such as the Zener Diode. Anything that maintains a constant drop can be used in parallel in order to maintain a constant voltage (provided that enough voltage is present). If you really wanted, you could put a bunch of diodes in series and add up the 7/10th volt drop of each diode to find the voltage you would get by putting a load in parallel.
     
  3. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    One trick with voltage dividers is to attenuate an input to an opamp.

    For example, a signal of a couple hundred volts is put on the top of a voltage divider. A signal the opamp can handle is tapped off and fed to the opamp. The amp could be a peak detector, zero crossing detector, differentiator, or whatever is needed.

    Folk who play with HV often use dividers to check voltage levels more safely.
     
  4. chesart1

    Senior Member

    Jan 23, 2006
    269
    1
    Voltage dividers are generally used in applications where the output of the divider is not compromised. Thingmaker3's example is excellent. The ideal operational amplifier inputs draw no current. Hence the input looks like an open circuit.

    As you continue your studies, you will find applications for voltage dividers in operational amplifier circuits, transistor circuits, power supply circuits, etc.

    When I was in my first electrical course, the teacher nicknamed me "the voltage divider" because I had some difficulty understanding the concept. When I raised my hand, he said, "the voltage divider man has a question." He only did this in one class, but I thought it was humorous.
     
  5. Dragon

    Active Member

    Sep 25, 2007
    42
    0
    cheddy

    I had a similar confusion when I first started out with voltage dividers. The example by thingmaker3 is perfect.

    As others have pointed out - application of voltage dividers is to attenuate the signal to a specific load. As for your apprehensions about disturbing the divider balance by adding anything further to a voltage divider, rest assured that in such situations the load always follows the divider at some parallel node, thereby eliminating any chances of disturbing the proportions across the divider network.

    In cases where the divider is connected in series to a load is that of an OPAMP. But as opamps have infinite input resistance, the proportions of voltages across the divider network stays the same.
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    To be more correct, ideal op amps have infinite input resistance. Real-world values are somewhat less perfect. Older op amps like the 741 have inputs resistances on the order of a couple of megaohms. Electrometer grade op amps go up to 10^15 ohms. It's one of the important parameters to be aware of when selecting op amps.
     
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