Variable resistance

I need to make pressure measurement circuit. Output of this circuit must be a variable resistance that must be between 10 ohm and 180 ohm acoording to applied pressure. pressure sensor gives 0- 20 mV output and I amplify it by an instrumentation amplifier to 0 - 5V level and sample it with an microcontroller. After microcontroller stage I must convert this measured value to an resistance value for output because Display circuits reads pressure values as a variable resistance between

10 ohm to 180 ohm. Variable resistance is referenced to ground from one of its terminals. Maybe this design can be implemented without microcontroller.

How can I make variable resistance by the means of microcontroller or without using microcontroller

Thanks

Reply to
evren.soykut
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Simplest is often PWM duty cycling which integrates into resistance.

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Many thanks,

Don Lancaster                          voice phone: (928)428-4073
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Reply to
Don Lancaster

If Don's pure PWM doesn't work you could use PWM and drive a FET gate with it. Make sure it's integrated well enough so the gate sees pure DC. However, since these have a large tolerance you need to drive via an opamp, use a FET array and hook a 2nd FET on the same die into the feedback. The 2nd FET forms a voltage divider with a resistor and then the uC can simply set a DC voltage (via the PWM) and this value translates into drain-source resistance.

Regards, Joerg

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Reply to
Joerg

You have the microcontroller, use a PWM output, switch between two stable known voltage values (Vdd/Vss may be good enough for you), LPF to get DC, feed that to a voltage-controlled current sink. Measure the voltage at the current source output pin using a spare ADC input. Periodically calculate and output the PWM duty cycle so that Iout = Vc/Rs, where Rs is the desired simulated sensor resistance and Vc is the voltage at the collector (or drain) of the current sink.

Best regards, Spehro Pefhany

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"it\'s the network..."                          "The Journey is the reward"
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Reply to
Spehro Pefhany

A 1K resistor with a CMOS 4016 or other switch in series with it looks like 1K at full duty cycle, 10K at 9:1 and 100K at 99:1 duty cycle.

So long as whatever is using the resistance can integrate the pwm. Usually a plain old capacitor will suffice.

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Many thanks,

Don Lancaster                          voice phone: (928)428-4073
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Reply to
Don Lancaster

Farnell list several interesting devices - Fairchild's opto-FETs (H11F1 and H11F3) which make very good isolated controllable resistors, though they won't get down to 180R, let alone 10R, and Vishay's IL300 optocouplr with two separate photodiodes, both driven by the same LED.

With the IL300 you use one photodiode for local feedback, while the other one sits on the other side of the isolation barrier. Cute.

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Bill Sloman, Nijmegen
Reply to
bill.sloman

Yes, a good old trick.

He needs to go down to something like *10* ohms, sounds like an automotive gauge or something like that. I'm thinking those things respond to the RMS current, not the average current, since they use a heater coil and a bimetal.

Best regards, Spehro Pefhany

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"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Reply to
Spehro Pefhany

Hello Bill,

These are nice but not so suitable for RF paths because of their large GS capacitance. A real MOSFET might be better in many cases if you can live within the limits for the gate breakdown voltage.

Regards, Joerg

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Reply to
Joerg

If I were emulating variable resistance I think I'd opt for a multiplying DAC, an OpAmp and a FET (or bipolar), so that the devices expecting a variable R actually saw the "real thing". I'm not sure some circuits would respond well to a PWM emulation unless designed for it in the first place.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Which is similar to some excellent advice you and other people gave me in a thread here about 3 years ago.

Google for thread title "Voltage Controlled Resistor?". The original question was posted by me on 11 Dec 02.

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Tony Williams.
Reply to
Tony Williams

Thank you everybody for your advices. This circuit will be used in automative application. Circuit must work in wide temperature rance maybe -4 F and 176 F (-20 C and 80 C) so if I use FET type solution Linearity and resistance change can occur. if I use this type of solution I will need a feedback mechanism

Maybe I can use a switched resistor network 1ohm,2 ohm,4,8,16,32,64,128,256 ohm maybe R-2R ladder network(I dont know how can I use it in this application)

I am waiting for your advices

thank you

Reply to
evren.soykut

At 10:35 am Message-ID:

That was a great thread, with several conversations going on at once, and by the 15th you had a working circuit you were happy with, although it did have a multiplier. :-)

I made a copy this morning, edited for brevity, posted below. The multiplier solution comes near the end of the text, where Google put it, although it was one of the early forks.

==============================================================

Subject: Voltage Controlled Resistor? Group: sci.electronics.design

From: Tony Williams Date: Wed, Dec 11 2002 10:35 am Message-ID: Email: Tony Williams

What's a good way of doing a precision VCR? I thought it was easy, but have been doodling on paper for half a day now. I could have sworn it could be done without a multiplier. Must be going senile.........

The spec; Vcontrol= 0-10V, Rout= 0-1k, over the range 0-10v or 0-10mA. 15-0-15 available.

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Tony Williams.

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Reply to
Winfield Hill

If this is simply to drive a gauge input then it may be simplified greatly if you make some measurements with a variable resistor between gauge and GND. Or just use 10, 22, 33, 47, 68, 82, 100, 120, 150, 180 ohm resistors, one at a time, record the voltage measurement across the resistor, that is from gauge terminal to GND; and note the gauge reading for each resistor, best estimate is good enough. Then report back with the tabulated results.

Reply to
Fred Bloggs

Sounds convoluted.

Do you know how the downstream circuit measures the 'resistance'?

It may simply be a current source looking at the resulting voltage - simply requiring a voltage input.

Could be an oscillator lookng for a resistance-set current - simply supply a current.

Wouldn't it be strange if the indicator turns out to need a 0-20mV input, or if your pressure sensng 'signal' originates in a 10 to 180 ohm transducer?

RL

Reply to
legg

Hello Jim,

The FET is nice but nowadays it's hard to find one that can cover the ranges above 100ohms nicely enough. They are all geared towards low RDSon these days and thus have large die and huge capacitances.

If performance is only required above a few MHz I prefer PIN diodes. Currently I am wrestling with these again, fighting rectifying effects and all that, plus scouring the dreadfully slow Philips web site for a SPICE model of their BAP50. Mostly the site trundles "waiting for stockquote blah blah blah...". I mean, why don't they just place the sub-circuit or model statements at the end of the data sheet?

Regards, Joerg

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Reply to
Joerg

You missed my point... there's an OpAmp plus a series source R to set PRECISE emulation... sort of like a current mirror.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Hello Jim,

Sure, I have used that as well in the past to obtain a linear range and/or precise setting. But it doesn't make the large capacitance go away :-(

Of course, I already know your answer. But I can't make my own devices, we discrete designers have to live with what the semi mfgs place into the vending machines.

Any chance that Lansdale will some day offer the 74HC equivalent of the CD4007 but with very low capacitances?

Regards, Joerg

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Reply to
Joerg

I doubt it. You could call Dale Lillard and ask. But Dale tends to build them just like they were in the old days, not as you'd like them to be today.

I just got an RFQ to reconstruct an obsolete AM/IF-strip/detector chip ;-) (Military use.)

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Hello Jim,

It would have to be at least like the SD5400 in RF performance. But the market would be small since most young EEs don't know how to design with this stuff anymore.

That's a surprise. Usually the defense folks insist on water-tight contracts with clauses such as the surrender of the mask plots in case of obsolescence or breach of delivery schedule so they can go out and have it fabbed elsewhere.

Regards, Joerg

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Reply to
Joerg

This is a strange one... ALL documentation has been tossed, though I did manage to scarf up a rough schematic.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

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