Post by thread:[EE]:Resistive circuit

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_OUTrubenTakeThisOuTpp.sbbs.se ============================== -- http://www.piclist.com hint: To leave the PICList .....piclist-unsubscribe-requestKILLspam@spam@mitvma.mit.edu

Ruben Jönsson wrote: {Quote hidden}> > 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. > 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 -- http://www.piclist.com hint: To leave the PICList .....piclist-unsubscribe-requestKILLspam.....mitvma.mit.edu

Try an FET these are better suited to the job Regards Art ----- Original Message ----- From: David VanHorn <EraseMEdvanhornspam_OUTTakeThisOuTCEDAR.NET> To: <PICLISTspam_OUTMITVMA.MIT.EDU> Sent: Tuesday, October 03, 2000 7:34 AM Subject: Re: [EE]:Resistive circuit {Quote hidden}> > > >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? > > -- > http://www.piclist.com hint: To leave the PICList > @spam@piclist-unsubscribe-requestKILLspammitvma.mit.edu > > -- http://www.piclist.com hint: To leave the PICList KILLspampiclist-unsubscribe-requestKILLspammitvma.mit.edu

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 _____________________________ What can one man* do? Help the hungry at no cost to yourself! at http://www.thehungersite.com/ (* - or woman, child or internet enabled intelligent entity :-)) From other worlds: http://www.changingourworld.com http://www.easttimor.com http://www.sudan.com From: Ruben Jönsson <RemoveMErubenjTakeThisOuTPP.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. -- http://www.piclist.com hint: To leave the PICList spamBeGonepiclist-unsubscribe-requestspamBeGonemitvma.mit.edu

Ruben Jönsson wrote: {Quote hidden}>> >> 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. 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 =================================== -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu

Date sent: Tue, 3 Oct 2000 19:30:13 +1000 Send reply to: staffEraseME.....blackrobotics.com From: Roman Black <EraseMEfastvidEZY.NET.AU> Organization: BlackRobotics (Fastvid) Subject: Re: [EE]:Resistive circuit To: RemoveMEPICLISTEraseMEEraseMEMITVMA.MIT.EDU {Quote hidden}> Ruben Jvnsson wrote: > > > > 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. > > > > > 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. 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}> > 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 > > -- > http://www.piclist.com hint: To leave the PICList > RemoveMEpiclist-unsubscribe-requestspam_OUTKILLspammitvma.mit.edu > ============================== Ruben Jvnsson AB Liros Elektronik Box 9124, 200 39 Malmv, Sweden TEL INT +46 40142078 FAX INT +46 40947388 RemoveMErubenTakeThisOuTspampp.sbbs.se ============================== -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestspamspamBeGonemitvma.mit.edu

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). Date sent: Tue, 3 Oct 2000 01:34:29 -0500 Send reply to: pic microcontroller discussion list <RemoveMEPICLISTKILLspamMITVMA.MIT.EDU> From: David VanHorn <dvanhornSTOPspamspam_OUTCEDAR.NET> Subject: Re: [EE]:Resistive circuit To: spamBeGonePICLISTSTOPspamEraseMEMITVMA.MIT.EDU {Quote hidden}> > > >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? > > -- > http://www.piclist.com hint: To leave the PICList > KILLspampiclist-unsubscribe-requestspamBeGonemitvma.mit.edu > ============================== Ruben Jvnsson AB Liros Elektronik Box 9124, 200 39 Malmv, Sweden TEL INT +46 40142078 FAX INT +46 40947388 EraseMErubenEraseMEpp.sbbs.se ============================== -- http://www.piclist.com hint: To leave the PICList @spam@piclist-unsubscribe-request@spam@spam_OUTmitvma.mit.edu

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 -- http://www.piclist.com hint: To leave the PICList .....piclist-unsubscribe-requestspam_OUTmitvma.mit.edu

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, =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Spehro Pefhany --"it's the network..." "The Journey is the reward" TakeThisOuTspeffKILLspamspaminterlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= -- http://www.piclist.com hint: To leave the PICList .....piclist-unsubscribe-requestRemoveMEmitvma.mit.edu

> > > 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 ? -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestspamBeGonemitvma.mit.edu

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@spam_OUTblackrobotics.com From: Roman Black <TakeThisOuTfastvidspamEZY.NET.AU> Organization: BlackRobotics (Fastvid) Subject: Re: [EE]:Resistive circuit To: PICLISTEraseMEMITVMA.MIT.EDU {Quote hidden}> Ruben Jvnsson 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. 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 > > -- > http://www.piclist.com hint: To leave the PICList > RemoveMEpiclist-unsubscribe-requestEraseMEspam_OUTmitvma.mit.edu > ============================== Ruben Jvnsson AB Liros Elektronik Box 9124, 200 39 Malmv, Sweden TEL INT +46 40142078 FAX INT +46 40947388 @spam@rubenRemoveMEEraseMEpp.sbbs.se ============================== -- http://www.piclist.com hint: PICList Posts must start with ONE topic: "[PIC]:" PIC only "[EE]:" engineering "[OT]:" off topic "[AD]:" ad's

> > 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....... -- http://www.piclist.com hint: PICList Posts must start with ONE topic: "[PIC]:" PIC only "[EE]:" engineering "[OT]:" off topic "[AD]:" ad's

>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 -- http://www.piclist.com hint: PICList Posts must start with ONE topic: "[PIC]:" PIC only "[EE]:" engineering "[OT]:" off topic "[AD]:" ad's