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From: arthurok at ix.netcom.com (ARTHUR OKUN )
Subject: Re: Building an MSD-CD
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You wrote:
>
>Okay, I went down to the library and got everything I could on high
voltage
>stuff...
>
>First, let's look at a spark gap. Essentially, you've got an
insulating gap
>(of air in this case). The conductivity of this gap is essentially
miniscule
>under normal conditions. When you hit a particular voltage (which
pretty much
>has to be determined experimentally) or higher, then the electrons
will go
>into a type of "avalanche" mode where the conductivity of the gap goes
way up
>(alternatively, the resistance goes way down). We won't have electrons
>accelerated fast enough to make a difference, so for our purposes, the
time
>it takes for the gap to go into breakdown/avalanche mode is about
10^-8 secs.
>(about 10 nanoseconds). After this occurs, the air affected in the gap
will
>be in a plasma-like state.
>
>Second, all capacitive high voltage impulse type circuits work by
feeding
>the output of a high voltage power source to the capacitor and
charging it up.
>When the voltage reaches the breakdown voltage of the spark gap, the
capacitor
>rapidly dumps the charge it held. So this means we really need a high
voltage
>DC-DC convertor (boost convertor). These types of circuits are also
known as
>charge pumps. These things look like this:
>
> +----------+---Inductor-----+-----Diode >|-------------+--------Spark
gap--+
> | | | |
|
> + | | |
|
>Battery Capacitor 1 Switch Capacitor 2
|
> - | | |
|
> | | | |
|
>
+----------+----------------+--------------------------+----------------
---+
>
>Okay, this is the basic circuit. I've deleted the usual control
circuitry
>since we're not going to have to worry about creating a DC voltage of
X at
>say +/-0.1 volts (this would be some sort of sensor or transformer on
the
>high voltage side). Capacitor 1 is just there to smooth out the
battery
>power.
>
>Okay, close the switch. The inductor starts charging up. Now open the
switch.
>The collapsing magnetic field in the inductor continues to conduct via
>capacitor 2. Continue toggling the switch until the spark gap reaches
>breakdown and drains the capacitor. This is pretty much identical to
the
>old flyback circuit in television sets. In practice, you'd use a high
voltage
>transistor (power FET or IGBT) in place of the switch and add some
digital
>controls to it for gating and such.
>
>If you are a mechanical engineer, the inductor and capacitor store
energy.
>Since the inductor is a coil of wire, it takes time for the flow in
the
>inductor to reach the peak. The diode is essentially a "one-way
valve". The
>Switch is a "2-way valve" and the battery is a pump. We build up a
good flow
>in the inductor, which acts like a piston; it builds up inertia (not
sure what
>the fluid term for this action is). Then open the switch (2-way
valve). The
>capacitor is more like a dam (holds whatever head we build up in it).
So
>the flow continues out of the inductor and builds up in the capacitor
as long
>as there is still inertia remaining in the inductor to drive the flow.
>Successive cycles of the switch build up a higher and higher head in
the
>capacitor until the "dam" collapses (the spark gap fires), causing the
charge
>that built up in the capacitor to flow out.
>
>There is another variation on this circuit, as follows.
>
> +----------+---Switch-------+ +--Diode >|-------------+--------Spark
gap--+
> | | | | |
|
> + | | | |
|
>Battery Capacitor 1 Transformer Capacitor 2
|
> - | | | |
|
> | | | | |
|
> +----------+----------------+
+-----------------------+-------------------+
>
>In this case, the switch-inductor circuit acts as before on the
transformer
>primary. When the field collapses, the transformer secondary feeds
charge
>into Capacitor 2. The difference is that the turns ratio of the
transformer
>gives you an additional voltage multiplication. Other advantages are
isolation
>and in DC-DC convertors, the transformer version can be smaller than
the
>inductor version (not sure if this applies for this application).
>
>There is another version called the buck-boost convertor. In this
case, you
>can get high negative voltages rather than positive voltages.
>
> +----------+---Switch-------+-----Diode |<-------------+--------Spark
gap--+
> | | | |
|
> + | | |
|
>Battery Capacitor 1 Inductor Capacitor 2
|
> - | | |
|
> | | | |
|
>
+----------+----------------+--------------------------+----------------
---+
>
>Notice that the only real change is the diode is reversed. In this
circuit,
>the battery is out of the loop when the switch is open. The inductor
conducts
>as before but it reverses the flow in the capacitor-spark gap circuit.
>
>There are also voltage multiplier ladders made out of spark gaps and
>capacitors and also voltage multiplier ladders made out of diodes and
>capacitors, but they are generally not as efficient in their power
usage as
>the circuits above.
>
>When the above circuits are used as DC-DC convertors, Capacitor 2 is
enormous
>so that the voltage ripple is minimized. The spark gap becomes a
normal load.
>Even computer "switching power supplies" use similar convertors to the
above
>circuits. A pair of resistors is attached on the "high voltage" side
or as
>an additional small secondary on the transformer. This is used as the
input
>to the control circuit. The control circuit usually varies the duty
cycle
>(on-time to off-time of the switch) although some versions vary the
frequency.
>The resulting voltages are as follows:
>
>Boost convertor: Vout=Vin/(1-Duty Cycle)
>Buck-Boost: Vout=-Vin*(Duty Cycle)/(1-Duty Cycle)
>Transformer Coupled Boost Convertor: Vout=Vin*N*(Duty Cycle)/(1-Duty
Cycle)
> where N is the turns ratio
>
it shouldnt be that difficult to build a simple dc-dc
converter(12v-~300vdc) and a coil switching circuit with cheap tv type
components the trick is to put a capacitor accross the output
transistor junction just like the condensor in an old fationed
kettering type ignition circuit thats also done in tv h.v.supplies
which i am familar with
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