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'[OT] MOSFET paranoia'
Hi Paul, this is what I think, I could be wrong.
The old theory says that changes in magnetic field generate current at
any electric conductor inserted in that field. I believe it is reversed;
"When crossing magnetic field lines, electrons in an electric conductor
will be pushed to one side of that conductor".
The stored energy in a steady magnetic field can be considered a result
of energy transformation, so the current you see at the coil after
removing power is in real back induced current. If this is not true,
than a loaded secondary coil stealing energy from the magnetic field
(previously generated by the primary coil) would be creating energy,
what is not possible.
A good example is feeding an auto-transformer bigger coil with 1A, the
same 1A will be crossing the smaller coil. Removing power and if the
reverse diode is installed only at the smaller coil, you will have a
current through the diode higher than 1A, so I think it is not just a
decay current, it is an "induced" one.
+V o----o o-------.
^ S bigger
| S induced
| o----------. current
| S | |
load S --- |
current S A v
I use to think that when a magnetic field is collapsing, the power used
to generate the field would be reapplied (with some losses) over the
coils. A changing magnetic field will push electrons away from their
atoms toward one side of the conductor. If there is a path it will be
keep pushing away (current), lower the load impedance higher the
current. It doesn't matter if there is just one or a thousand of wire
turns, with load it will try to consume all the stored energy from the
Without load, electrons will be just accumulating (voltage) more and
more, creating a high voltage at the extremes of the coil, then, after
the field collapsed completely electrons return back to their original
atoms (well, something like that), wasting energy as heat. So the
collapsing field energy will be consumed in any way, with or without a
The "induced current" here is the same as moving a permanent magnet in
the middle of a coil.
"Paul B. Webster VK2BZC" wrote:
> That is woolly. There is no "back induced current", just one that
> Paul B.
'[OT] MOSFET paranoia'
Jeffrey D Spears
|I saw a demo in an ElectroMagnetics course that illustrated this. The
TA took an aluminium tube, about a meter and a half long and 3/4" ID.
First he dropped a cylindrical plug made of wood down the thing and it
slid right down like we would expect. Next he took a similiar sized
plug with a permanent magnet embedded inside and dropped it down the
tube. It took about ten seconds to traverse the length and drop out
The changing magnetic field, changing as the magnet falls down, induces
a current that circulates through the tubing. The current, which varies
at each point as the magnet falls induces a secondary magnetic field that
opposes the original one. All this serves to buoy the magnet as it
slides down the tube.
One interesting point made by this illustration is the induced magnetic
field must oppose the original one. If otherwise, the magnet would
shoot out the bottom at an increasing rate of speed (ala Cannon!) and
defy every principle of conservation of energy written by man.
On Mon, 24 Apr 2000, Wagner Lipnharski wrote:
Jeffrey D. Spears
University of Michigan
College of Engineering
``Double-E, can't spell gEEk without it!''
-Captain Gerald M. Bloomfield II, USMC
>shoot out the bottom at an increasing rate of speed (ala Cannon!) and
>defy every principle of conservation of energy written by man.
Men didn't write the laws of physics, or there probably would be such screwups.
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At 04:17 PM 5/1/00 -0400, Andrew Kunz wrote:
>>shoot out the bottom at an increasing rate of speed (ala Cannon!) and
>>defy every principle of conservation of energy written by man.
>Men didn't write the laws of physics, or there probably would be such
They'd just insist it was an error in the documentation anyway.
Are you an ISP? Tired of spam?
http://www.spamwhack.com A pre-emptive strike against spam!
Where's Dave? http://www.findu.com/cgi-bin/find.cgi?kc6ete-9
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I have not followed the recent MOSFET/motor switching threads
too closely, but do recall talk about PICs being reset by motor
switching, etc. I ran across the following design note while
surfing for info on noise in ADCs:
"Resistors Protect Microcontrollers From Capacitively Coupled Spikes".
16F84 was being used to control gates of MOSFETs [IRL530] which
provided two-phase variable-frequency drive to a motor. Switching
spikes were being fed back thru the parasitic capacitances of the
MOSFETs to the PIC and resetting it.
10K resistors inserted between the PIC and MOSFET gates fixed
the problem. Note - they don't show a lot of other protection
[catching diodes/etc] here, but use of the series Rs looks like
a good design idea in such ckts.
> "Resistors Protect Microcontrollers From Capacitively Coupled Spikes".
> 16F84 was being used to control gates of MOSFETs [IRL530] which
> provided two-phase variable-frequency drive to a motor. Switching
> spikes were being fed back thru the parasitic capacitances of the
> MOSFETs to the PIC and resetting it.
> 10K resistors inserted between the PIC and MOSFET gates fixed
> the problem. Note - they don't show a lot of other protection
> [catching diodes/etc] here, but use of the series Rs looks like
> a good design idea in such ckts.
I am really going to regret bringing this up but it is on my
That is a large resistance (10k) in series with typ 3nF
gate capacitance. I think it is fine the the app shown
because the frequency is low.
A very rough calibrated eye ball and rusty brain reckoning
would suggest that you are into serious linear (and hot
mosfet) territory much above 3 to 5 kHz.
I suppose one could calculate the highest value that
would suit ones application freq and see if that is enough
to protect the micro, otherwise zener protection etc or a
mosfet driver or another pre-output mosfet stage will be
Its just I have never seen this problem before using the
good old tried and true industry standards of 100 ohm
series resistors, gate to drain zener/tranzorb clamping
and back-emf diodes and I have been involved in a fair bit
of motor/transformer work up to 100 watts.
Uni of Auckland
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