voltage controlled resistor

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

Sounds like a fet

"Ray King" wrote in news:TZH2i.6$ snipped-for-privacy@bignews2.bellsouth.net:

Yes. Try the H11F1 optoFET. That can vary from >300 Mohms down to a few tens of ohms or less, when its LED is fed by an input voltage via a series resistor. I haven't yet tried it for linearity, but it's a nice idea because it isolates input and output. If you use the output to drive another FET, you can probably arrange a near-zero ohms to >300 Mohms resistor.

Hmm. how about a Mosfet ? or something in the line of a 4066 which does not get you in the low order of R but can be some what controlled via voltage.

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On 16 May 2007 10:43:47 -0700, snipped-for-privacy@gmail.com put finger to keyboard and composed:

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

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The "Vactrol"?

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Michael A. Terrell
Central Florida

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

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

On Thu, 17 May 2007 06:25:34 GMT, "Michael A. Terrell" put finger to keyboard and composed:

The name doesn't ring a bell, but some research suggests that may be it. Thanks.

- Franc Zabkar

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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.

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On 17 May 2007 14:19:55 GMT, snipped-for-privacy@FreeNet.Carleton.CA (Michael Black) put finger to keyboard and composed:

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

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

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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

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

I just got an email 2 days ago that says the H11F1 was just announced as discontinued:

"You have purchased the following part number(s) from Digi-Key within the last two years. The manufacturer has announced this part(s) will become obsolete. Please click on the link below for more information on possible substitutes.

FAIRCHILD OPTOELECTRONICS GROUP Description: OPTOCOUPLER FET BILAT 6-DIP Manufacturer Part Number: H11F1 Digi-Key Part Number: H11F1-ND"

------------

Maybe it is just the DIP that is gone, I haven't looked into it.

On Thu, 17 May 2007 22:11:57 GMT, "Michael A. Terrell" put finger to keyboard and composed:

"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

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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

mw wrote in news:Fr63i.3130$ snipped-for-privacy@newssvr19.news.prodigy.net:

They're multisourced, I think. I doubt it's entirely obsolete. It better hadn't be, anyway.

I looked at Fairshild's site... the H11F1, H11F2, and H11F3 are all listed as lifetime buy. I didn't find any recommendations about alternatives, except on their obsolete search they recommend "H11F1M", which doesn't exist.

Nice...