Resistor In Supply Line

Discussion in 'General Electronics Chat' started by sjgallagher2, Feb 20, 2015.

  1. sjgallagher2

    Thread Starter Member

    Feb 6, 2013
    111
    7
    I've been analyzing a lot of circuits lately, and I've noticed that they sometimes have resistors in the supply line in between stages, like in this metal detector circuit from talkingelectronics.com:

    [​IMG]

    the 330R resistor on top is what I mean. I've seen it in transistor amplifiers as well. What effect does this have on the circuit? I feel as though it would make the analysis much more complex, with voltage drops and all. Can somebody shed some light for me? It would be much appreciated. Thanks in advance
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    The resistor works in combination with the 5.6 Volts zener on the far left of the schematic.
    It makes the voltage for the oscillator more stable, so the oscillator will not drift from voltage changes on the battery (to a certain level).

    Bertus
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,107
    3,038
    Just to add a small point, the resistor limits the current passing through the zener, to protect it. You choose the value to compromise between competing factors; you want to minimize current wasted through the zener if the circuit is not using any - meaning the zener must absorb it all, while allowing for the maximum current needed by the circuit to pass through the resistor without dropping the voltage below the zener voltage. That would cause the loss of voltage regulation.

    In the example above, the maximum current would be (9V-5.6V)/330Ω=10mA. The zener will keep regulating the circuit voltage as long as the current it needs stays in the range of 0-10mA, and the zener+circuit will draw 10mA all the time.
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,361
    In the above usage, the zener is used to provide a regulated DC voltage. The resistor is to limit the current through the zener diode and protect the diode from overheating.

    In other usage, the resistor R (or sometimes an inductor) is used in conjunction with a filter capacitor C to create an RC low pass filter to clean up the power supply to preamp stages of an amplifier.
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    From a battery life point-of-view it would be better to use a series regulator to give a stable voltage, since that would use only the current required by the oscillator stage, but a resistor and zener diode is a cheaper solution.
     
  6. sjgallagher2

    Thread Starter Member

    Feb 6, 2013
    111
    7
    Okay I understand a little better now. But my next question is obviously why not put the resistor directly in series with the zener? I see that it is in series with the zener, but it also has an effect on all parallel loads in between the zener and the resistor. The only reason for the resistor seems to be there is to limit current in the zener, but it also has some small effects on voltage, because of where it is placed. I just need to understand why they put it where they did, as opposed to further left or right, is what I'm getting at haha. Thanks again for all the replies.
     
    Last edited: Feb 20, 2015
  7. WBahn

    Moderator

    Mar 31, 2012
    17,743
    4,789
    The purpose of the zener is to get a stable voltage for the oscillator. Thus the top of the zener is the power supply for the oscillator circuit. To the right of the resistor you have a 9V unregulated DC supply that will vary in voltage as the battery dies. To the left of it you have a 5.6V regulated supply that will maintain a nearly constant voltage, provided the oscillator circuit doesn't draw too much current, until the battery is well past anything resembling "dead".
     
  8. sjgallagher2

    Thread Starter Member

    Feb 6, 2013
    111
    7
    Okay, so if I have this right, only the oscillators need a regulated voltage, in order to keep the frequency stable, but the rest of the circuitry doesn't, because it's more robust with regard to changes in supply voltage?
     
  9. WBahn

    Moderator

    Mar 31, 2012
    17,743
    4,789
    That's pretty much it. The right hand side is just a small amplifier to drive the speaker and also just an LED indicator. They don't really care too much about the exact voltage.
     
Loading...