puzzling grounded input

Guest
In this circuit:
http://kitsrus.com/projects/k48.pdf

Why is the other end of RV1 grounded?

Thanks,

Michael
 
On Nov 14, 10:24 am, mrdarr...@gmail.com wrote:
In this circuit:http://kitsrus.com/projects/k48.pdf

Why is the other end of RV1 grounded?

Thanks,

Michael

Never mind... I think it has something to do with the input
impedance...

Michael
 
On Fri, 14 Nov 2008 10:32:50 -0800 (PST), mrdarrett@gmail.com wrote:

On Nov 14, 10:24 am, mrdarr...@gmail.com wrote:
In this circuit:http://kitsrus.com/projects/k48.pdf

Why is the other end of RV1 grounded?

Thanks,

Michael


Never mind... I think it has something to do with the input
impedance...
---
Only incidentally.

Think about it like this:

At the top of the pot the input signal will be at its maximum amplitude
since that's connected to the output of the signal source, and at the
bottom of the pot it'll be at 0 volts, since that's ground. Anywhere in
between it'll be somewhere between max and 0.

What you have with a potentiometer is what looks like two resistors in
series with the slider at their junction, like this:

VIN>---+
|
[R1]
|
+------->VOUT
|
[R2]
|
GND>---+------->GND


Since it's a 10k pot, let's assume the slider is located half-way
between the two ends, like this:


EIN>---+
|
[5k]
|
+------->VOUT
|
[5k]
|
GND>---+------->GND


Then, if we relabel the circuit for convenience:


E1
|
[R1]
|
+---E2
|
[R2]
|
GND


we can say:


E1 * R2 1V * 5kR
E2 = --------- = ---------- = 0.5V
E1 + R2 1V + 5kR


If you solve that equation for the slider being located anywhere along
the element and remember that both sides of the pot have to add up to
10k, you'll find that E2 will increase more and more as the slider gets
closer and closer to the E1 end of the pot and that E2 will decrease
more and more as the slider gets closer and closer to the GND end of the
pot.

So what is it?

It's a volume control! :)

JF
 
On Fri, 14 Nov 2008 14:52:35 -0600, John Fields
<jfields@austininstruments.com> wrote:

On Fri, 14 Nov 2008 10:32:50 -0800 (PST), mrdarrett@gmail.com wrote:

On Nov 14, 10:24 am, mrdarr...@gmail.com wrote:
In this circuit:http://kitsrus.com/projects/k48.pdf

Why is the other end of RV1 grounded?

Thanks,

Michael


Never mind... I think it has something to do with the input
impedance...

---
Only incidentally.

Think about it like this:

At the top of the pot the input signal will be at its maximum amplitude
since that's connected to the output of the signal source, and at the
bottom of the pot it'll be at 0 volts, since that's ground. Anywhere in
between it'll be somewhere between max and 0.

What you have with a potentiometer is what looks like two resistors in
series with the slider at their junction, like this:

VIN>---+
|
[R1]
|
+------->VOUT
|
[R2]
|
GND>---+------->GND


Since it's a 10k pot, let's assume the slider is located half-way
between the two ends, like this:


EIN>---+
|
[5k]
|
+------->VOUT
|
[5k]
|
GND>---+------->GND


Then, if we relabel the circuit for convenience:


E1
|
[R1]
|
+---E2
|
[R2]
|
GND


we can say:


E1 * R2 1V * 5kR
E2 = --------- = ---------- = 0.5V
E1 + R2 1V + 5kR
---
Tsk, tsk, tsk...


E1 * R2 1V * 5kR
E2 = --------- = ---------- = 0.5V
R1 + R2 5kR + 5kR

JF
 
On Fri, 14 Nov 2008 10:24:19 -0800 (PST), mrdarrett@gmail.com wrote:

In this circuit:
http://kitsrus.com/projects/k48.pdf

Why is the other end of RV1 grounded?

Thanks,

Michael
A volume control is a variable voltage divider. If the bottom end of
RV1 was not grounded, RV1 would just be a variable series resistor,
and would have very little effect on the volume.



--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca
 
On Nov 14, 1:18 pm, Peter Bennett <pete...@somewhere.invalid> wrote:
On Fri, 14 Nov 2008 10:24:19 -0800 (PST), mrdarr...@gmail.com wrote:
In this circuit:
http://kitsrus.com/projects/k48.pdf

Why is the other end of RV1 grounded?

Thanks,

Michael

A volume control is a variable voltage divider.

Oh...

If the bottom end of
RV1 was not grounded, RV1 would just be a variable series resistor,

That's all I thought it was...

and would have very little effect on the volume.

Ah...

Thanks John and Peter!


--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
GPS and NMEA info:http://vancouver-webpages.com/peter
Vancouver Power Squadron:http://vancouver.powersquadron.ca

Michael
 
On Nov 14, 1:24 pm, mrdarr...@gmail.com wrote:
In this circuit:http://kitsrus.com/projects/k48.pdf

Why is the other end of RV1 grounded?

Thanks,

Michael
yeah, like everybody says, and you're on the right track with your
input impedance idea; tacking the far end to ground stabilizes the
input impedance so that the input signal is well behaved with respect
to where you set the pot at. on the other hand, if you just had a
variable resistor in series, like with the thing ungrounded, the input
signal source seeing a wildly changing impedance both in value and in
reactivity (from mostly resistance to mostly capacitative) will very
likely change its frequency response etc, unless it's nicely designed
to ignore that kind of thing.
 
<mrdarrett@gmail.com> wrote in message
news:a4c97e0b-4b14-41dc-b5f2-169fb85d0d7d@s9g2000prg.googlegroups.com...
In this circuit:
http://kitsrus.com/projects/k48.pdf

Why is the other end of RV1 grounded?

Thanks,

Michael
One quick question regarding the labeling conventions of the resistors in
the schematic. Why are some labled this way: 1K5 or 2K7

Is that 1.5K , 2.7K ohms? Makes sense, I just haven't seen that before.

Scott
 
On 2008-11-15, sbrehler <sbrehler@mndspring.com> wrote:
mrdarrett@gmail.com> wrote in message
news:a4c97e0b-4b14-41dc-b5f2-169fb85d0d7d@s9g2000prg.googlegroups.com...
In this circuit:
http://kitsrus.com/projects/k48.pdf

Why is the other end of RV1 grounded?

Thanks,

Michael

One quick question regarding the labeling conventions of the resistors in
the schematic. Why are some labled this way: 1K5 or 2K7

Is that 1.5K , 2.7K ohms? Makes sense, I just haven't seen that before.
Exactly right. one reason for this is it makes printed schematics
easier to read.


Look at C5 if I scale the schematic to "125%" the '.'
almost disappears, by using a letter instead of a '.' the values are
more easily read, (they could have labeled C5 100nF, I don't know why
they didn't)

they should have used vector graphics instead of pixels for
the schematic then it would scale better - but that's a different
matter

also the font they chose has the '.' skewed to the left which is
unfortunate.

bye.
 
Jason,

Thanks for the clarification!

Scott
"Jasen Betts" <jasen@xnet.co.nz> wrote in message
news:gfm9ch$1jd$1@reversiblemaps.ath.cx...
On 2008-11-15, sbrehler <sbrehler@mndspring.com> wrote:

mrdarrett@gmail.com> wrote in message
news:a4c97e0b-4b14-41dc-b5f2-169fb85d0d7d@s9g2000prg.googlegroups.com...
In this circuit:
http://kitsrus.com/projects/k48.pdf

Why is the other end of RV1 grounded?

Thanks,

Michael

One quick question regarding the labeling conventions of the resistors in
the schematic. Why are some labled this way: 1K5 or 2K7

Is that 1.5K , 2.7K ohms? Makes sense, I just haven't seen that before.

Exactly right. one reason for this is it makes printed schematics
easier to read.


Look at C5 if I scale the schematic to "125%" the '.'
almost disappears, by using a letter instead of a '.' the values are
more easily read, (they could have labeled C5 100nF, I don't know why
they didn't)

they should have used vector graphics instead of pixels for
the schematic then it would scale better - but that's a different
matter

also the font they chose has the '.' skewed to the left which is
unfortunate.

bye.
 

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