Author Topic: 5 pin thumbwheel variable resistor pinout.  (Read 12195 times)

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Offline josephpate

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5 pin thumbwheel variable resistor pinout.
« on: October 21, 2015, 03:19:24 am »
I have this simple volume thumbwheel  that I need to desolder and replace with a front panel mount pot.

http://imgur.com/a/Ca4Eg

It has two large solder joints that, as far as I can tell, are just for support.  Then there's 5 other pins which presumably are in in/out pins for both audio channels.

I found this image on google:
http://cdn.instructables.com/F6K/QVPA/GAPUS878/F6KQVPAGAPUS878.LARGE.jpg

Would my part have this same pin out?  I tried figuring it out with my multimeter but none of the readings seemed to make sense (maybe when its out of the circuit it will.)

Also, what's up with the ground pin?  I should only need left and right input and output pins right?
 

Offline Connoiseur

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Re: 5 pin thumbwheel variable resistor pinout.
« Reply #1 on: October 21, 2015, 04:06:45 am »

Also, what's up with the ground pin?  I should only need left and right input and output pins right?

Ground pin is provided because it is a potentiometer and not merely a variable resistor. See the attached schematic.
 

Offline josephpate

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Re: 5 pin thumbwheel variable resistor pinout.
« Reply #2 on: October 21, 2015, 04:25:36 am »
So what would the ground pin be used for in this scenario (audio)? 
 

Online blueskull

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Re: 5 pin thumbwheel variable resistor pinout.
« Reply #3 on: October 21, 2015, 04:58:50 am »
Grounding one side of the pot gives you a variable resistive ladder, so you can adjust volume.
 

Offline Connoiseur

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Re: 5 pin thumbwheel variable resistor pinout.
« Reply #4 on: October 21, 2015, 05:19:47 am »
The potentiometer outputs are fed to the output power amplifier stage(s). Potentiometer is used to supply scaled (scaling varies from 0 to 1) version of the audio signal; which works by the principle of voltage division.
You can't have voltage division unless you connect the potentiometer between input voltage and ground.
https://en.wikipedia.org/wiki/Voltage_divider
 

Offline Richard Crowley

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Re: 5 pin thumbwheel variable resistor pinout.
« Reply #5 on: October 21, 2015, 05:26:10 am »
If you notice your first photo, you will see the larger copper track going to the left-most pin. That appears to correlate with the ground node.

We are assuming that your unidentified gadget is stereo.
 

Offline josephpate

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Re: 5 pin thumbwheel variable resistor pinout.
« Reply #6 on: October 21, 2015, 09:33:10 pm »
Its actually an el-cheapo USB audio interface, but same difference I guess.

http://www.amazon.com/gp/product/B0023BYDHK/ref=pd_lpo_sbs_dp_ss_1?pf_rd_p=1944687662&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B000KW2YEI&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=0R3176QGNGRK4CY5MBRB

So Is there some reason that a voltage divider would work better than a variable resistance, considering that this is a crappy headphone volume knob? 

Either way it doesnt really matter for my project, since I'm just gonna wire in a new pot where that other one is.
 

Offline Richard Crowley

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Re: 5 pin thumbwheel variable resistor pinout.
« Reply #7 on: October 21, 2015, 09:52:38 pm »
Is there some reason that a voltage divider would work better than a variable resistance,
It comes down to a voltage divider either way. With a "variable resistance" you are dependent on the input impedance of the destination to form the "bottom half" of the voltage divider. By using a potentiometer (a variable voltage-divider) you have DIRECT control over the level. Otherwise a "variable resistor" may have no effect at all on the level if you have a high-impedance destination. Or the variable resistor could have almost no practical range if operating into a destination impedance that was very low.

Using a potentiometer is the method that 99.999999999999999% of all audio gear uses for audio level control.  It is completely impractical to use a simple variable series resistor.
 

Offline Cliff Matthews

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Offline josephpate

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Re: 5 pin thumbwheel variable resistor pinout.
« Reply #9 on: October 21, 2015, 10:53:30 pm »
Is there some reason that a voltage divider would work better than a variable resistance,
It comes down to a voltage divider either way. With a "variable resistance" you are dependent on the input impedance of the destination to form the "bottom half" of the voltage divider. By using a potentiometer (a variable voltage-divider) you have DIRECT control over the level. Otherwise a "variable resistor" may have no effect at all on the level if you have a high-impedance destination. Or the variable resistor could have almost no practical range if operating into a destination impedance that was very low.

Using a potentiometer is the method that 99.999999999999999% of all audio gear uses for audio level control.  It is completely impractical to use a simple variable series resistor.

I see.  Thanks for the info!
 


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