Zener Resistor Location

Discussion in 'General Electronics Chat' started by cl10Greg, Aug 29, 2014.

  1. cl10Greg

    Thread Starter Active Member

    Jan 28, 2010
    49
    0
    Hello Everyone,

    Embarrassingly enough (kinda but I do believe that you should ask anything you don't know) I am an engineer that doesn't understand the concept of the zener diode that well and clamping. So I know that the zener clamps at a zener voltage and requires a set amount of current to do the clamping. What I don't understand is why the resistor always between the power and the zener? I know it's there to limit the current to the zener but that also limits the current to the load as well. So you would have to size the resistor huge if you are using a large current. So the obvious question to me is "why not just move the resistor to the other side?" Does the zener fail that fast during the start of conduction that you're not allowed to? Are there some other system variables at work? Please let me know.
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    move the resistor to what side? Either it is in series with the zener and limits the current, or it is in parallel with the zener and just wastes power.
    Say your supply is 12V and you want to regulate it to 9V using zener. If you put 30 ohms in series, then you have 100mA that will be shared between your load and the zener, in such ratio that the voltage is 9V. If you make the resistor smaller, then you will have more current available for your load, but also you will waste more power when the load is light.
     
  3. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    and if you move the resistor to the load side, your zener will go bad. putting a 9 volt zener across a 12 volt supply without a limiting resistor will cause too much current through the zener.
     
  4. crutschow

    Expert

    Mar 14, 2008
    12,995
    3,229
    Normally a clamp zener has a voltage higher than the normal voltage so it limits any abnormally high voltage excursions. When that excursion occurs the zener current must be limited to avoid zapping the zener, so that requires a series impedance of some sort, depending upon the current/energy capabilities of the source. If the energy of the peak voltages are high and you need a low series resistance then you need a high power zener to handle the temporary surge current. TVS (Transient Voltage Suppression) diodes are zeners with very high peak power capabilities that are specifically designed for clamping short, high energy voltage excursions.
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Greg, four different ways of hooking up a Zener with a current limiting resistor, four different outputs, which does what you want? (The source voltage is the same in all four cases).

    Only V(z1) and V(z4) look useful to me...

    200.jpg
     
  6. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,501
    380
    Option #3 can also used when working from a fixed DC supply.

    eg: Dropping 12V down to a nominal ~9V by using a 3.3Vz
     
Loading...