# Function of the Coil - Any Differences in Quality?



## South Pender (Jul 2, 2008)

As I try to learn just how a quartz watch works, I'm puzzled by the function of the copper cylindrical coil you see in all movements. My first question, then, is just what is its function? I assume it has something to do with taking the electrical energy from the battery and transmitting it to the movement, but I'm very hazy about this. Second, is the coil a component that can affect accuracy--in the sense that a higher-quality coil could be expected to be found in the better HEQ watches? Oh, one last question: Is the tuning-fork quartz crystal found near the coil? Is the crystal encased in a cylindrical enclosure?


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## M4tt (Jan 18, 2007)

South Pender said:


> As I try to learn just how a quartz watch works, I'm puzzled by the function of the copper cylindrical coil you see in all movements. My first question, then, is just what is its function? I assume it has something to do with taking the electrical energy from the battery and transmitting it to the movement, but I'm very hazy about this. Second, is the coil a component that can affect accuracy--in the sense that a higher-quality coil could be expected to be found in the better HEQ watches? Oh, one last question: Is the tuning-fork quartz crystal found near the coil? Is the crystal encased in a cylindrical enclosure?


As I understand it, the coil drives the stepper motor by inducing an alternating magnetic field in the stepper motor. As such the quality of the coil might effect the smartness with which the drive train moves but, assuming it produces enough torque to move the train, it isn't going to effect the accuracy.

As in the SD the timekeeping is done by the crystal and divided down (by twos) from the operating frequency to a usable pulse to run the drive train. By the time it gets to the coil and the stepper motor the timebase is established.

The little aluminium cylinder does indeed contain a small quartz tuning fork in most, but not all watches. This resonant body may come in other shapes such as lenticular (lentil like). However, the tuning fork with a 32k frequency is overwhelmingly the most common. Only the earliest quartz do not also come with a mosfet (the little black cube) divider circuit which divides the frequency of the quartz down by repeated division by two. It is entertaining and educational to work your way through a quartz watch doing the necessary maths for the divider circuit (and the teeth of the gear chain) to start at a quartz signal of 32,768Hz (thats two to the fifteen Hz) and end up with a tick every second.


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## Bruce Reding (May 5, 2005)

M4tt said:


> This resonant body may come in other shapes such as lenticular (lentil like). However, the tuning fork with a 32k frequency is overwhelmingly the most common.


To add to this comment, I believe that it's true to say that _all_ crystals in movements that are in production today are tuning forks.


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## M4tt (Jan 18, 2007)

Too true, and Concorde doesn't fly to New York any more. What happened to the future? We were supposed to be living on the Moon by now!


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## South Pender (Jul 2, 2008)

Thanks, guys. So the coil's function is with the stepper motor. I had thought (obviously mistakenly) that it perhaps provided the necessary electrical charge to get the quartz crystal resonating. Does this come instead from the battery itself? Or from the battery via the IC? Can someone explain that part of QC operation?


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## shandy (Feb 13, 2008)

You may find this of interest!





I used to watch Tim and Rex's TV shows all the time when I lived in the UK, part boffins, part nutty proffesors! but they explain things really well!


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## M4tt (Jan 18, 2007)

The Mosfet and the IC are functionally the same thing. The function is to divide down 32,768 by divisions of two in fifteen (or more) steps:
*
4,194,304 *(4mhz - try dividing by 128!)*
...
f2,4? *(but what is the actual frequency?)*
...
f8192?
...
32,768* (A second, remember)
16,384 (GP, FL, JLC)
8,192
4,096
2,048
1,024
512
(looking familiar?)
256
128
64
32
16
8
4
2 (9F)
1 ... tick...tick ...tick

It's a simple job, but someone's got to do it. As for those that use a tuning fork and index wheel, Well, you can do the maths for the beta 21 (Omega 1300 or Omega f8192 to make it easy) or the Bulova accuquartz (anyone know the quartz frequency?) what about the slightly more challenging Omega Marine Chronometer f2.4?

There is no theoretical reason (but usually a few practical ones) that the division above cannot be achieved mechanically - at least some of it is witht the beta 21 and the accuquartz. It doesn't _have _to be base two either this is just convenient and conventional.

So what about the two odd ones out - any offers?


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## South Pender (Jul 2, 2008)

M4tt said:


> The Mosfet and the IC are functionally the same thing. The function is to divide down 32,768 by divisions of two in fifteen (or more) steps:
> 
> *4,194,304 *(4mhz - try dividing by 128!)
> *...*
> ...


Very helpful, M4tt. I've seen this terminology before about "dividing down" the resonance, but really don't understand what that means. As I understand it, the crystal resonates at 32,768 "beats" (or times) each second. The IC then "divides this down" in some way. So let's start with the resonating crystal. The IC somehow is able to detect a one-second pulse and send the signal to the stepper motor to advance the second hand by one division. Can you describe the events that take place between the resonating crystal and the advancing of the second hand?


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## psycofant (Aug 9, 2008)

> As I understand it, the coil drives the stepper motor by inducing an alternating magnetic field in the stepper motor. As such the quality of the coil might effect the smartness with which the drive train moves but, assuming it produces enough torque to move the train, it isn't going to effect the accuracy.


The coil _looks like_ an inductor, but beyond that, who can say? Have you seen a circuit diagram of a quartz movement? If so, please provide a link as I'd like to see one too. I mean no disrespect here, but (in the interest of discussion) I don't know how you can say that the coil drives the stepper motor.



> Only the earliest quartz do not also come with a mosfet (the little black cube) divider circuit which divides the frequency of the quartz down by repeated division by two. It is entertaining and educational to work your way through a quartz watch doing the necessary maths for the divider circuit (and the teeth of the gear chain) to start at a quartz signal of 32,768Hz (thats two to the fifteen Hz) and end up with a tick every second.


A MOSFET is a single transistor; the little black cube is an integrated circuit which contains many transistors.


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## psycofant (Aug 9, 2008)

> Very helpful, M4tt. I've seen this terminology before about "dividing down" the resonance, but really don't understand what that means. As I understand it, the crystal resonates at 32,768 "beats" (or times) each second.


Yes, using a power of 2 makes the division easy. The way this is done is that the IC has a register (counter) which counts each beat or pulse from the crystal. This count would be stored digitally in binary (1s and 0s, _i.e._, base 2), which makes dividing by any power of 2 extremely easy because you need only look at the most significant bit.

As an analogy/example, let's say I have a register that counts in decimal and can hold any number from 0 to 999, that is, it is three-digits long. If this counter keeps going up, for me to detect every thousandth tick, all I have to do is to watch the topmost digit and see when it changes from 9 back to 0 because this is when the 999 rolls over back to 000.

Going back to the 32k clock, then what we need is a 15-bit register and you need only to look at the most significant bit and detect when it changes from 1 back to 0. The technical term is that you need a falling-edge detector on the most significant bit (MSB). This will give you a pulse a second which is then sent to the stepper motor driver.



> The IC then "divides this down" in some way. So let's start with the resonating crystal. The IC somehow is able to detect a one-second pulse and send the signal to the stepper motor to advance the second hand by one division. Can you describe the events that take place between the resonating crystal and the advancing of the second hand?


"Block diagram":

32k clock --> counter --> (falling-edge dector on MSB) --> (stepper motor driver circuitry) --> stepper motor --> seconds hand --> gear train to other hands


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## Eeeb (Jul 12, 2007)

South Pender said:


> Very helpful, M4tt. I've seen this terminology before about "dividing down" the resonance, but really don't understand what that means. As I understand it, the crystal resonates at 32,768 "beats" (or times) each second. The IC then "divides this down" in some way. So let's start with the resonating crystal. The IC somehow is able to detect a one-second pulse and send the signal to the stepper motor to advance the second hand by one division. Can you describe the events that take place between the resonating crystal and the advancing of the second hand?


You create a counter (a register to use the h/w term) into which you add 1 every oscillation of the crystal. It is a binary counter. Computers are full of them.

0000000000000001 after the first

0000000000000010 after the second

0000000000000011 after the third

0000000000000100 after the forth ... ad. nausium...

Another part of the hard ware examines a certain bit... in this example, let us examine the very first bit in this register... it is now a "0"

When it becomes a "1" you trigger the coil so that it will advance the second hand.

It will become a "1" once the crystal has oscillated 32768 times. Since the crystal is cut to oscillate 32768 times per second, it becomes a "1" once a second has passed.

After you have triggered the coil so the gear train advances 1 second, you zero the digit and start waiting for it to turn into a "1" again.

This should give you an idea how it works... I hope I made it clear. If not, ask again!


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## psycofant (Aug 9, 2008)

Looks like Eeeb and I said the same thing but using different approaches. 

Eeeb, I think you have one too many 0s in your register. For a 32k clock, it should be 15 bits in length, not 16.


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## Eeeb (Jul 12, 2007)

Technical discussion of no relevance to others:

Read the discussion of how the circuit works... it triggers on the bit becoming 1 and then zeros it... I thought it a simpler explanation. So it needs the 'extra' bit.


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## psycofant (Aug 9, 2008)

Ah, ok. I get it now. We're not quite talking about the same scheme then. I think mine's simpler, though. ;-)


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## Eeeb (Jul 12, 2007)

But mine is more easily explained! Yours is the way it would really be done.


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## Pawl_Buster (Mar 12, 2007)

psycofant said:


> The coil _looks like_ an inductor, but beyond that, who can say? Have you seen a circuit diagram of a quartz movement? If so, please provide a link as I'd like to see one too. I mean no disrespect here, but (in the interest of discussion) I don't know how you can say that the coil drives the stepper motor.
> 
> A MOSFET is a single transistor; the little black cube is an integrated circuit which contains many transistors.


By definition, a coil is an inductor. When any inductor has a voltage applied to it then removed, it induces a current in the circuit. It doesn't matter if it is coils in a transformer, a generator, an alternator, a motor or just some wire wrapped around a nail; the principle is the same...only the application changes.

You are right about the MOSFET being a single transistor but often it is simply the final drive stage of a more complicated integrated circuit. For simplicity sake many of these integrated circuits are just called MOSFETs


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## M4tt (Jan 18, 2007)

As I have said before, there are 10 sorts of people,those who understand base two and those who don't.

Now, can anyone tell me how it might be managed in an analogue circuit or even in base n?

Actually, that's just being a smartarse. What I am really interested in is how 
the beta 21 and the 2.4 divide down. I would sit down and work it out but I know someone can just tell me and I am still in a world of half unpacked boxes and a beloved who really expects...


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## psycofant (Aug 9, 2008)

> Now, can anyone tell me how it might be managed in an analogue circuit or even in base n?


One possible (not necessarily the most practical) would be to take the count in the register, feed it into a digital-to-analog convertor (DAC) and then the analog output voltage is then fed to a comparator with a reference voltage at its other input. When the analog voltage from the DAC crosses the reference voltage, that gives you the trigger signal to fire the stepper once to get the seconds hand to its next tick.

Base n isn't really any more complicated than binary for this type of application; it's just that binary is the most convenient "signal system" to work with.



> Actually, that's just being a smartarse. What I am really interested in is how the beta 21 and the 2.4 divide down. I would sit down and work it out but I know someone can just tell me and I am still in a world of half unpacked boxes and a beloved who really expects...


What are their frequencies? I don't know anything about these movements, but from the wording in your question, I take it that they are not powers of 2. Using a frequency of 2^n Hz is still the most convenient because it's really easy to do binary division or look at the bit #n of a counter register as I explained in an earlier post. This is why 32 kHz (actually 32,768 Hz) is as common as it is.

You can always take the value in the counter and compare it to a fixed value and every time it crosses that value, you generate an output pulse (seconds tick) and reset the counter to 0. This would work for any quartz frequency.

Actually, if you consider the 196k clock rates (actually 196,608 Hz) used in some of the HEQ movements, it's clearly not a power of two. (2^16 + 2^17 = 196k) One could ask why they do not just use 256k?


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## psycofant (Aug 9, 2008)

Alpha-Getty said:


> By definition, a coil is an inductor. When any inductor has a voltage applied to it then removed, it induces a current in the circuit. It doesn't matter if it is coils in a transformer, a generator, an alternator, a motor or just some wire wrapped around a nail; the principle is the same...only the application changes.


I found this: http://www.explainthatstuff.com/quartzclockwatch.html

According to the picture, about two-thirds of the way down the page, the coil thing is identified as being the stepper motor. So perhaps our answer is that it's part of the stepper motor's stator.


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## ppaulusz (Feb 11, 2006)

M4tt said:


> *...*
> *f2,4? *(but what is the actual frequency?)...


_*2,359,296Hz*_


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## ppaulusz (Feb 11, 2006)

ppaulusz said:


> *2,359,296hz*


2^21 + 2^18 = 2,097,152 + 262,144 = 2,359,296


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## Pawl_Buster (Mar 12, 2007)

psycofant said:


> I found this: http://www.explainthatstuff.com/quartzclockwatch.html
> 
> According to the picture, about two-thirds of the way down the page, the coil thing is identified as being the stepper motor. So perhaps our answer is that it's part of the stepper motor's stator.


Ultimately all the electronics are part of the motor but in this case, the coil is one of two primary parts of the stepper. The coil, when activated, creates a magnetic fiel which attracts or pushed an impulse wheel that turns a few degrees. The impulse wheel is connected to the gears which eventually drive the hands.
The quartz crystal vibrates at 32,768 times per second and when this signal is sent through the integrated circuitry, it is divided down to one pulse per second. This one pulse is used to trigger the MOSFET which applies a voltage to the coil which in turn generates the magnetic field which pushes or pulse the impulse wheel.
The coil is just a coil but when put together and in the right proximity to an impulse wheel, it becomes part of a stepper motor :-!


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## M4tt (Jan 18, 2007)

ppaulusz said:


> 2^21 + 2^18 = 2,097,152 + 262,144 = 2,359,296


Thanks gentlemen, that's one curiosity answered.


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## M4tt (Jan 18, 2007)

Thanks ppaulez supplies the frequency elsewhere in this thread now - but it is also answered too. 

Just got to comment on the fantastic quality of the replies here today.


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## ron (Dec 30, 2006)

*Re: To M4tt : Accuquartz calculations*

Hi M4tt, On the great tunning fork site of Rob http://members.iinet.net.au/~fotoplot/acc.htm you can find the calculations for that odd tunning fork movt.with quartz.See technical info serie's 224.
Love the tunning fork cal's. a lot, and hope to find a 720 oneday;-)

greetings Ronald


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## M4tt (Jan 18, 2007)

*Re: To M4tt : Accuquartz calculations*

That's brilliant Ron, thanks for finding that - be warned that the f720 is a bit of a beast and upsets cats, dog, and small children with its high pitched wail...


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## canipus (Sep 7, 2008)

Alpha-Getty said:


> Ultimately all the electronics are part of the motor but in this case, the coil is one of two primary parts of the stepper. The coil, when activated, creates a magnetic fiel which attracts or pushed an impulse wheel that turns a few degrees. The impulse wheel is connected to the gears which eventually drive the hands.
> 
> The coil is just a coil but when put together and in the right proximity to an impulse wheel, it becomes part of a stepper motor :-!


Where did you get that from?

The coil is part of the driver circuit for the stepper motor. The stepper motor is self contained with its own field winding and miniature permanent magnet rotor. The magnetic field from the outside coil winding has nothing to do with directly turning a wheel or any mechanical motion. The magnetic field that creates rotation in the stepper motor is the field from the stepper's winding acting against its permanent polarised magnet. What the outside coil/inductor does is create a bipolar energy pulse or to put it more accurately it acts as an energy translator between the battery and the field current requirement of the stepper motor driven synchronously by the output of the clock driver silicon. The coil is bipolar driven by the output of the divider and every time the output hits binary zero, a reverse electro motive force (EMF) is built up across the inductor due to the collapse of its magnetic field. This reverse EMF is bipolar because the drive to the inductor and hence the magnetic field is opposite polarity on each successive binary zero. But to clarify, the magnetic field of this inductor plays no direct part in pushing or pulling the rotation of the motor. What happens is the back EMF creates a flow of bidirectional current (bipolar EMF), through the stepper winding IN THE STEPPER MOTOR. The resultant magnetic field from this flow of current in the stepper winding then creates a force that is opposite to the permanent magnet field that is also inside the motor. The mechanical (magnetic), force is thus created that causes the rotor to rotate 180 degrees on each pulse (360 degrees every two pulses from the clock divider output).

All the outside coil is doing is acting as a bridge or translator to convert the battery voltage / stepper pulse current via the coils' back EMF. The current availability from the pulse coming out of the clock divider is miniscule and by itself is not sufficient to drive current through the stepper field winding. With no current in the stepper field winding not enough of a magnetic field can be created to effect rotation. By passing the clock pulse into the coil inductance, the resultant back EMF (stored reservoir energy in the inductor), effectively multiplies up the energy that is translated into sufficient current to drive the rotor field winding.

The permanent magnetic in the stepper is very small and by design does not need to be very strong. It is very easy to demagnetise this magnet by placing the quartz movement close to another source of magnetism. If the stepper's little magnet is demagnetised, the stepper motor is scrap and so is your quartz movement and hence your watch. Hence do not take your quartz watch close to hi-fi / home theatre speakers, medical magnetic resonance CAT scanners or other magnetic areas. Mechanical watches can also have their balance wheels impacted by magnets that causes the wtch to run fast several minutes per hour. However, a mechanical watch can be taken apart (at a price), the wheels demagnetised, then re-assembled. problem fixed. Not so with your quartz movement. Ruin the magnet in your stepper motor and your quartz movement is scrap.


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## fry (May 11, 2008)

Impressive.. and good reading.


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## Pawl_Buster (Mar 12, 2007)

canipus said:


> Where did you get that from?
> 
> The coil is part of the driver circuit for the stepper motor. The stepper motor is self contained with its own field winding and miniature permanent magnet rotor. The magnetic field from the outside coil winding has nothing to do with directly turning a wheel or any mechanical motion. The magnetic field that creates rotation in the stepper motor is the field from the stepper's winding acting against its permanent polarised magnet. What the outside coil/inductor does is create a bipolar energy pulse or to put it more accurately it acts as an energy translator between the battery and the field current requirement of the stepper motor driven synchronously by the output of the clock driver silicon. The coil is bipolar driven by the output of the divider and every time the output hits binary zero, a reverse electro motive force (EMF) is built up across the inductor due to the collapse of its magnetic field. This reverse EMF is bipolar because the drive to the inductor and hence the magnetic field is opposite polarity on each successive binary zero. But to clarify, the magnetic field of this inductor plays no direct part in pushing or pulling the rotation of the motor. What happens is the back EMF creates a flow of bidirectional current (bipolar EMF), through the stepper winding IN THE STEPPER MOTOR. The resultant magnetic field from this flow of current in the stepper winding then creates a force that is opposite to the permanent magnet field that is also inside the motor. The mechanical (magnetic), force is thus created that causes the rotor to rotate 180 degrees on each pulse (360 degrees every two pulses from the clock divider output).
> 
> ...


You are absolutely correct...I was just trying to give a simple explanation.

I have to differ with you on one point though, there are some simple quartz clock motors that are a simple coil and stepper wheel. Since they are usually powered by a single AA battery or two, they can provide enough power for a simple transistor amplifier to deliver sufficient current to the coil to 'step' the motor. This is not the case with a watch which at best has a large button cell that would not last very long if it had to directly drive the stepper coil.


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## canipus (Sep 7, 2008)

Alpha-Getty said:


> ... there are some simple quartz clock motors that are a simple coil and stepper wheel. Since they are usually powered by a single AA battery or two, they can provide enough power for a simple transistor amplifier to deliver sufficient current to the coil to 'step' the motor. This is not the case with a watch which at best has a large button cell that would not last very long if it had to directly drive the stepper coil.


yes, right - as you correctly state I was limiting the explanation to quartz wrist watch movements.


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## Pawl_Buster (Mar 12, 2007)

canipus said:


> yes, right - as you correctly state I was limiting the explanation to quartz wrist watch movements.


None the less, your detailed explanation was excellent :-!


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