What does this arrow mean on simple schematic?

Thread Starter

newbie217

Joined Apr 12, 2009
52
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!
 

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Thread Starter

newbie217

Joined Apr 12, 2009
52
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.
 

bertus

Joined Apr 5, 2008
21,448
Hello,

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



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
 

rjenkins

Joined Nov 6, 2005
1,013
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.
 

CDRIVE

Joined Jul 1, 2008
2,219
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!
 

DickCappels

Joined Aug 21, 2008
7,547
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.
 

CDRIVE

Joined Jul 1, 2008
2,219
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?
 

bertus

Joined Apr 5, 2008
21,448
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
 

rjenkins

Joined Nov 6, 2005
1,013
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.
 

hgmjr

Joined Jan 28, 2005
9,029
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?
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
 

CDRIVE

Joined Jul 1, 2008
2,219
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.
 
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