Re: Basic Stamp Controlled Spark Gap

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

At 05:42 PM 5/12/2003 -0600, you wrote:
>Original poster: "Jeremy Scott by way of Terry Fritz 
><teslalist-at-qwest-dot-net>" <supertux1-at-yahoo-dot-com>
>
>Hi,
>
>I'm designing a basic stamp controlled rotary spark
>gap that will be able to decide for itself the
>optimal timing for gap electrode presentations.
>
>Mechanics:
>
>The gap will have 1/4" tungsten electrodes, either
>four or eight of them depending on what it's capable
>of spinning.
>
>The following will be controlled or read by the stamp:
>(Feel free to comment if you know of a better way!)
>
>Tachometer feed back - 1/4" a hole in the disc will
>break the beam of IR light and drive a phototransistor
>hooked up to an input pin. This will create a stream
>of pulses that can be measured. RPM and electrode face
>time will be calculated in the stamp. The hole will
>be some fixed number of degrees away from an
>electrode, the basic stamp will use this (along with
>RPM) to calculate exactly 'when' a rotating electrode
>passes
>a fixed point.

If the motor has a shaft that sticks out the other end, put your optical 
encoder there.. keeping it away from the high voltage/high current 
circuitry of the primary.  You might also be able to pick up pulses from 
the internal cooling fan of the motor with a suitable sensor.  Most motors 
have some sort of fins cast into one or the other end of the rotor.


>Speed control -  H-Bridge push/pull type circuit
>using power MOSFETS and an output from the stamp.
>This and the tachometer feedback will keep the
>motor running at a consistent speed by varying
>the frequency of AC. This should compensate for
>any desyncronizing drag of the disc. How fast
>it rotates is yet to be determined. Initially
>planning on 1800RPM with eight electrodes for
>up to 240BPS.

Since the motor is unidirectional, all you need is one switching device.. 
you don't need 4 quadrant operation (i.e. reverse or braking)...



>Phase control - R/C type servos will rotate the
>'fixed'
>pair of electrodes (connections to the power supply)
>anywhere from 0 to 90 degrees in fractional degree
>increments.

Why not shift the rotor motor speed and phase... rather than moving the 
electrodes. no extra servos needed. Imagine you've got the motor synced up 
at 1800 rpm and at phase=0... if the motor is slowed to 1799 rpm, it will 
roll through 360 of phase shift relative to power line in 1 minute.. 6 
degrees/second.. so, if you wanted to run at 18 degrees behind, you just 
run at 1799 for 3 seconds.. in reality, this is just a phase locked loop, 
and you don't need the step functions...





>Timing signal -- this is the hard part, it involves
>two things: a) figuring out where in the capacitor's
>charge cycle the spark gap is firing,

An optical detector will do quite nicely for telling when the gap fires. 
very bright light, fast rise time.



>and b)
>quantifiying the capacitor voltage when this happens.

Yes, this is a bit of a challenge..