Transcranial Magnetic Stimulation (TMS)
is a technique to stimulate brain tissue directly through the skull. It works by sending a very high current pulse
through a coil that is located on the subject's scalp. The fast-rising edge of the pulse induces a
current in the brain tissue, causing neuron stimulation.
This device uses a capacitor bank, high
current SCR, trigger circuit, figure-8 (or butterfly) coil, and high voltage
charging circuit.
hey everyone I
thought I'd show you what
I've been working on I was inspired to
do this project by those ultracapacitors
that I showed in a previous video so I
mentioned maybe using them to build a
coil gun or a spot welder but another
idea got into my mind this is a
transcranial magnetic stimulation device
and I'll let you look it up if you're
curious about the details but basically
it's just a very large capacitor bank
that's discharged through a coil and by
holding the coil near your head you can
induce an electric field in the brain
tissue which causes a neuron stimulation
so let me just show you how it's built
this is a trigger circuit and we've got
a smaller capacitor five five five timer
and a switch to start the timer and
transistor to dump current into the gate
of this monster silicon controlled
rectifier so these hockey-puck SCR s can
handle huge amounts of current and even
better they can handle huge amounts of
rates of change of current so I'll put a
link to the specs on this one up in the
video
I think it's rated at 1200 amps and it
can it can block about four and a half
kilo volts the capacitor bank is made up
of two strings of five capacitors each
capacitor is 400 volts at 470 micro
farad's and I'm charging the capacitor
bank through here and on the other end
of this cable I've got the high voltage
supply for one of the parts of the
electron microscope so I can choose a
voltage anywhere from 500 to 2,000 volts
and these resistors here limit the
charging rate and so far this is
actually posed the biggest problem with
the circuit so far so let me turn it on
I'm going to turn on the power voltage
supply and wake my meter back up we're
monitoring the charge voltage through
this high voltage probe so 1 millivolt
equals 1 volt so since we've got one
volt here we've got a thousand volts on
the
passenger bank and if I hit the trigger
I get a nice large crack as the coil
shakes and then the voltage comes back
up to about a thousand volts so the is
the idea is that you would hold that if
the reason that we're using this
butterfly shaped coil by the way one of
these goes clockwise the other goes
counterclockwise is that it creates a
peak magnetic field in the middle here
it's a much sharper focus so that
instead of just using one of these coils
which produces sort of a cloud of
stimulation with this you can actually
move the focus around quite a bit more
easily so I'll show me actually using
this thing in a future video
I haven't actually gotten very much
neuron stimulation yet because I haven't
taken it over a thousand volts there's a
few more refinements I want to make like
I say those charged resistors are the
biggest problem so far but I have been
measuring the discharge current just by
measuring the voltage flow through the
ground part of the circuit here you can
see I've got the oscilloscope probes
just right across that piece of copper
wire so by measuring the voltage
differential across there I can measure
the amount of current during the
discharge so I used my milli ohm meter
to figure out that it's one milliamp
across here then I can just multiply the
the voltage by a thousand and get amps
all right well let's run another you can
see it takes about a second or three to
charge I'll give you some round numbers
that I found in the literature in case
you're curious what the circuit is doing
this combination coil here should have
an inductance of about twenty micro
henries a 10 for each coil and I didn't
measure that it's just sort of
calculated and also according to the
literature the capacitor bank should be
about 100 microfarads at two kilovolts
although it could be much higher the
rate of current change in the literature
is often cited to be around 10 amps per
microsecond so and the peak current is
often about between five and ten
thousand amps
so the post period lasts a couple
hundred microseconds and peaks at you
know five to ten thousand amps this
should produce a field of maybe a few
thousand Tesla's per second it a change
in magnetic field of that and that's
that change in magnetic field is
actually what induces the the current in
inside the brain tissue so the reason
that the ultra capacitors wouldn't work
for this project is because the
inductance of this coil resists current
flow through it resists a change in
current flow and to overcome that
inductance we need a really high voltage
so the ultracapacitor bank only got up
to like twenty or thirty volts or
something and even if you have a
essentially no resistance between the
capacitors and this coil if you close
the switch there's there's not going to
be nearly enough current flowing because
the inductance actually prevents it so
the benefit with or mean that the
necessity of having a higher voltage is
to overcome the inductance of the coils
there same thing with a coil gun it
actually would not work with the ultra
capacitors because the inductance of the
coil inside the coil gun would just
overcome that voltage from the ultra
capacitors so you really need a higher
driving voltage in order to get a really
high high speed changing magnetic field
okay well in the next video I hope to
refine this a little bit more and show
you it in use so stay tuned for that
okay see you next time bye
s
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