What does this arrow mean on simple schematic?

Discussion in 'The Projects Forum' started by newbie217, Aug 29, 2009.

  1. newbie217

    Thread Starter Active Member

    Apr 12, 2009
    52
    0
    Hello all,

    I'm new here and trying to get started in electronics and diy projects. One thing I am noticing a lot on some audio project schematics is the use of an arrow, like the one circled on the 100k resistor (see image). What does this mean exactly? Thanks!
     
    • Amp.JPG
      Amp.JPG
      File size:
      47.1 KB
      Views:
      562
  2. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    The part you circled is a potentiometer.
    A potentiometer can be used as a variable voltage divider.
    See this page of the eBook for more info:
    http://www.allaboutcircuits.com/vol_1/chpt_6/1.html

    The potentiometer part starts about half way down.

    Greetings,
    Bertus
     
  3. newbie217

    Thread Starter Active Member

    Apr 12, 2009
    52
    0
    Hello,

    Oh, I see. I'm still not quite clear why some pots are 3 terminal devices? In the previous example, one end of the pot connects to ground. The other end connects to V+ (like a regular resistor).

    The wiper (shaft) is manually controlled to adjust the resistance. What is the third terminal for? And why does it need to connect to the other 100k resistor in the circuit? Thanks again.
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    The potentiometer in the schematic you gave is to set the gate voltage for the FET.

    [​IMG]

    The 100K resistor to ground is to protect the FET when the potentiometer fails.
    Perhaps others have other explanations, but this is the way I see it.

    Greetings,
    Bertus
     
  5. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    A potentiometer is a three terminal device; the two ends of the fixed resistance, plus the 'wiper' terminal from the moving contact.

    A 'Variable resistor' is actually a potentiometer using just the wiper and one end terminal.

    Have a look here:
    http://en.wikipedia.org/wiki/Potentiometer

    The picture of the big wirewound pot allows you to see how it works.
    The two end terminals are the ends of a fixed resistance (of whatever value the pot is).

    The third (wiper) terminal is arraged so it's connection to the resistor can be moved around to any point along it.

    If you put a voltage across the track, the voltage at the wiper is proportional to it's physical position; mid way = 50%, quarter way = 25% etc.
     
  6. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    In addition to the information already provided, here is a link to a short write-up on potentiometers that can give you another perspective on the component.

    hgmjr
     
  7. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    A Potentiometer can be wired as a variable resistor using only 2 terminals, the wiper & either fixed outer terminal. When wired like this it is technically referred to as a Rheostat but I must admit that it's a term that dates me. When all three terminals are used it is technically termed a Potentiometer because it forms a voltage divider that can be black boxed as two series resistors. On the other hand if you ever come across an old Rheostat it will have only 2 terminals; the wiper and one fixed terminal. Bottom line: A Pot can be a Rheostat but a Rheostat can't be a Pot. ;)

    It's times like this that I feel like a Dinosaur!
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    CDrive: I can relate to that!
     
  9. DickCappels

    Moderator

    Aug 21, 2008
    2,664
    634
    That's an interesting circuit -kind of a bias control and a volume control in the same pot. That 100k resistor could be better used if put in series with the wiper. My guess, Bertus, is that the 100k resistor is an artifact of the original design, though it could serve the purpose you mentioned.
     
  10. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    That is an odd circuit! I've never seen a VReg used like that and I must admit that it's counter intuitive and very unorthodox,... to me, that is. :confused: Anyone care to comment on what the designer had in mind?
     
  11. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    The LM317 acts as an current source.
    The current will be about 1.25/5 = 250 mA.
    The FET is acting as a voltagecontrolled resistor.
    This in combination with the constant current source will give a voltage variation dependend on the input voltage.
    The FET is in linear mode and MUST be cooled.

    Greetings,
    Bertus
     
  12. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    It's a type of constant current source (or sink, in that case).

    If you hold a known, contant voltage across a fixed resistance, you must be putting a constant current through the resistor.
     
  13. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
  14. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    cdrive,

    I too was initially confused by the voltage regulator's connection in the circuit under discussion. Things cleared up considerably when I considered that a constant current is being delivered to the load resistor and therefore a constant current must be delivered through the Vin terminal.

    hgmjr
     
  15. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Thanks for the link bertus and the constant current explanation. BTW, the first single stage NPN amplifier he shows can't be class A (as he states) and the Collector signal can't be a true reproduction of the Base signal.
     
  16. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello CDRIVE,

    You are right. The first circuit looks like an emittor follower to me.

    Greetings,
    Bertus
     
  17. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Yeah, with no foward bias! Shure would like to know how he can get Class A from that?
     
Loading...