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'[EE]:Resistive circuit'
2000\10\03@021627 by ISO-8859-1?Q?Ruben_J=F6nsson?=

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Hello all,

I have a circuit with a potentiometer (value 1kohm) in it. This
potentiometer is to be set when the equipment is first installed in
its environment. I am looking at a way to replace this potentiometer
with an automatic setting, done every time the circuit is powered up.
I thought I had a solution with a digital potentiometer in reostat
configuration (2 terminal) and a PIC to control it. The problem is
that the wiper current of these circuits have to be lass than 1 mA (I
need about 10mA).

So my question is, does anyone know of a two terminal circuit (plus
power and control) that acts as a variable resistor where the
resistance is set by a voltage level or a variable resistor with less
current through it?

I'm sure there must be a nice litle OP-amp or transistor curcuit that
does this but I just can't seem to figure it out.


==============================
Ruben Jvnsson
AB Liros Elektronik
Box 9124, 200 39 Malmv, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
spam_OUTrubenTakeThisOuTspampp.sbbs.se
==============================

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2000\10\03@023530 by David VanHorn

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>
>So my question is, does anyone know of a two terminal circuit (plus
>power and control) that acts as a variable resistor where the
>resistance is set by a voltage level or a variable resistor with less
>current through it?

Maybe I'm being dense, but isn't this the definition of a transistor?

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2000\10\03@043109 by staff

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Ruben Jönsson wrote:
{Quote hidden}

Sounds like you to need to establish WHAT the pot normally connects to.
If it is used in some type of resistive divider to set a voltage you
can probably use a transistor when you know more of the spec of the
target circuit, ie whether it sinks or sources current.

As a curiosity, many modern cheap hifi units have a motorised
volume control, you may have seen these. You use the remote control
and the volume knob on the hifi winds itself up or down. Pretty
nifty! I have a couple of these I have pulled from dead hifi units.
I always thought they might be handy for times when I have no option
but to use a pot, and it needs to be electrically isolated etc.
They are usually 500k or 250k. And log probably, being volume
pots and all. :o)
-Roman

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2000\10\03@064927 by Arthur Brown

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Try an FET these are better suited to the job

Regards Art

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Sent: Tuesday, October 03, 2000 7:34 AM
Subject: Re: [EE]:Resistive circuit


{Quote hidden}

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2000\10\03@065251 by Russell McMahon

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Ruben,


This should be easy enough to achieve if you will describe your end
application slightly more completely.

It seems that the actual pot current is your end target (rather than the
more normal output voltage).
If you can describe what you are trying to achieve (eg 0 to 10mA variable
into a 100r resistor which is earth referenced at one end (or whatever))
then someone here will certainly be able to suggest a simple method.


regards,




     Russell McMahon
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From: Ruben Jönsson <RemoveMErubenjTakeThisOuTspamPP.SBBS.SE>
I have a circuit with a potentiometer (value 1kohm) in it. This
potentiometer is to be set when the equipment is first installed in
its environment. I am looking at a way to replace this potentiometer
with an automatic setting, done every time the circuit is powered up.
I thought I had a solution with a digital potentiometer in reostat
configuration (2 terminal) and a PIC to control it. The problem is
that the wiper current of these circuits have to be lass than 1 mA (I
need about 10mA).

So my question is, does anyone know of a two terminal circuit (plus
power and control) that acts as a variable resistor where the
resistance is set by a voltage level or a variable resistor with less
current through it?

I'm sure there must be a nice litle OP-amp or transistor curcuit that
does this but I just can't seem to figure it out.

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2000\10\03@072237 by Simon Nield

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as one of the previous posters said, best thing is probably to work out what the circuit actually
needs and try to supply that instead of trying to create the right sort of variable resistive
divider.

otoh: a handy part for this sort of application is the h11f1. it's as close as i have seen to a
simple voltage controlled resistor. never used it myself, but i have seen it used in a couple of
audio compressor-limiter-noisegate type designs.
worth a quick look anyway.

Regards,
Simon

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2000\10\03@103948 by Dan Michaels
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Ruben Jönsson wrote:
{Quote hidden}

As others have said, can try BJT or FET ckt. I use a ckt like this to control the frequency of an XR2206 VCO chip - requires resistance varying over about 1K to 10M, referenced to gnd.
This is a shunt scheme, and isn't going to work if you need a series R.

I found NPN inverter was easier to control than FET ckt. Open-collector
is the "output" node, 390 ohm resistor to gnd in emitter lead [provides
feedback --> temperature stability]. 3.3K base to gnd, 43K base to PIC.
Mine uses PWM for control, so there is also a 10 uF cap from base to gnd. PWM = 0 - 1023 swishes the effective R over the full range, and sweeps the VCO frequency over 500:1.
In your case, you would want smaller Rs in the ckt - esp in the emitter
for higher current. You need to play with the 43K : 3.3K "ratio" to set the range of control for the specific case. [my ckt is actually referenced to -6v, rather than gnd, but it doesn't really matter - just requires a different "ratio"].

regards,
- Dan Michaels
Oricom Technologies
http://www.users.uswest.net/~oricom
===================================

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2000\10\03@104128 by ISO-8859-1?Q?Ruben_J=F6nsson?=

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Date sent:              Tue, 3 Oct 2000 19:30:13 +1000
Send reply to:          staffEraseMEspam.....blackrobotics.com
From:                   Roman Black <EraseMEfastvidspamEZY.NET.AU>
Organization:           BlackRobotics (Fastvid)
Subject:                Re: [EE]:Resistive circuit
To:                     RemoveMEPICLISTEraseMEspamEraseMEMITVMA.MIT.EDU

{Quote hidden}

It is used as a current damper in the feedback winding in a
oscillator for a metal detector. The pot is adjusted until the
oscillator stops and then turned back (about one turn) until the
oscillator starts again. If a metal object is now put into the field
of the inductors the oscillator stops again, indicating metal. The
problem is that the pot has to be set when it is mounted (in a
conveyor belt) because the surrounding metal affects the detector,
otherwise making its sensitivity very poor.

This is, since long, a proven design which works very well and I was
thinking that I could improve its functionality by removing the
trimming in a simple way. Every metaldetector unit contains between 3
and up to 9 oscillator circuits, I think, depending on the width of
the belt, and it is very easy to get a poor sensitivity if the pots
aren't set accordingly.

{Quote hidden}

==============================
Ruben Jvnsson
AB Liros Elektronik
Box 9124, 200 39 Malmv, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
RemoveMErubenTakeThisOuTspamspampp.sbbs.se
==============================

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2000\10\03@144243 by ISO-8859-1?Q?Ruben_J=F6nsson?=

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Forgot to mention that it has to work with both positive and negative
voltages (0V on one terminal and +- 1V oscillating at 150kHz on the
other terminal).

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Subject:                Re: [EE]:Resistive circuit
To:                     spamBeGonePICLISTSTOPspamspamEraseMEMITVMA.MIT.EDU

{Quote hidden}

==============================
Ruben Jvnsson
AB Liros Elektronik
Box 9124, 200 39 Malmv, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
EraseMErubenspamEraseMEpp.sbbs.se
==============================

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2000\10\03@144816 by David VanHorn

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At 08:46 AM 10/3/00 +0200, you wrote:
>Forgot to mention that it has to work with both positive and negative
>voltages (0V on one terminal and +- 1V oscillating at 150kHz on the
>other terminal).

I'm starting to like the motor driven pot.
You could maybe use an LDR optocoupler.

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2000\10\03@150922 by staff

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Ruben Jönsson wrote:
>
> Forgot to mention that it has to work with both positive and negative
> voltages (0V on one terminal and +- 1V oscillating at 150kHz on the
> other terminal).
>
You have some special needs there Ruben, but I have a suggestion.
About 18 years back in an industrial elec subject I learned about
magnetic amplifiers. What you need is something reliable to
adjust the impedance in your tuned circuit. Like a pot. Semiconductors
are all going to be difficult to implement in that circuit.

Imagine a small transformer like an audio coupling transformer etc.
One winding is used to replace the pot. With the other (control) winding
disconnected it will have a low impedance to your 150kHz ac
signal. Now, if you connect DC (not ac) to the control winding
the core of the transformer goes into saturation and it offers a
high impedance to the ac going through the other winding.

So by adjusting the simple DC supply to the transformer you
can adjust the amount of core saturation and use that to control
ac current through the main winding. I have seen huge ones of these
control AC megawatts from quite a small dc input. Was common about
50 years ago before things like thyristors came to save us all.

Now you may have to purchase some small toroids or ferrite cores,
and wind them yourself, because at 150kHz they won't need many
turns. That is probably an advantage. It should be pretty easy
to tune the thing by adding/removing turns. Keep the dc control
winding with more turns to keep current to a more PIC-like
level. You might also need to fiddle with some ac damping
(snubbers, zeners etc) on the control winding but it shouldn't be
too hard. Since you stated it was only milliamps through the pots
you will only need tiny transformers and very small current to
saturate a tiny core like that.

This solution offers excellent industrial reliability and isolation
and once you have done the homework on the first one will be very easy
to wind the rest and has a fairly cheap parts count. If you are
already working with metal detector coils you probably have good
grounding in the magnetic formulas to give you rough turns to
start with.

That's the way I would do it! 1940s magnetic amp theory. :o)
-Roman

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2000\10\03@152522 by jamesnewton

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Very interesting... How does that compare cost, weight, size and loss wise
to the controllers (ECC's?) used in RC cars/boats or medium small robots?
Other than the fact that they have DC supplies and this is best for AC.

Doesn't the core have to be pretty big to handle any reasonable current?
Doesn't the AC power signal get injected into the DC control signal? Where
do you find the equations on making something like that?

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{Original Message removed}

2000\10\03@161511 by Spehro Pefhany

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At 06:08 AM 10/4/00 +1000, you wrote:

>signal. Now, if you connect DC (not ac) to the control winding
>the core of the transformer goes into saturation and it offers a
>high impedance to the ac going through the other winding.

Minor nit, it's the other way around, the inductance of the winding
drops greatly when the core is saturated (it starts to look like an
air core coil).

This is a good suggestion and might well work for the application
the OP has in mind. The current source used to power the control
winding should have enough compliance (+/-) to handle the induced
voltage or have a choke in series with it.

Best regards,
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2000\10\03@181937 by Jinx

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> > > So my question is, does anyone know of a two terminal circuit (plus
> > > power and control) that acts as a variable resistor where the
> > > resistance is set by a voltage level or a variable resistor with
> > > less current through it?

An audio filter I made once had an oscillator driving the control pin
of a 4066 switch. The effective resistance of the switch changed
by varying the mark:space ratio of the oscillator. More ON time =
lower resistance. The 4066 has a book value of 80 ohms, so that's
the minimum you'd get, and it can pass 25mA when fully on. Perhaps
you could try an experiment with a 4066 and a 555. You could
parallel the four switches in the package to lower the resistance and
get the current rating. As the switches are electrically isolated you
might not have a problem with that signal you want to pass. For that
matter, would an opto-transistor do, perhaps using a similar M:S type
system ? I don't think they make opto-FETs do they ?

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2000\10\04@014051 by ISO-8859-1?Q?Ruben_J=F6nsson?=

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Thanks a lot for all your input.

What at first seemed like an easy task with a digitally controlled
pot has now become a bit more complicated.

I am going to have a closer look at the transformer, the h11fx part
and the 4066 approach. Plus a couple of ideas of my own. Initially I
was just going to replace the pot with a resistive circuit but now I
may have too look at other possibilities, to see if I can achieve the
same result by fiddling with other parts of the circuit.

Maby the simplest is to get a 10kohm digitally controlled put and
parallell it with a lower value resistor to get the currents down to
accepted levels.

Date sent:              Wed, 4 Oct 2000 06:08:37 +1000
Send reply to:          spamBeGonestaff@spam@spamspam_OUTblackrobotics.com
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Organization:           BlackRobotics (Fastvid)
Subject:                Re: [EE]:Resistive circuit
To:                     PICLISTEraseMEspamMITVMA.MIT.EDU

{Quote hidden}

I'm afraid not. This is an old design, done long before I started my
electronic career.

>
> That's the way I would do it! 1940s magnetic amp theory. :o)
> -Roman
>
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==============================
Ruben Jvnsson
AB Liros Elektronik
Box 9124, 200 39 Malmv, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
@spam@rubenRemoveMEspamEraseMEpp.sbbs.se
==============================

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2000\10\04@041419 by Alan B. Pearce

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>I don't think they make opto-FETs do they ?

They are made, RS Components have them H11F1 from Isocom Components (RS #111-194 cost GBP2.31 ea in 1-24 lots.)

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2000\10\04@063609 by Jinx

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> > I don't think they make opto-FETs do they ?

> They are made, RS Components have them H11F1 from
> Isocom Components (RS #111-194 cost GBP2.31 ea in 1-24 lots.)

Oh yeah, see them. Should've bothered to stop mid-sentence and
whip out the catalogue. The description says ON = 200 ohms, 60mA
and that "With 99.9% linearity, this device gives distortion-free
control of low-level AC up to 50VRMS and DC analogue signals.
It is ideal for use as a remote variable resistor or an analogue switch"

Sounds like a good candidate for the metal detector tuner. Bit pricey
though, NZ$11.32. But if it does the job, no fuss, no mess.......

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2000\10\04@151053 by Peter L. Peres

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If it does not need to be very linear then use a JFET of appropriate
polarity and Vg0. Steer the gate with a DAC output or rectified PWM.
Because of the high impedance you can use a 'analog memory' with a
capacitor driven only part of the time with a tristateable pin througha
small resistor. Other than that, change your circuit to use a controlled
current source instead of the resistor. These are easier to get by.

hope this helps,

Peter

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2000\10\04@151103 by Peter L. Peres

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>Maybe I'm being dense, but isn't this the definition of a transistor?

Er, yes, if it is a JFET or VFET. BJTs are controlled current sources ...
see my previous posting.

Also, has anyone noticed how hard it is to make a 'controlled resistor'
vs. a controlled current source ? $0.001 parts are hard to replace. Tsk,
tsk.

Peter

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2000\10\04@151115 by Peter L. Peres

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> oscillating +/- 1V

You want a P-JFET. Since your app is industrial you will have no troubles
finding one with low enough Vg0 (to be able to turn it off with 5V). You
want one with Vg0 of about 3V. There will be some parametric distortion of
the signal (which should appear as a higher 2nd harmonic in the output
imho) but it should work ok in your application. If for any reason this
makes the oscillator unstable then you have to change the feedback path to
attenuate higher frequencies more. But this is not likely to be needed.

Peter

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2000\10\04@151120 by Peter L. Peres

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>how does it compare

Magamps do not really compare with normal controllers, they are not very
efficient, they are heavy, their amplification factor is lousy, they have
every kind of loss imaginable, including eddy current, hysterezis and
resistive, BUT you can build a magamp the size of a small truck without
any problems (except you need to own the power company to provide the
power supplies), it will work forever (I mean it) and it will withstand
abuse that no other kind of amplifier will withstand. By this I mean
thermal overload, g's, radiation, freezing, anything you can think of. As
long as you do not reach the Curie point of the core and the insulation
does not break down, the thing will work. Even at 400 degrees C ambient
(continuous).

Magamps are still used in some types of equipment (they used to be popular
in color TVs up to 10 years ago, as pincushion correction modulators). The
only type of magamp with a relatively bright future is the Josephson type
junction or loop (superconductive type). If material science will make
some progress then one might see these in everyday life.

And last: it is possible to change the Q of a core (like in a metal
detector) using two small magnets. One is fixed and one can be moved vs.
the other. This gives a settable magnetic field which can be chosen such
that it saturates the core just enough to allow an oscillator to chug
along until a metal piece appears and stops it.

Just curious: what is on the conveyor belt ? Fish, construction materials
? ;-)

Peter

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2000\10\04@151125 by Peter L. Peres

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>matter, would an opto-transistor do, perhaps using a similar M:S type
>system ? I don't think they make opto-FETs do they ?

They do, but they are hard to buy off the shelf. If you really want to you
can use the LED and LDR in a small black plastic tube scheme. This is
surprisingly linear (resistance to LED current is better than 3% !). I
once made a 4-20 mA galvanically insulated potentiometer like this. Works
a treat.

Peter

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2000\10\04@165107 by iso-8859-1?Q?Ruben_J=F6nsson?=

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Thanks for Your much apreciated input.

----- Original Message ----- From: Peter L. Peres <EraseMEplpspam@spam@ACTCOM.CO.IL>
To: <@spam@PICLISTspam_OUTspam.....MITVMA.MIT.EDU>
Sent: Wednesday, October 04, 2000 9:24 PM
Subject: Re: [EE]:Resistive circuit


>
> Just curious: what is on the conveyor belt ? Fish, construction materials
> ? ;-)
>
Mostly recycled plastic parts going into a grinder. The grinder doesn't like metal objects.

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2000\10\04@170117 by Oliver Broad

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Approaching this from a different angle: what you're really trying to vary
is gain I believe, if the circuit is built around an opamp you could replace
it with an OTA and control the transconductance.

{Original Message removed}

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