#286353 - 04/09/2006 23:06
Silly basic electronics question.
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carpal tunnel
Registered: 20/12/1999
Posts: 31600
Loc: Seattle, WA
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Let's say I have a variable resistor that tapers between "approximately zero resistance" and "about 90 ohms resistance".
Is there any way to add regular resistors to the circuit (in some fancy pattern) that will change it so that it goes between "approximately zero resistance" and "a huge gigantic insane amount of resistance" but still tapers smoothly between those two extremes?
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#286354 - 04/09/2006 23:35
Re: Silly basic electronics question.
[Re: tfabris]
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carpal tunnel
Registered: 20/12/1999
Posts: 31600
Loc: Seattle, WA
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Hm. Researching it more, it appears that I'm simply wiring the thing up wrong. I've just got a knobby thingy that I want to alter the volume of a line-level circuit. I'm only using two of the terminals and I think I'm supposed to be using all three terminals to make it work right.
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#286355 - 05/09/2006 02:10
Re: Silly basic electronics question.
[Re: tfabris]
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pooh-bah
Registered: 20/01/2002
Posts: 2085
Loc: New Orleans, LA
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Normally, the first terminal is ground, the second is variable resistance, and the third is full resistance. So normally, only pins 1 and 2 are used. Not all 3. And in answer to your first question, No. Get a bigger pot. Adding a resistor in series will just slide your impedance. In other words, you add a 100 Ohm resistor in, you get a pot that goes from 100 to 190 Ohms.
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#286356 - 05/09/2006 02:12
Re: Silly basic electronics question.
[Re: lectric]
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pooh-bah
Registered: 20/01/2002
Posts: 2085
Loc: New Orleans, LA
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Oh, and you could also use pin 3 to have an opposite resistance path. So when you turn the pot up, you get less and less resistance between pin 2 and 3 while getting more and more resistance between pins 1 and 2. Think of it like a fader.
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#286357 - 05/09/2006 06:20
Re: Silly basic electronics question.
[Re: tfabris]
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veteran
Registered: 01/10/2001
Posts: 1307
Loc: Amsterdam, The Netherlands
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Quote:
Hm. Researching it more, it appears that I'm simply wiring the thing up wrong. I've just got a knobby thingy that I want to alter the volume of a line-level circuit. I'm only using two of the terminals and I think I'm supposed to be using all three terminals to make it work right.
If that is what you actually want to do, then you want to use it as a voltage divider circuit, not as a variable resistance. So ground pin 1, connect the output of the previous stage to pin 3, and take the signal to the following stages from pin 2. But this assumes the first stage can drive a resistance/impedance of the total of your pot.
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#286358 - 05/09/2006 11:55
Re: Silly basic electronics question.
[Re: julf]
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carpal tunnel
Registered: 29/08/2000
Posts: 14496
Loc: Canada
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Quote:
But this assumes the first stage can drive a resistance/impedance of the total of your pot.
I think it also assumes that shorting the inputs won't melt anything. 
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#286359 - 05/09/2006 12:36
Re: Silly basic electronics question.
[Re: julf]
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carpal tunnel
Registered: 20/12/1999
Posts: 31600
Loc: Seattle, WA
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Quote:
So ground pin 1, connect the output of the previous stage to pin 3, and take the signal to the following stages from pin 2.
Yup. This is, as I learned, the correct way to wire up a volume control, to give you a full zero-to-infinity sweep on the knob.
I didn't understand this before, because it seemed to me that at the max-resistance end of the knob's travel, I was fully connecting the signal to ground. As opposed to just putting a lot of resistance between source and destination. It seemed to me like connecting signal to ground so directly like that would be a bad idea. But it seems to work. Clearly I don't have the electronics theory background to understand why. I guess one way to look at it is that it's like a shunt for the electrons instead of an on/off valve.
Quote:
I think it also assumes that shorting the inputs won't melt anything.
Exactly. It seemed to me that wiring all three pins like that *WAS* shorting the inputs. I guess it turns out that for a line-level volume circuit, that's exactly what you want to do.
Thanks for the help, everyone.
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#286360 - 05/09/2006 13:35
Re: Silly basic electronics question.
[Re: tfabris]
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carpal tunnel
Registered: 29/08/2000
Posts: 14496
Loc: Canada
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Quote:
. It seemed to me that wiring all three pins like that *WAS* shorting the inputs.
No, it *IS* shorting the inputs, at one of the two extremes of adjustment of the potentiometer.
Cheers
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#286361 - 05/09/2006 13:38
Re: Silly basic electronics question.
[Re: tfabris]
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veteran
Registered: 01/10/2001
Posts: 1307
Loc: Amsterdam, The Netherlands
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Quote:
I guess one way to look at it is that it's like a shunt for the electrons instead of an on/off valve.
Perhaps a better way to look at it is that of a voltage divider. You get the full output voltage of the driver stage over the full resistance of the pot, but the centre tap taps into it at a variable point - exactly halfway it gets exactly half the voltage (assuming a linear pot and no influence from the imput impedance of the following stage, but...)
Quote:
It seemed to me that wiring all three pins like that *WAS* shorting the inputs.
Indeed, but for most *input* stages that is perfectly OK.
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#286362 - 05/09/2006 14:08
Re: Silly basic electronics question.
[Re: julf]
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carpal tunnel
Registered: 20/12/1999
Posts: 31600
Loc: Seattle, WA
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Quote:
Indeed, but for most *input* stages that is perfectly OK.
So what you're saying is, if I wired it backwards, I'd be shorting the wrong end of things and could damage something?
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#286363 - 05/09/2006 14:20
Re: Silly basic electronics question.
[Re: tfabris]
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carpal tunnel
Registered: 13/07/2000
Posts: 4180
Loc: Cambridge, England
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Quote:
So what you're saying is, if I wired it backwards, I'd be shorting the wrong end of things and could damage something?
Well, yes. Shorting the next device's input (the output of your circuit) to ground is obviously OK, as that's the same as there being silence (0V) on the signal. Shorting the previous device's output (the input of your circuit) to ground is typically very bad, as the other device will be continually dumping current into that output trying to drive it high, but never managing.
Peter
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#286364 - 05/09/2006 14:35
Re: Silly basic electronics question.
[Re: tfabris]
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carpal tunnel
Registered: 29/08/2000
Posts: 14496
Loc: Canada
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Quote:
Quote:
Indeed, but for most *input* stages that is perfectly OK.
So what you're saying is, if I wired it backwards, I'd be shorting the wrong end of things and could damage something?
What we're saying is, the thing you are connecting from, as the input to the variable resistor, has to be designed to tolerate having its output (the input to the variable resistor) shorted to ground. Which is what happens when you twist the knob all the way to the left (or right, depending which way round it gets connected).
In the case of, say, a passive microphone as the input, no problem.
In the case of, say, the speaker outputs of an empeg, you'll damage the empeg.
Cheers
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#286365 - 05/09/2006 14:49
Re: Silly basic electronics question.
[Re: mlord]
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carpal tunnel
Registered: 13/07/2000
Posts: 4180
Loc: Cambridge, England
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Quote:
[What we're saying is, the thing you are connecting from, as the input to the variable resistor, has to be designed to tolerate having its output (the input to the variable resistor) shorted to ground. Which is what happens when you twist the knob all the way to the left (or right, depending which way round it gets connected).=
No, I'm not saying that. You fasten the full resistance (pins 1-3) across the input (between the input and ground), and take the output from the moving tap (pin 2). Only your circuit's output, not its input, ever gets shorted to ground. The input needs to tolerate being grounded through 90ohm, but any line level circuit should tolerate that. It doesn't need to tolerate being grounded through 0ohm.
Peter
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#286366 - 05/09/2006 17:26
Re: Silly basic electronics question.
[Re: peter]
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carpal tunnel
Registered: 29/08/2000
Posts: 14496
Loc: Canada
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Ahh.. yup, you're right. I had it backwards in my mind.
Thanks!
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#286367 - 05/09/2006 19:34
Re: Silly basic electronics question.
[Re: peter]
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carpal tunnel
Registered: 20/12/1999
Posts: 31600
Loc: Seattle, WA
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Quote:
Shorting the previous device's output (the input of your circuit) to ground is typically very bad,
/me double checks the pinouts on the pot...
PHEW.
Okay, I did it right. Had a fifty-fifty chance and I just happened to have done it right.
Thanks again, folks.
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