Building an MSD-CD

[Paul E. Campbell]

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