Wiring a potentiometer correctly

Discussion in 'General Electronics Chat' started by Will777, Sep 12, 2010.

  1. Will777

    Thread Starter Member

    Sep 12, 2010
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    Hi all,

    I am building a field strength meter, at this stage on breadboard but later on stripboard.

    This is the circuit diagram:

    [​IMG]

    I am struggling to understand the schematic regarding the 100K pot on the left marked "sensitivity".

    As I understand it the wire coming from the base terminal of the BC547 transistor needs to be connected to terminal A of the pot, then the terminal B of the pot is to be connected to the 47K resistor. What about the wiper? I see with this pot the wiper is pointing back into the pot? Does this mean both terminal B and the wiper needs to be connected to the 47K resistor at the top?

    Thank you for any help with this, I am stuck on this issue.
     
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  2. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
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    yes. You just short the wiper to the "B" terminal at the potentiometer, as shown below.

    [​IMG]
     
  3. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
    92
    Yes.

    The wiper will short across the top end of the of the potentiometer so that it works as a variable resistor (as opposed to a variable voltage divider). The reason to connect the top terminal of the potnetiometer even though that top end will be bypassed by the wiper includes the following.

    The wiper will be more reliable and less noisy because if it ever breaks connection due to wear or damage it can tend to arc and create more damage. With the parallel connection of some limited resistance that arcing becomes unlikely. For noise there is a big difference between the wiper bumping open and opening that whole circuit line, compared to falling back only to the full potentiometer resistance.

    Also keep in mind that components are more secure and reliable when all of their pins are soldered to the board. If you want that top pin soldered to the board why bother creating a disconnected pad for it when there are the advantages already described to using it as a fallback value for wiper deadspotting and bouncing.
     
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  4. marshallf3

    Well-Known Member

    Jul 26, 2010
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    So you found that circuit too? I took it as a base design and modified it into a tester for the transmitters built into automotive keyfobs.

    Had to change a few things to get the frequency range up and I set my input up as a switchable tuned tank circuit too, European cars are ~ 434 MHz, North American & Japanese ~ 315 MHz.

    It worked out very well.

    What I really liked was the way they designed the detection circuit.
    http://www.talkingelectronics.com/projects/FieldStrengthMeterMkl/FieldStrengthl.html
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    I suggest that you skip the breadboard. They really do not work well with high-frequency circuits, as they simply have far too many parasitics (inductance and capacitance) to work well at anything approaching RF.

    By breadboard, I am referring to something like this:

    [​IMG]

    "Dead-bug" aka free-form style works very well for RF prototyping. This method gives you a big ground plane to work with. You can "dead bug" with a double-sided board, and have both power and ground planes for best performance.
    Keep your wiring as short as reasonably possible. I'm talking very short.

    They specified 3mm for the diameter of the coils, however they did not say whether it was the internal, external, or average diameter, which is unfortunate. (eta: it's 3mm internal, 3.4mm external; so 0.2mm wire which is AWG-32)

    A number 32 drill is 0.120"; 3mm is ~0.118", and would make a good form for winding the coils. You might also use a 1/8" drill; it measures .125" of course.
    Try using AWG 22 or AWG 24 magnet wire to wind the coil. (eta:nope, use AWG32. Radio Shack's 3-pack of magnet wire only goes to AWG-30; you'd have to wind the coils a bit differently to compensate) You can tune them to a higher frequency (lower nH value) by spreading the coils slightly. A little bit at a time does it. If you distort the coil, you will radically change its' properties.
    [eta] After printing out their finished project and measuring it with a caliper, I've determined that they used AWG 32 magnet wire. Seems I was off by quite a bit.

    Best as I can measure/calculate, the 15T inductor is about 161nH, and the 13T inductor is actually 162nH, due to it's shorter length! Anyway, that should be a decent starting place for experimentation.

    Note carefully the orientation of the two inductors on the finished project. You should make your prototype match that layout as closely as possible. Inductors' magnetic fields cause interactions between the inductors. Placement can be quite critical.

    Note that the "23A" battery is typically called an "A23" in the States. It's a 12v battery that measures 10mm by 29mm.
     
    Last edited: Sep 12, 2010
  6. Will777

    Thread Starter Member

    Sep 12, 2010
    48
    1
    Wow, thank you so much to all the friendly advice and help guys.

    This is my second project that I am doing on stripboard, sourcing the components myself - a bit daunting - kits are much easier! But I am learning new things every day - and the electronics bug has bitten me hard!

    SgtWookie:
    I have given up the idea of prototyping on the breadboard - thanks. I will move on to stripboard and will definitely note position of the inductors. They say elsewhere in the design that the coil is 3mm diameter and that the diameter of the wire used is not important - I used 1mm enamelled wire for mine (sorry, metric measurements over here in South Africa :) ) I am very interested in RF electronics, I've built a few FM transmitter kits and need this field strength meter to help me with a pulsed FM tracking transmitter I did. BTW, the web page for this design is http://www.talkingelectronics.com/projects/FieldStrengthMeterMkl/FieldStrengthl.html

    I found the 12V battery - they call it a "lighter/camera" battery over here - made a holder out of a paperclip.

    marshallf3:
    Yeah, I've been looking for a long time for a good design for a field strength meter - this one looked good. They have quite a few great circuits on there - I intend building a few of the tracking transmitters next.

    Potato Pudding:
    Thanks for a great explanation - makes a lot of sense.

    eblc1388:
    Pictures are worth a thousand words! Great, thanks.

    I suspect the fact that I have LONG leads on all components and that I used breadboard contributes to my problems currently.

    One last question - they suggest you use paperclips bent to fit the hole of a multimeter jack socket to connect the circuit to a multimeter - I have single-core leads with banana plugs - would this affect the working at all? As SgtWookie said with RF electronics leads need to be short?
     
  7. eblc1388

    Senior Member

    Nov 28, 2008
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    They are fine, even better than paper clips.

    Just keep the lead reasonably short and everything will be fine.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Your leads should be as short as possible.

    1mm wire is too large in diameter. You need 0.2mm magnet wire if you are trying to replicate the original circuit exactly.

    However, to compensate, you can use a #32, 1/8" or 3mm drill with your 1mm magnet wire, and wind on 12 turns in a single layer as closely-spaced as possible, instead of 13 or 15 turns. That should give you about 165nH; separating the coils VERY slightly afterwards will decrease the inductance.

    The coils will be about 1/2" long, or 12.7mm after winding.

    I typically use an Xacto knife to separate the coils if necessary after soldering.

    Note the direction of wind of the original coils, as well as their placement (click link:) http://www.talkingelectronics.com/p...l/images/Field_Strength_Meter_Mk1_Side-on.jpg

    Have a look at this image: http://www.token.com.tw/inductor/image/tcac-2.jpg

    Note the different winding direction, and different termination styles. You need to match the wind direction and termination style used in the project image. It all makes a difference.
     
    Last edited: Sep 12, 2010
  9. Will777

    Thread Starter Member

    Sep 12, 2010
    48
    1
    Great - I will shorten my leads as much as possible
     
  10. Will777

    Thread Starter Member

    Sep 12, 2010
    48
    1
    Ok, great.

    I have taken the breadboard layout apart and will start the stripboard layout tonight, I think I do have 0.2mm magnet wire so I will redo the coils.

    I am learning a lot about inductance and coils - this is definitely a very interesting project.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    I updated my post while you were replying. Please review it, as some important information for you to consider has been added.
     
  12. Will777

    Thread Starter Member

    Sep 12, 2010
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    Noted, I will take that into consideration when redoing the coils - thanks
     
  13. marshallf3

    Well-Known Member

    Jul 26, 2010
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    As a general field strength meter it isn't really a finely tuned circuit, just a broadband RF amp and detector so the coils don't have to be exact for it to work. Also, as I'm sure one of the above articles pointed out (haven't read them yet) you'll want to minimize mutual inductance between the two coils. I chose to mount one at a right angle to the other and had no problems.
     
  14. Will777

    Thread Starter Member

    Sep 12, 2010
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    In terms of the point of keeping leads as short as possible - I have a question regarding construction techniques - esp stripboard (with the horizontal tracks) vs normal perfboard (with just the holes and soldering spots).

    Are any of these preferable for RF circuits - and do the long horizontal tracks of stripboard also influence the circuit as a long lead on a component might do?

    I've also noted that some articles talk about careful soldering technique when building an RF circuit - what I understand is to not use to much solder?
     
  15. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Sad answer is it depends on where the copper strips are, you're probably best using board with just holes and no copper strips or even copper rings around the holes.

    Laying out and constructing RF circuits is a science in itself. Every wire and component becomes a potential inductor, an antenna and/or a capacitance to everything else.

    The schematic, as drawn, is already a pretty good basis for the parts layout but as in the pictures keep one of the coils at a right angle to the other to minimize stray inductive coupling.
     
  16. Will777

    Thread Starter Member

    Sep 12, 2010
    48
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    Cool, thanks - I have some perf-board with no tracks - going to attempt to build it on that
     
  17. SgtWookie

    Expert

    Jul 17, 2007
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    Remember me mentioning parasitic inductance and capacitance earlier?

    Well, even a straight piece of wire has inductance.

    You can use this calculator to figure out how much for a given wire diameter: http://www.consultrsr.com/resources/eis/induct5.htm

    It's an eye-opener as to how quickly it adds up. A 1" length of AWG 32 wire has almost 28nH inductance. I measured a short piece of paperclip on a very expensive material analyzer several years back; it had 15nH inductance at 15MHz. This turned out to be critical knowledge for testing, as an engineer was using this piece of paper clip to zero the instrument, which resulted in an offset in the readings of wound inductors.

    My point here is that the "dead bug" construction technique on a double-sided copper-clad results in a ground plane and a power plane; and an infinite plane has virtually zero inductance. This is important for reduction of unwanted parasitics.
     
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  18. Will777

    Thread Starter Member

    Sep 12, 2010
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    SgtWookie,

    An eye-opener indeed! I realize now how many mistakes I've made with RF circuits till now, mistakes that came out when I translated circuit diagrams into stripboard layouts.

    Great calculator - thx.
     
  19. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    After you have the inductors placed AND set as you want them. I would suggest putting a small, very small piece of paper(Tissue), rolled into a pin shape inside the inductor and then using a candle drop some hot wax onto the coil and paper form. This will hold them securely and still allow you to further tweak them if needed.

    (once things are set and ready to be enclosed, wires and other 'stuff' can intrude on the circuit components(coil) and bend or deform them. You won't see it either as the enclosure prevents it.)

    Just a hack from lessons learned the hard way. Good luck with the project.



    Kermit
     
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  20. SgtWookie

    Expert

    Jul 17, 2007
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    And now you know "the rest of the story". :)

    You should try working in the GHz range sometimes. It seems more like magic than science, until you really start understanding things.
     
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