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voltage controlled resistor

Does anyone know of an IC chip or discrete component intended to be
used as a variable resistor whose resistance value may be controlled
by varying its voltage input/signal
 
F

Franc Zabkar

Jan 1, 1970
0
Does anyone know of an IC chip or discrete component intended to be
used as a variable resistor whose resistance value may be controlled
by varying its voltage input/signal

About 20 years ago I encountered a device consisting of a light source
(LED ?) and a light dependent resistor. IIRC the part came in a
package that was not much bigger than a TO92 transistor. I think
similar schemes are used to provide noiseless volume controls for
musical instruments, eg the expression pedals for pianos and organs.

- Franc Zabkar
 
M

Michael A. Terrell

Jan 1, 1970
0
Franc said:
About 20 years ago I encountered a device consisting of a light source
(LED ?) and a light dependent resistor. IIRC the part came in a
package that was not much bigger than a TO92 transistor. I think
similar schemes are used to provide noiseless volume controls for
musical instruments, eg the expression pedals for pianos and organs.



The "Vactrol"?


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
 
M

Michael Black

Jan 1, 1970
0
Franc said:
About 20 years ago I encountered a device consisting of a light source
(LED ?) and a light dependent resistor. IIRC the part came in a
package that was not much bigger than a TO92 transistor. I think
similar schemes are used to provide noiseless volume controls for
musical instruments, eg the expression pedals for pianos and organs.
And the poster hasn't really given enough detail.

Those things, and you could make them yourself if you had the light
dependent resistors, tend to be the simplest solution, because the
existing circuit only sees a variable resistor. I gather though
that they tended to be not so linear vs control voltage, and I don't
recall them coming in a wide range of values.

Since the original poster didn't specify what he was trying to do,
getting a result is not going to be easy. The FETs mentioned by
some, those work, but you have to retrofit them onto the existing
design, and I thought there were linearity issues too, as in adding
distortion to the circuit if not done right. Linearity in the control
voltage versus resistance too, I think it was Siliconix who had a whole
application note about that.

Someone mentioned 4066s. The trick would be to pulse modulate those
to get the varying resistance. The problem then is that you end up
with junk on the signal, unless, as Don Lancaster pointed out decades
ago, you are controlling a low pass filter that takes out the junk.

One could use an attenuator made up of a forward biased diode, I've
seen those but I don't know how well they worked.

One could always change the circuit, so instead of a stage of amplification
and a volume control, you put in a gain control stage. So any sort of
stage intended for AGC or voltage controlled amplificaton would fit in.

Michael
 
R

Roger Hamlett

Jan 1, 1970
0
Does anyone know of an IC chip or discrete component intended to be
used as a variable resistor whose resistance value may be controlled
by varying its voltage input/signal
It really does depend massively, on the linearity you require, whether you
need to handle voltages that are unipolar or bipolar, and the actual
current levels involved.
The simple FET, is the commonest solution.
For AC, a pair wired as a 'bilateral switch', is the normal solution (the
4066, contains a small pair of FETs that can be used this way).
Historically, the Cds photo cell, was also sometimes used.
If noise is not a problem, for higher powers, simple PWM control of a
on/off switch is probably a 'better' solution.
Generally, most solutions go 'non linear' at some point. You can achieve
much wider control ranges, using something like the IL300 opto (or it's HP
equivalent). The 'point' about this type of opto, is that two of the sense
elements, are integrated into the single package, allowing accurate
feedback control of the resistance achieved.
For pure AC, the 'ecstatically wound choke', can be used, where a DC
voltage fed through one winding, moves the choke into saturation.
The number of possible solutions, is 'myriad', and a bit more data on what
is required, might help in making a better suggestion.

Best Wishes
 
M

Michael A. Terrell

Jan 1, 1970
0
Franc said:
The name doesn't ring a bell, but some research suggests that may be
it. Thanks.


That was the brand I used to replace in guitar amps, back in the '60s
and '70s. They were a neon lamp and photoresistor in a hard black
plastic tube. Some could be cut open and replace the neon when it
failed. I have no idea if the brand is still around. Its changed hands
several times over the years.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
 
F

Franc Zabkar

Jan 1, 1970
0
And the poster hasn't really given enough detail.

Those things, and you could make them yourself if you had the light
dependent resistors, tend to be the simplest solution, because the
existing circuit only sees a variable resistor. I gather though
that they tended to be not so linear vs control voltage, and I don't
recall them coming in a wide range of values.

I would hazard a guess that these devices were mostly used in volume
control applications where the user provided the feedback. Hence
linearity may not have been a big issue.

- Franc Zabkar
 
M

Michael A. Terrell

Jan 1, 1970
0
Franc said:
I would hazard a guess that these devices were mostly used in volume
control applications where the user provided the feedback. Hence
linearity may not have been a big issue.


There were incandescent versions for remote volume control
applications. Some Public Address amplifiers had sockets to add remote
volume controls.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
 
H

Homer J Simpson

Jan 1, 1970
0
About 20 years ago I encountered a device consisting of a light source
(LED ?) and a light dependent resistor.

Filament lamp (flashlight) and LDR - often just in a piece of shrink tubing.
 
F

Franc Zabkar

Jan 1, 1970
0
That was the brand I used to replace in guitar amps, back in the '60s
and '70s. They were a neon lamp and photoresistor in a hard black
plastic tube. Some could be cut open and replace the neon when it
failed. I have no idea if the brand is still around. Its changed hands
several times over the years.

http://optoelectronics.perkinelmer.com/catalog/Category.aspx?CategoryName=VTL5C+Series

"PerkinElmer Optoelectronics has been a leading manufacturer of analog
optical isolators for over twenty years and makes a broad range of
standard parts under its trademark VACTROL®."

"The analog optical isolator (AOI) also uses an optical link between
input and output. The input element is an LED and the output element
is always photoconductive cell or simply photocell. Together, the
coupled pair act as an electrically variable potentiometer. Since the
output element of the AOI is a resistor, the voltage applied to this
output resistor may be DC and/or AC and the magnitude may be as low as
zero or as high as the maximum voltage rating."

- Franc Zabkar
 
R

Ray King

Jan 1, 1970
0
Hi,

If you use a comparator or opamp output into the gate of a mosfet. ( do not
forget the 10k or higher series gate resistor).
Add a small value ( compared to the resistance range you are looking for )
resistor in the source of the fet. The negative comparator input terminal to
the source of the fet and the positive terminal to a reference voltage. The
variable resistance will be across the series connection of the fet and
source resistor.

Ray
 
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