Do Wiggle Stick Meters Wear Out?

[John Woodgate]

I read in alt.binaries.schematics.electronic that Chuck Harris
<cfharris@erols.com> wrote (in <3ff34855$0$4761$61fed72c@news.rcn.com&gt
about 'Do Wiggle Stick Meters Wear Out?', on Wed, 31 Dec 2003:

I would guess that the difference has something to do with how big
your capacitor is, vs how big the coil is. Perhaps if the coil is
really small in inductance, and the capacitor is very large in
capacitance you get enough loss that the coil cannot pass the energy
back to the capacitor before it is mostly all gone?

Not really; you are on the right lines insofar as a low value of L/C
make oscillation less likely, as I show below. But you haven't actually
mentioned the key word. The crucial factor is the *resistance* of the
coil, as I tried to indicate before. If R =>sqrt(L/C) the current won't
oscillate. It didn't.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!

 

[Sam Goldwasser]

John Woodgate <jmw@jmwa.demon.contraspam.yuk> writes:

I read in alt.binaries.schematics.electronic that Sam Goldwasser
sam@saul.cis.upenn.edu> wrote (in <6wwu8efnio.fsf@saul.cis.upenn.edu&gt
about 'Do Wiggle Stick Meters Wear Out?', on Tue, 30 Dec 2003:

A taut band movement suspends the coil and needle on a very thing
tensioned steel band which both provides the electrical connections
and restoring force.

I was given a classic Cambridge galvanometer with a broken taut-band
suspension. The original band was phosphor bronze, not steel. I replaced
it with 3 mil stainless steel wire (yes, from a wire recorder from the
same source as the galvo!). It was about 10 times less sensitive.

I had to repair a taut band suspension because I had acceidentally
trashed its mounting (don't ask). Fortunately, the band itself
was intact and could be unsoldered from the remaining debris. It's
not fun but is possible, more so I'd say than for a typical jeweled
movement simply because there is not hairspring to cause problems.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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[Sam Goldwasser]

John Woodgate <jmw@jmwa.demon.contraspam.yuk> writes:

I read in alt.binaries.schematics.electronic that BFoelsch
BFoelsch@snet.ditch.this.net> wrote (in <ppmdnfbD2IbnlG6i4p2dnA@giganew
s.com&gt about 'Do Wiggle Stick Meters Wear Out?', on Wed, 31 Dec 2003:

In a past life I frequently worked on small devices with small magnets. The
specific manufacturer with whom I worked preached long and loud about NOT
breaking the magnetic circuit during disassembly, putting keepers on before
disassembly, and the need to remagnetize after re-assembly. I am talking
about 1950 era magnets here.

How much truth is there to this concern. Was it valid in 1950?

It's still valid for some materials. It depends on the coercivity.
Ferrite magnets usually have such a huge coercivity that it isn't
necessary to keeper them.

When in doubt, use a keeper. I've seen servo and stepper motor PM magnets die
if the rotor is just pulled out.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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[Chuck Harris]

John Woodgate wrote:

I read in alt.binaries.schematics.electronic that Chuck Harris
cfharris@erols.com> wrote (in <3ff34855$0$4761$61fed72c@news.rcn.com&gt
about 'Do Wiggle Stick Meters Wear Out?', on Wed, 31 Dec 2003:


I would guess that the difference has something to do with how big
your capacitor is, vs how big the coil is. Perhaps if the coil is
really small in inductance, and the capacitor is very large in
capacitance you get enough loss that the coil cannot pass the energy
back to the capacitor before it is mostly all gone?


Not really; you are on the right lines insofar as a low value of L/C
make oscillation less likely, as I show below. But you haven't actually
mentioned the key word. The crucial factor is the *resistance* of the
coil, as I tried to indicate before. If R =>sqrt(L/C) the current won't
oscillate. It didn't.

Yes, but it also won't put much in the way of current into the
coil. You might as well use a straight DC supply, and just press
the button for an instant.

I big honking diode would work better. Charge your capacitor up to
400V or so, and dump it into a coil that has a diode in series.
The current will quickly ramp up to some maximum value. There will
be a nasty back emf spike, but such is life.

-Chuck

 

[Jim Weir]

Everything wears out. The Rockies and the Sierra are dwindling down at about an
inch every 1000 years. If you mean in your lifetime, probably not unless you
start to bang the needle off the peg every so often.

Jim

 

[DarkMatter]

On 31 Dec 2003 19:12:30 -0500, Sam Goldwasser <sam@saul.cis.upenn.edu>
Gave us:

When in doubt, use a keeper. I've seen servo and stepper motor PM magnets die
if the rotor is just pulled out.

I'm having a tough time with this one.

 

[Sam Goldwasser]

DarkMatter <DarkMatter@thebarattheendoftheuniverse.org> writes:

On 31 Dec 2003 19:12:30 -0500, Sam Goldwasser <sam@saul.cis.upenn.edu
Gave us:

When in doubt, use a keeper. I've seen servo and stepper motor PM magnets die
if the rotor is just pulled out.

I'm having a tough time with this one.

In what way?

I think one example was a "Slo-Syn" motor. I removed the rotor out of
curiosity. When it was put back in, the motor had lost 75 percent of
its torque. Definietly a weak magnet. I probably still have it and
an identical good unit.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
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[DarkMatter]

On 31 Dec 2003 20:43:57 -0500, Sam Goldwasser <sam@saul.cis.upenn.edu>
Gave us:

DarkMatter <DarkMatter@thebarattheendoftheuniverse.org> writes:

On 31 Dec 2003 19:12:30 -0500, Sam Goldwasser <sam@saul.cis.upenn.edu
Gave us:

When in doubt, use a keeper. I've seen servo and stepper motor PM magnets die
if the rotor is just pulled out.

I'm having a tough time with this one.

In what way?

I think one example was a "Slo-Syn" motor. I removed the rotor out of
curiosity. When it was put back in, the motor had lost 75 percent of
its torque. Definietly a weak magnet. I probably still have it and
an identical good unit.

It was the sliding action, perhaps. I'm still finding it hard to
believe that it just didn't get overheated, and went above it's curie
point temperature, causing loss, and you didn't catch the torque loss.

Are you saying that you made a before and after measurement with it,
when you did it? Or was it "in for repair" already?

 

[John Woodgate]

I read in alt.binaries.schematics.electronic that Chuck Harris
<cfharris@erols.com> wrote (in <3ff36a53$0$4764$61fed72c@news.rcn.com&gt
about 'Do Wiggle Stick Meters Wear Out?', on Wed, 31 Dec 2003:

Yes, but it also won't put much in the way of current into the
coil.

It put enough.

You might as well use a straight DC supply, and just press
the button for an instant.

Oh, yes A 4 A DC supply is much easier that a 34 mA one.

I big honking diode would work better. Charge your capacitor up to
400V or so, and dump it into a coil that has a diode in series.
The current will quickly ramp up to some maximum value.

This capacitor has infinite capacitance?

There will
be a nasty back emf spike, but such is life.

I don't really think you understand what happens.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!

 

[Sam Goldwasser]

DarkMatter <DarkMatter@thebarattheendoftheuniverse.org> writes:

On 31 Dec 2003 20:43:57 -0500, Sam Goldwasser <sam@saul.cis.upenn.edu
Gave us:

DarkMatter <DarkMatter@thebarattheendoftheuniverse.org> writes:

On 31 Dec 2003 19:12:30 -0500, Sam Goldwasser <sam@saul.cis.upenn.edu
Gave us:

When in doubt, use a keeper. I've seen servo and stepper motor PM magnets die
if the rotor is just pulled out.

I'm having a tough time with this one.

In what way?

I think one example was a "Slo-Syn" motor. I removed the rotor out of
curiosity. When it was put back in, the motor had lost 75 percent of
its torque. Definietly a weak magnet. I probably still have it and
an identical good unit.

It was the sliding action, perhaps. I'm still finding it hard to
believe that it just didn't get overheated, and went above it's curie
point temperature, causing loss, and you didn't catch the torque loss.

Are you saying that you made a before and after measurement with it,
when you did it? Or was it "in for repair" already?

Yes, at least by hand. I didn't actually use instruments. They were
identical before, very different after. Neither was subjected to any
abuse.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
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[Chuck Harris]

John Woodgate wrote:

I big honking diode would work better. Charge your capacitor up to
400V or so, and dump it into a coil that has a diode in series.
The current will quickly ramp up to some maximum value.


This capacitor has infinite capacitance?

And who said it did?

There will
be a nasty back emf spike, but such is life.


I don't really think you understand what happens.

You think there won't be a back emf spike when the diode
stops conducting? Hmmm?

-Chuck Harris

 

[DarkMatter]

On 01 Jan 2004 10:07:05 -0500, Sam Goldwasser <sam@saul.cis.upenn.edu>
Gave us:

DarkMatter <DarkMatter@thebarattheendoftheuniverse.org> writes:

On 31 Dec 2003 20:43:57 -0500, Sam Goldwasser <sam@saul.cis.upenn.edu
Gave us:

DarkMatter <DarkMatter@thebarattheendoftheuniverse.org> writes:

On 31 Dec 2003 19:12:30 -0500, Sam Goldwasser <sam@saul.cis.upenn.edu
Gave us:

When in doubt, use a keeper. I've seen servo and stepper motor PM magnets die
if the rotor is just pulled out.

I'm having a tough time with this one.

In what way?

I think one example was a "Slo-Syn" motor. I removed the rotor out of
curiosity. When it was put back in, the motor had lost 75 percent of
its torque. Definietly a weak magnet. I probably still have it and
an identical good unit.

It was the sliding action, perhaps. I'm still finding it hard to
believe that it just didn't get overheated, and went above it's curie
point temperature, causing loss, and you didn't catch the torque loss.

Are you saying that you made a before and after measurement with it,
when you did it? Or was it "in for repair" already?

Yes, at least by hand. I didn't actually use instruments. They were
identical before, very different after. Neither was subjected to any
abuse.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the feedback form on the S.E.R FAQ Web sites.

I'd bet that a check on the "magnetized state" of the rotor would

show that it had become magnetized thereby offsetting what work gets
done upon re-assembled.

If you demag the rotor, then return it, the torque should return.

Just one possibility that immediately came to mind. I'm not saying
that is what happened, just throwing it out to see who salutes it.

 

[DarkMatter]

On Thu, 01 Jan 2004 11:01:25 -0500, Chuck Harris <cfharris@erols.com>
Gave us:

John Woodgate wrote:

I big honking diode would work better. Charge your capacitor up to
400V or so, and dump it into a coil that has a diode in series.
The current will quickly ramp up to some maximum value.


This capacitor has infinite capacitance?

And who said it did?


There will
be a nasty back emf spike, but such is life.


I don't really think you understand what happens.

You think there won't be a back emf spike when the diode
stops conducting? Hmmm?

-Chuck Harris

When one dumps a cap into a coil, there is a current inrush. When
one attaches a constant voltage DC source to a coil, there is a
current inrush, and then a standing magnetic field is exhibited until
the source is removed.

The collapse of the flux will be slower from the cap than if a DC
source is applied, a standing field is exhibited, and then said source
is immediately released. That second method will exhibit a MUCH
higher slew rate on the collapsing flux, and a much higher potential
will be "seen" at the coil terminals.

A cap makes for a slow collapse, and the back emf is nowhere near as
high. As the cap discharges, the voltage decreases to zero. The coil
does not recharge the cap, even without the diode. At least not
enough to make a difference. Ther are a few perturbations, but it
looks similar to a slightly under damped feedback loop, with each
reverb being vastly smaller than the previous...then to zero.

 

[Jim Adney]

On Wed, 31 Dec 2003 17:24:05 -0800 DarkMatter
<DarkMatter@thebarattheendoftheuniverse.org> wrote:

On 31 Dec 2003 19:12:30 -0500, Sam Goldwasser <sam@saul.cis.upenn.edu
Gave us:

When in doubt, use a keeper. I've seen servo and stepper motor PM magnets die
if the rotor is just pulled out.

I'm having a tough time with this one.

I don't claim to fully understand the cause, but I do know that it can
certainly happen with certain kinds of magnets.

Sturmey-Archer, the maker of the 3-speed bicycle hubs that you're
probably familiar with, also made a hub with a generator built into
it. If you pull the armature out of the multipole circular magnet it
will lose most of its field. I've never done this, but I have the
factory manuals that explain to only slip the armature out as you slip
the keeper in.

I also have a NOS magnet, shipped from the factory with a keeper in
place.

-
-----------------------------------------------
Jim Adney jadney@vwtype3.org
Madison, WI 53711 USA
-----------------------------------------------

 

[Chuck Harris]

Damn! I wasted all that money going to engineering school!

Inductors resist a change in current. When you apply a voltage to an
inductor, the current is initially *zero* and it rises linearily until
it reaches V/Rcoil, or the saturation of the core if you have one.

When you clamp a charged capacitor to an inductor, you create a tank
circuit. The capacitor and the inductor pass energy back and forth.
They would do so forever, but for radiation losses, and losses in the
coil and inductor.

Either I am not understanding what you meant to say, or you need to
study the basic circuit elements, and resonant circuits some more
Mr. DarkMatter.

-Chuck Harris



DarkMatter wrote:

On Thu, 01 Jan 2004 11:01:25 -0500, Chuck Harris <cfharris@erols.com
Gave us:


John Woodgate wrote:

I big honking diode would work better. Charge your capacitor up to
400V or so, and dump it into a coil that has a diode in series.
The current will quickly ramp up to some maximum value.


This capacitor has infinite capacitance?

And who said it did?


There will
be a nasty back emf spike, but such is life.


I don't really think you understand what happens.

You think there won't be a back emf spike when the diode
stops conducting? Hmmm?

-Chuck Harris



When one dumps a cap into a coil, there is a current inrush. When
one attaches a constant voltage DC source to a coil, there is a
current inrush, and then a standing magnetic field is exhibited until
the source is removed.

The collapse of the flux will be slower from the cap than if a DC
source is applied, a standing field is exhibited, and then said source
is immediately released. That second method will exhibit a MUCH
higher slew rate on the collapsing flux, and a much higher potential
will be "seen" at the coil terminals.

A cap makes for a slow collapse, and the back emf is nowhere near as
high. As the cap discharges, the voltage decreases to zero. The coil
does not recharge the cap, even without the diode. At least not
enough to make a difference. Ther are a few perturbations, but it
looks similar to a slightly under damped feedback loop, with each
reverb being vastly smaller than the previous...then to zero.

 

[DarkMatter]

On Wed, 31 Dec 2003 12:38:03 -0500, "BFoelsch"
<BFoelsch@snet.ditch.this.net> Gave us:

OK - related question.

In a past life I frequently worked on small devices with small magnets. The
specific manufacturer with whom I worked preached long and loud about NOT
breaking the magnetic circuit during disassembly, putting keepers on before
disassembly, and the need to remagnetize after re-assembly. I am talking
about 1950 era magnets here.

How much truth is there to this concern. Was it valid in 1950?

Equipment in question was audio recording devices; cutter heads, pickups,
microphones, etc.

Take a look at the link I posted earlier in this thread, and you'll
see (read and see) how important it is.

Of course it is still valid. Valid for all magnetic media.

Why else would we actually develop different magnetic alloys unless
properties, and characterizations of any and all known and attainable
principles were not the goal?

 

[DarkMatter]

On Thu, 01 Jan 2004 13:21:37 -0500, Chuck Harris <cfharris@erols.com>
Gave us:

-Chuck Harris

Losses, losses, losses. I think I said that.

 

[DarkMatter]

On Thu, 01 Jan 2004 13:21:37 -0500, Chuck Harris <cfharris@erols.com>
Gave us:

Either I am not understanding what you meant to say, or you need to
study the basic circuit elements, and resonant circuits some more
Mr. DarkMatter.

It will cycle a few times, but will essentially stabilize at a DC
voltage, with a standing field on the coil, and slowly leak down from
there on high turn count, high DC resistance coils, and on low turn
coils, even faster. Continuous self oscillation requires a pump.

Resonate that.

 

[Watson A.Name - "Watt Sun]

In article <n9t6vv848age1jhu7b9nr5s6j1mlh1vna6@4ax.com>, jim@rst-
engr.com mentioned...

Everything wears out. The Rockies and the Sierra are dwindling down at about an
inch every 1000 years. If you mean in your lifetime, probably not unless you
start to bang the needle off the peg every so often.

Jim

Yeah, I'd guess as much, now that I've read the other followup about a
jeweled watch escapement moving back and forthe five times a second
for decades.

About the mountains.. I was watching some program on PBS this morning
that went along with a guy who survived Mt. St. Helens "eruption", if
you can call seven tenths of a cubic mile of a mountain blowing away
an eruption. ;-)

They took a hike back around (what's left of) the mountain. After
almost 24 years, the landscape still looks as bleak as the moon in
some places.

--
@@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@h@e@r@e@@
###Got a Question about ELECTRONICS? Check HERE First:###
http://users.pandora.be/educypedia/electronics/databank.htm
My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 <at> hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
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[DarkMatter]

On Thu, 1 Jan 2004 13:30:40 -0800, Watson A.Name - "Watt Sun, Dark
Remover" <alondra101@hotmail.com> Gave us:

In article <n9t6vv848age1jhu7b9nr5s6j1mlh1vna6@4ax.com>, jim@rst-
engr.com mentioned...

Everything wears out. The Rockies and the Sierra are dwindling down at about an
inch every 1000 years. If you mean in your lifetime, probably not unless you
start to bang the needle off the peg every so often.

Jim

Yeah, I'd guess as much, now that I've read the other followup about a
jeweled watch escapement moving back and forthe five times a second
for decades.

About the mountains.. I was watching some program on PBS this morning
that went along with a guy who survived Mt. St. Helens "eruption", if
you can call seven tenths of a cubic mile of a mountain blowing away
an eruption. ;-)

They took a hike back around (what's left of) the mountain. After
almost 24 years, the landscape still looks as bleak as the moon in
some places.

Yet another man was the last to be heard from at the time of the
eruption.

He was on the radio phone with:

"Yes, there it goes... Yes, it's very big... Yes, it's going to get
me too..."

Poor guy. Fertilizer for thinking he was far enough away at five
miles.

Trees combed over like a giant hair brush came through for miles.

Atomic blast force levels many times that of hiroshima, or even our
biggest Cobalt Hydrogen devices ever got... many times over.