magnetic field

[CWatters]

"qude" <qmdynamics@yahoo.com> wrote in message
news:1119172222.190468.82400@f14g2000cwb.googlegroups.com...

Thanks for the answers but I want understand the physics side.
So what is the function of voltage and current in terms of
electron movement?? Let's focus on the physics side of it.

An electron has charge and current is defined as the number of unit charge
(Q) flowing per unit Time (T)

I = Q/T

Ohms law says

I = V/R

so

Q/T = V/R

or

Q = T * V/R

eg Increase V and more units of charge flow past in a given time.
or
Increase R and fewer units of charge flow past in a given time

 

[CWatters]

"qude" <qmdynamics@yahoo.com> wrote in message
news:1119223515.039007.79790@g47g2000cwa.googlegroups.com...

Suppose you have a current source with magnetic field
and you want to reduce it by adding a metal near it
to induce opposite magnetic field to cancel the one
from the source (partially). What coil configuration
must you use? Is this possible?

All electrical circuits have a return path. The best you can do is to use a
second wire for the return path and to twist it around the signal wire. You
now have equal currents flowing in opposite directions in close proximity.
The magnetic fields almost cancel out.

 

[sue jahn]

"qude" <qmdynamics@yahoo.com> wrote in message news:1119216943.195356.8180@g44g2000cwa.googlegroups.com...

right?

Correct!
<< In conclusion, ***all*** magnetic fields encountered in
nature are generated by circulating currents. There is no fundamental
difference between the fields generated by permanent magnets
and those generated by currents flowing around conventional
electric circuits. In the former case the currents which generate
the fields circulate on the atomic scale whereas, in the latter case
the currents circulate on a macroscopic scale
(i.e., the scale of the circuit). >>
http://farside.ph.utexas.edu/teaching/302l/lectures/node62.html

This means the 12volts/18ampere setup produce larger
magnetic field magnitude (from the current flow) than
the 110volts/2Ampere setup (noting that both produce the
same 220 Watts power), right??

Indeed.

Or does the larger 110 volts in the latter give more push
to the 2 ampere resulting in similar magnetic field
magnitude for both setups??
No. Fewer charges need to move past a point in

a second to transfer the same energy because they are
moving faster. Tho' charges have energy, (angular momentum)
it is not created nor destroyed. The energy that does
the work is the mass(equivalent energy) of the charge in motion.

If the answer is that the 12volts/18 ampere indeed
produce larger magnetic field, do designers find it
necessary to use larger voltage instead of larger
current to prevent magnetic field interference?

Hmmm... Frequently higher voltages are used to avoid
thick wire. I can't think of an example. There are other
techniques that are more effective, but in principle you
are correct.

If the answer is

But it isn't. You got it right above.

Sue...


that both produce the same magnetic

field magnitude, then it means in larger voltage
such as 110 volts, it pushes the 2 ampere more so
it moves faster compared to the 18Ampere with 12
volts that don't push it faster. If not, how can
the magnetic field magnitude in both be the same
(assuming they are the same).

Thanks.

qude

 

[Dave]

"qude" <qmdynamics@yahoo.com> wrote in message
news:1119273009.300137.199390@z14g2000cwz.googlegroups.com...

CWatters wrote:
"qude" <qmdynamics@yahoo.com> wrote in message
news:1119223515.039007.79790@g47g2000cwa.googlegroups.com...


Suppose you have a current source with magnetic field
and you want to reduce it by adding a metal near it
to induce opposite magnetic field to cancel the one
from the source (partially). What coil configuration
must you use? Is this possible?

All electrical circuits have a return path. The best you can do is to use
a
second wire for the return path and to twist it around the signal wire.
You
now have equal currents flowing in opposite directions in close
proximity.
The magnetic fields almost cancel out.

I'm asking whether putting a separate wire without voltage or
current source with certain coil configuration can cause
opposite magnetic field to be produced by the induced current,
and what is the magnitude of it, Anyone knows?

qude

yes it can. magnitude depends on configuration and incident field. i.e. a
plate of aluminum can almost completely cancel a 60hz magnetic field due to
induced currents.

 

[Dave]

"qude" <qmdynamics@yahoo.com> wrote in message
news:1119277654.267938.166170@g43g2000cwa.googlegroups.com...

Dave wrote:
"qude" <qmdynamics@yahoo.com> wrote in message
news:1119273009.300137.199390@z14g2000cwz.googlegroups.com...


CWatters wrote:
"qude" <qmdynamics@yahoo.com> wrote in message
news:1119223515.039007.79790@g47g2000cwa.googlegroups.com...


Suppose you have a current source with magnetic field
and you want to reduce it by adding a metal near it
to induce opposite magnetic field to cancel the one
from the source (partially). What coil configuration
must you use? Is this possible?

All electrical circuits have a return path. The best you can do is to
use
a
second wire for the return path and to twist it around the signal
wire.
You
now have equal currents flowing in opposite directions in close
proximity.
The magnetic fields almost cancel out.

I'm asking whether putting a separate wire without voltage or
current source with certain coil configuration can cause
opposite magnetic field to be produced by the induced current,
and what is the magnitude of it, Anyone knows?

qude


yes it can. magnitude depends on configuration and incident field. i.e.
a
plate of aluminum can almost completely cancel a 60hz magnetic field due
to
induced currents.

Is it always cancellation.. or can you make the configuration such
that the induced field would add up to the source increasing
the magnetic field (or does the law of conservation of field
applies here).

qude
in antenna design you can configure conductors to increase the field in one

direction but it always is decreased in some other direction because of
conservation of energy. i haven't really thought of that for low frequency
work, but would assume the same would apply, while reducing the field by
cancellation is one direction it must increase in some other area, minus a
bit for losses from resistive heating in the material used for the
shielding.

 

[Gerald Robinson]

"Dave" <noone@nowhere.com> wrote in message
news:e7OdnXX818iSSSvfRVn-3g@crocker.com...

"qude" <qmdynamics@yahoo.com> wrote in message
news:1119277654.267938.166170@g43g2000cwa.googlegroups.com...


Dave wrote:
"qude" <qmdynamics@yahoo.com> wrote in message
news:1119273009.300137.199390@z14g2000cwz.googlegroups.com...


CWatters wrote:
"qude" <qmdynamics@yahoo.com> wrote in message
news:1119223515.039007.79790@g47g2000cwa.googlegroups.com...


Suppose you have a current source with magnetic field
and you want to reduce it by adding a metal near it
to induce opposite magnetic field to cancel the one
from the source (partially). What coil configuration
must you use? Is this possible?

All electrical circuits have a return path. The best you can do is to
use
a
second wire for the return path and to twist it around the signal
wire.
You
now have equal currents flowing in opposite directions in close
proximity.
The magnetic fields almost cancel out.

I'm asking whether putting a separate wire without voltage or
current source with certain coil configuration can cause
opposite magnetic field to be produced by the induced current,
and what is the magnitude of it, Anyone knows?

qude


yes it can. magnitude depends on configuration and incident field.
i.e. a
plate of aluminum can almost completely cancel a 60hz magnetic field due
to
induced currents.

Is it always cancellation.. or can you make the configuration such
that the induced field would add up to the source increasing
the magnetic field (or does the law of conservation of field
applies here).

qude
in antenna design you can configure conductors to increase the field in
one direction but it always is decreased in some other direction because
of conservation of energy. i haven't really thought of that for low
frequency work, but would assume the same would apply, while reducing the
field by cancellation is one direction it must increase in some other
area, minus a bit for losses from resistive heating in the material used
for the shielding.

How would you calculate the losses?

In an earlier question I am asking about using the heating effect?

 

[Don Kelly]

"qude" <qmdynamics@yahoo.com> wrote in message
news:1119308009.684198.139120@f14g2000cwb.googlegroups.com...

Dave wrote:
"qude" <qmdynamics@yahoo.com> wrote in message
news:1119277654.267938.166170@g43g2000cwa.googlegroups.com...


Dave wrote:
"qude" <qmdynamics@yahoo.com> wrote in message
news:1119273009.300137.199390@z14g2000cwz.googlegroups.com...


CWatters wrote:
"qude" <qmdynamics@yahoo.com> wrote in message
news:1119223515.039007.79790@g47g2000cwa.googlegroups.com...


Suppose you have a current source with magnetic field
and you want to reduce it by adding a metal near it
to induce opposite magnetic field to cancel the one
from the source (partially). What coil configuration
must you use? Is this possible?

All electrical circuits have a return path. The best you can do is
to
use
a
second wire for the return path and to twist it around the signal
wire.
You
now have equal currents flowing in opposite directions in close
proximity.
The magnetic fields almost cancel out.

I'm asking whether putting a separate wire without voltage or
current source with certain coil configuration can cause
opposite magnetic field to be produced by the induced current,
and what is the magnitude of it, Anyone knows?

qude


yes it can. magnitude depends on configuration and incident field.
i.e.
a
plate of aluminum can almost completely cancel a 60hz magnetic field
due
to
induced currents.

Is it always cancellation.. or can you make the configuration such
that the induced field would add up to the source increasing
the magnetic field (or does the law of conservation of field
applies here).

qude
in antenna design you can configure conductors to increase the field in
one
direction but it always is decreased in some other direction because of
conservation of energy. i haven't really thought of that for low
frequency
work, but would assume the same would apply, while reducing the field by
cancellation is one direction it must increase in some other area, minus
a
bit for losses from resistive heating in the material used for the
shielding.

But it's not always the case, isn't it. Suppose you turn a connected
wire in parallel with the current of each in opposite direction.
There would be cancellation of the magnetic field outside the
parallel wires and nothing in between them. This means cancellation
outside indeed occur. Where does the conservation of energy came into
play here? There is no evidence it is increased elsewhere with
the same magnitude as the ones outside that is cancelled.

qude

There seems to be a great deal of confusion involved here. More so in your
first message.

Note that inducing a voltage in a parallel wire (and this voltage producing
a current), the best you will get is that the external field will be the
same as with the original wire only as all you have is a crude transformer.

In the case of the wire being physically parallel with a return wire
carrying the same current (e.g. a lampcord), partial cancellation (never
full unless the wires can occupy the same point in space) will occur outside
the wires. However, between the wires, the field will be not be "nothing"
but will be enhanced as the two currents are then additive with regard to
the magnetic field. Sketch the field around a current into the page and that
due to a current out of the page. Bring the sketches together- note the
relative directions of the fields in different locations. I can give you a
set of equations for calculation of the field due to a number of parallel
current carrying wires if you wish.
As to conservation of energy- there is no problem. The total field energy is
dependent on the current and the position of the wires. If the position
changes, then the field changes but there is work input or output to change
the position In fact, conservation of energy can be used to calculate forces
in such a situation. (Conservation of energy leads to : change in electrical
energy in =change in magnetic field energy + change in mechanical energy +
losses). This can be and is expressed in a number of ways in Electromagnetic
Energy Conversion texts.


--
Don Kelly
dhky@peeshaw.ca
remove the urine to answer

 

[Dave]

"qude" <qmdynamics@yahoo.com> wrote in message
news:1119330643.140180.186910@f14g2000cwb.googlegroups.com...

Dave wrote:
"qude" <qmdynamics@yahoo.com> wrote in message
news:1119273009.300137.199390@z14g2000cwz.googlegroups.com...


CWatters wrote:
"qude" <qmdynamics@yahoo.com> wrote in message
news:1119223515.039007.79790@g47g2000cwa.googlegroups.com...


Suppose you have a current source with magnetic field
and you want to reduce it by adding a metal near it
to induce opposite magnetic field to cancel the one
from the source (partially). What coil configuration
must you use? Is this possible?

All electrical circuits have a return path. The best you can do is to
use
a
second wire for the return path and to twist it around the signal
wire.
You
now have equal currents flowing in opposite directions in close
proximity.
The magnetic fields almost cancel out.

I'm asking whether putting a separate wire without voltage or
current source with certain coil configuration can cause
opposite magnetic field to be produced by the induced current,
and what is the magnitude of it, Anyone knows?

qude


yes it can. magnitude depends on configuration and incident field. i.e.
a
plate of aluminum can almost completely cancel a 60hz magnetic field due
to
induced currents.


To avoid confusion.

This is what I'm describing. A plate of aluminium put over a wire
carrying 60 hz AC or fluctuating dc. Using the right hand rule,
the current is clockwise looking from the source at the left. Now
what I wanna understand is how does the induced current in
the plate of aluminum (not in contact with it but put on top of
it at a distance) produce magnetic vector that is in opposite to
that of the wire. Does this something to do with Lenz law?
What's the proof that induced current and magnetic field
in the aluminum plate is opposite to that of the wire... which
part of the plate the current flows?

Thanks guys.

qude

to aid in figuring this out in your mind replace the plate with a whole
bunch of little wire loops. as the field changes the db/dt induces a
current flow in each of the little loops that will oppose the incident
field. this is a simple case of induced current from a changing magnetic
field. it gets much more complicated when you realize there are an infinite
number of these loops and they are all connected to each other when it
becomes a solid plate, but the small loop model can still be used to give
good estimates of fields.

 

[Dave]

"qude" <qmdynamics@yahoo.com> wrote in message
news:1119359115.474113.237760@o13g2000cwo.googlegroups.com...

Dave wrote:
"qude" <qmdynamics@yahoo.com> wrote in message
news:1119330643.140180.186910@f14g2000cwb.googlegroups.com...


Dave wrote:
"qude" <qmdynamics@yahoo.com> wrote in message
news:1119273009.300137.199390@z14g2000cwz.googlegroups.com...


CWatters wrote:
"qude" <qmdynamics@yahoo.com> wrote in message
news:1119223515.039007.79790@g47g2000cwa.googlegroups.com...


Suppose you have a current source with magnetic field
and you want to reduce it by adding a metal near it
to induce opposite magnetic field to cancel the one
from the source (partially). What coil configuration
must you use? Is this possible?

All electrical circuits have a return path. The best you can do is
to
use
a
second wire for the return path and to twist it around the signal
wire.
You
now have equal currents flowing in opposite directions in close
proximity.
The magnetic fields almost cancel out.

I'm asking whether putting a separate wire without voltage or
current source with certain coil configuration can cause
opposite magnetic field to be produced by the induced current,
and what is the magnitude of it, Anyone knows?

qude


yes it can. magnitude depends on configuration and incident field.
i.e.
a
plate of aluminum can almost completely cancel a 60hz magnetic field
due
to
induced currents.


To avoid confusion.

This is what I'm describing. A plate of aluminium put over a wire
carrying 60 hz AC or fluctuating dc. Using the right hand rule,
the current is clockwise looking from the source at the left. Now
what I wanna understand is how does the induced current in
the plate of aluminum (not in contact with it but put on top of
it at a distance) produce magnetic vector that is in opposite to
that of the wire. Does this something to do with Lenz law?
What's the proof that induced current and magnetic field
in the aluminum plate is opposite to that of the wire... which
part of the plate the current flows?

Thanks guys.

qude

to aid in figuring this out in your mind replace the plate with a whole
bunch of little wire loops. as the field changes the db/dt induces a
current flow in each of the little loops that will oppose the incident
field. this is a simple case of induced current from a changing magnetic
field. it gets much more complicated when you realize there are an
infinite
number of these loops and they are all connected to each other when it
becomes a solid plate, but the small loop model can still be used to give
good estimates of fields.


I'm inquiring because I saw a product at a shop which has this
cooper plate. It is supposed to lower emissions of magnetic
field from appliances by simply being put near them. I wonder
how many percentage of magnetic field are suppressed when say
put directly on top a computer monitor or toaster, any idea?
It's for those who are very EM sensitive or electrosensitives.

p6

no, its for those who have more money than brains. at the edges the fields
wrap around so for a small shield like that it would only reduce the field
slightly for a small distance on the other side of it. even when shielding
transformer vaults it requires not only doing the whole wall between the
transformer and the equipment being protected, but a large part of the
floor, ceiling and other walls... preferably wrapping the whole room.

 

[Impmon]

On 21 Jun 2005 13:58:18 -0700, "xoyo" <xiaojun.wang@gmail.com> wrote:

Hi,

Our lab is planning to buy a large screen display (about 40"),
to demonstrate the experiment result. The video input will be
from PC.

Could anyone give me some suggestion on which one to buy?

Probably plasma. I doubt there's any LCD in 40" range. I do know at
certain size, LCD ends up becoming more expensive to manufacture than
plasma.
--
When you hear the toilet flush, and hear the words "uh oh", it's already
too late. - by anonymous Mother in Austin, TX
To reply, replace digi.mon with phreaker.net

 

[Dave Garnett]

Our lab is planning to buy a large screen display (about 40"),
to demonstrate the experiment result. The video input will be
from PC.

Could anyone give me some suggestion on which one to buy?

Thanks a lot!

Buy a projector - big pictures from a small box !!




Posted Via Nuthinbutnews.Com Premium Usenet Newsgroup Services
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** SPEED ** RETENTION ** COMPLETION ** ANONYMITY **
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[CWatters]

"Dave" <noone@nowhere.com> wrote in message
news:re2dnYDqPISYjyXfRVn-vA@crocker.com...

I'm inquiring because I saw a product at a shop which has this
cooper plate. It is supposed to lower emissions of magnetic
field from appliances by simply being put near them. I wonder
how many percentage of magnetic field are suppressed when say
put directly on top a computer monitor or toaster, any idea?
It's for those who are very EM sensitive or electrosensitives.

p6


no, its for those who have more money than brains. at the edges the
fields
wrap around so for a small shield like that it would only reduce the field
slightly for a small distance on the other side of it. even when
shielding
transformer vaults it requires not only doing the whole wall between the
transformer and the equipment being protected, but a large part of the
floor, ceiling and other walls... preferably wrapping the whole room.

yeah there are lots of suspect products on the market that claim to "protect
you from harmful radiation" - but if they worked whole industries would be
out of business. It can be a bitch preventing EMC escaping from electronic
devices like computers. I've done my time down a saltmine trying to design
screening. Extreemly frustrating it can be.

 

[Terry Bull]

Since both are available at that size, and you are driving it with the 15pin
VGA/XGA output, then either would do.

The latest LCD are better than earlier models for movement, but still
'laggy' so if you plan on showing fast movement then use a plasma.

If your pictures are still/frozen for a long time, then beware of the
burn-in problems that can occur with plasma.

hope that helps
Andrew


"xoyo" <xiaojun.wang@gmail.com> wrote in message
news:1119387498.866519.18700@z14g2000cwz.googlegroups.com...

Hi,

Our lab is planning to buy a large screen display (about 40"),
to demonstrate the experiment result. The video input will be
from PC.

Could anyone give me some suggestion on which one to buy?

Thanks a lot!

 

[CWatters]

"Dave Garnett" <dave.garnett@metapurple.co.uk> wrote in message
news:42b90cce$1_1@spool9-east.superfeed.net...

Our lab is planning to buy a large screen display (about 40"),
to demonstrate the experiment result. The video input will be
from PC.

I would go for a 40" lCD with the highest resolution you can find. 1280 x
768 seems to be standard on 40" models. A large screen doesn't always mean
any more pixels though. I also look for one with a Digital input. Amazon has
several.

There is also a 45" model from Sharp with a 1,920 x 1,080 resolution..

http://www.amazon.com/exec/obidos/tg/detail/-/B0002TX3KS/ref=pd_sbs_e_4/002-1749504-9809609?v=glance&s=electronics

 

[Dave]

"Gerald Robinson" <gerald@robinson5333.freeserve.co.uk> wrote in message
news:d96url$skf$1@newsg4.svr.pol.co.uk...

"Dave" <noone@nowhere.com> wrote in message
news:e7OdnXX818iSSSvfRVn-3g@crocker.com...

"qude" <qmdynamics@yahoo.com> wrote in message
news:1119277654.267938.166170@g43g2000cwa.googlegroups.com...


Dave wrote:
"qude" <qmdynamics@yahoo.com> wrote in message
news:1119273009.300137.199390@z14g2000cwz.googlegroups.com...


CWatters wrote:
"qude" <qmdynamics@yahoo.com> wrote in message
news:1119223515.039007.79790@g47g2000cwa.googlegroups.com...


Suppose you have a current source with magnetic field
and you want to reduce it by adding a metal near it
to induce opposite magnetic field to cancel the one
from the source (partially). What coil configuration
must you use? Is this possible?

All electrical circuits have a return path. The best you can do is
to use
a
second wire for the return path and to twist it around the signal
wire.
You
now have equal currents flowing in opposite directions in close
proximity.
The magnetic fields almost cancel out.

I'm asking whether putting a separate wire without voltage or
current source with certain coil configuration can cause
opposite magnetic field to be produced by the induced current,
and what is the magnitude of it, Anyone knows?

qude


yes it can. magnitude depends on configuration and incident field.
i.e. a
plate of aluminum can almost completely cancel a 60hz magnetic field
due to
induced currents.

Is it always cancellation.. or can you make the configuration such
that the induced field would add up to the source increasing
the magnetic field (or does the law of conservation of field
applies here).

qude
in antenna design you can configure conductors to increase the field in
one direction but it always is decreased in some other direction because
of conservation of energy. i haven't really thought of that for low
frequency work, but would assume the same would apply, while reducing the
field by cancellation is one direction it must increase in some other
area, minus a bit for losses from resistive heating in the material used
for the shielding.



How would you calculate the losses?

very carefully. calculating induced currents and losses is not the easiest
problem, especially in solid plates. it may be a bit easier in wire loops.

In an earlier question I am asking about using the heating effect?

yes, if a current is induced by the magnetic field then there will be some
loss due to heating in the material.

 

[John Larkin]

On 22 Jun 2005 06:28:24 -0700, "redbelly" <redbelly98@yahoo.com>
wrote:

I'm looking for a basic, easy-to-build constant current source to drive
thermistors, nominally at 0.1 mA. The thermistors would be the common
10 k-ohm @ 25 C variety.

I spent time Googling this topic without success. One site mentioned
that an LM317 can be used to generate a current, but the current would
have to be a minimum of 5 or 10 mA. I need a 0.1 mA source.

I'd like to build something, preferably for under $10.00, with better
than 0.5% accuracy (that should correspond to temperature readings
within 0.1 or 0.2 C). My plan is to have several thermistors in series
driven with the circuit, and then I can read off the voltage of each
thermistor with an A/D hooked up to a PC, and be able to monitor
temperature readings over several hours or up to a day.

TIA,

Mark

Why constant current? A better way is to have a voltage reference
driving a resistor and then the thermistor


A
|
vref---+-----resistor-----+--------B
|
|
thermistor
|
|
gnd


So, measure Va relative to ground, then Vb, and do the math.

Putting the thermistors in series might get tricky, especially if you
use a constant-current source and the temperature swing is wide... you
might run out of voltage. It might be better to use a multiplexer to
select one thermistor at a time.

John

 

[John Popelish]

redbelly wrote:

I'm looking for a basic, easy-to-build constant current source to drive
thermistors, nominally at 0.1 mA. The thermistors would be the common
10 k-ohm @ 25 C variety.

I spent time Googling this topic without success. One site mentioned
that an LM317 can be used to generate a current, but the current would
have to be a minimum of 5 or 10 mA. I need a 0.1 mA source.

I'd like to build something, preferably for under $10.00, with better
than 0.5% accuracy (that should correspond to temperature readings
within 0.1 or 0.2 C). My plan is to have several thermistors in series
driven with the circuit, and then I can read off the voltage of each
thermistor with an A/D hooked up to a PC, and be able to monitor
temperature readings over several hours or up to a day.

I agree with John Larkin that the way to go is just to use a resistor

divider and some math. The only advantage of the current source would
be its linearity, but the thermistor has a very nonlinear response to
temperature, so that is out the window. If you select a series
resistor that is about equal to the thermistor's resistance in the
middle of the temperature range you are interested in, the divider
voltage has its largest voltage change per degree in that band of
temperature, and is more approximately linear than if the thermistor
were driven with a constant current.

All that said, if you still want to use a current source, you can make
one with a current reference chip, LM334Z
http://cache.national.com/ds/LM/LM134.pdf
a diode and two resistors, as shown on page 7.

Of course, the LM334Z is also a temperature sensor all by itself, and
a linear one, at that, since it produces a current proportional to
absolute temperature, if you just program it with a resistor. So you
could replace your thermistor with a resistor, and have the LM334Z
sense the temperature. But the signal is smaller than you would get
with the thermistor.

 

[CWatters]

"redbelly" <redbelly98@yahoo.com> wrote in message
news:1119446904.126135.292690@f14g2000cwb.googlegroups.com...

I'm looking for a basic, easy-to-build constant current source to drive
thermistors, nominally at 0.1 mA. The thermistors would be the common
10 k-ohm @ 25 C variety.

Can't get much simpler..

http://zebu.uoregon.edu/~rayfrey/431/notes6.pdf

 

[CWatters]

"CWatters" <colin.watters@pandoraBOX.be> wrote in message
news:qFhue.127203$qm.6955067@phobos.telenet-ops.be...

"redbelly" <redbelly98@yahoo.com> wrote in message
news:1119446904.126135.292690@f14g2000cwb.googlegroups.com...
I'm looking for a basic, easy-to-build constant current source to drive
thermistors, nominally at 0.1 mA. The thermistors would be the common
10 k-ohm @ 25 C variety.

Can't get much simpler..

http://zebu.uoregon.edu/~rayfrey/431/notes6.pdf

If you want to know how it works....

The base is held at a constant voltage by R1&R2. Therefore the Emitter is
held at a constant voltage (=VBASE-VBE).

Therefore the emitter current is constant = (VBASE-VBE)/RE

Therefore the collector current is constant (IC=IE)

You can make a similar circuit using a PNP transistor if you want one lead
of the thermistor grounded.

 

[Dave]

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Suppose you have a current source with magnetic field
and you want to reduce it by adding a metal near it
to induce opposite magnetic field to cancel the one
from the source (partially). What coil configuration
must you use? Is this possible?

All electrical circuits have a return path. The best you can do
is
to
use
a
second wire for the return path and to twist it around the
signal
wire.
You
now have equal currents flowing in opposite directions in close
proximity.
The magnetic fields almost cancel out.

I'm asking whether putting a separate wire without voltage or
current source with certain coil configuration can cause
opposite magnetic field to be produced by the induced current,
and what is the magnitude of it, Anyone knows?

qude


yes it can. magnitude depends on configuration and incident field.
i.e.
a
plate of aluminum can almost completely cancel a 60hz magnetic
field
due
to
induced currents.


To avoid confusion.

This is what I'm describing. A plate of aluminium put over a wire
carrying 60 hz AC or fluctuating dc. Using the right hand rule,
the current is clockwise looking from the source at the left. Now
what I wanna understand is how does the induced current in
the plate of aluminum (not in contact with it but put on top of
it at a distance) produce magnetic vector that is in opposite to
that of the wire. Does this something to do with Lenz law?
What's the proof that induced current and magnetic field
in the aluminum plate is opposite to that of the wire... which
part of the plate the current flows?

Thanks guys.

qude

to aid in figuring this out in your mind replace the plate with a
whole
bunch of little wire loops. as the field changes the db/dt induces a
current flow in each of the little loops that will oppose the incident
field. this is a simple case of induced current from a changing
magnetic
field. it gets much more complicated when you realize there are an
infinite
number of these loops and they are all connected to each other when it
becomes a solid plate, but the small loop model can still be used to
give
good estimates of fields.


I'm inquiring because I saw a product at a shop which has this
cooper plate. It is supposed to lower emissions of magnetic
field from appliances by simply being put near them. I wonder
how many percentage of magnetic field are suppressed when say
put directly on top a computer monitor or toaster, any idea?
It's for those who are very EM sensitive or electrosensitives.

p6


no, its for those who have more money than brains. at the edges the
fields
wrap around so for a small shield like that it would only reduce the
field
slightly for a small distance on the other side of it. even when
shielding
transformer vaults it requires not only doing the whole wall between the
transformer and the equipment being protected, but a large part of the
floor, ceiling and other walls... preferably wrapping the whole room.

I checked further. Is it possible the plate of cooper or
aluminum can be designed so that (by induction) it can produced
a third waveform, by some kind of circuit. ES or Electrosensitives
have so called neutralizing frequency and these swedish designers
created some kind of copper plate in which when you put it near
any source of magnetic field, it can produce a third waveform
oscilating at a certain frequency. For example. You are tasked
to design such circuit that is powered by induction (with power
requirement even 0.01 mA or less) and oscilating at 20 Hz. How
would you do that?

rfid tags pick up one rf frequency and then use that to power their
circuitry to respond on a different frequency. anti-theft labels in retail
stores are even simpler and do a similar job but without the smart data of
an rfid tag. adding a diode or two to a simple loop of wire you can double
the frequency. you can also generate subharmonics of the exciting current
with a properly tuned resonant circuit and a diode or two.

Also is it only magnetic field that can be used for contactless
induction. How about electric field. Can electric field be used
for contactless induction too? The former is Faraday law of
induction, what's the second called (if at all)?

yes, an electric field can be used by having two plates instead of a coil.
as the electric field changes it will produce a voltage difference between
the plates that can be used to power devices. i think some power utilities
use this for 'unpowered' devices on high voltage lines. many years ago one
of the 'popular' magazines published a free power motor that ran on the
clear air field gradient.