Re: Basic Stamp Controlled Spark Gap

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

Original poster: "Crow Leader by way of Terry Fritz <teslalist-at-qwest-dot-net>" <tesla-at-lists.symmetric-dot-net>

Ooh, it's exciting to hear others are working on computer controlled coils
too. I'm steering the way of an old laptop and RS/232 digital I/O controller
for the brains.


Sent: Monday, May 12, 2003 6:42 PM
Subject: Basic Stamp Controlled Spark Gap


 > 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

This will probably need to be modulated so interference is not an issue.
1/4" may even be a bit large of a hole if you want real fine adjustment of
your timing or phase.

 > 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.
 >
 > 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

I made a leg of a H bridge and scaling it times 4 and making a real PCB for
it was more of a hassle than just getting a ready made bridge. It was not
shielded and has only "crashed" once so far. I'm sure if I put it in a box
and kept it more than a few inches from my spark gap, it would be fine
forever. Speed control has so far just been a linear taper variable
resistor.

 > motor running at a consistent speed by varying
 > the frequency of AC. This should compensate for

For a PWM drive, you need want a fixed chopping rate, and a variable on/off
timing for each clock cycle. Mine was 2.4kHz because it sounded cool when
connected to a speaker. The real controller I ended up using is also 2.4KHz,
but some are up in the tens of kHz.

 > 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.
 >
 > 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.
 >
 > 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, and b)
 > quantifiying the capacitor voltage when this happens.
 >
 > I believe this has to be done with an analog to
 > digital converter which constantly 'reads' a rectified
 > voltage from the capacitor. (Also the voltage across
 > the secondary windings of power transformer)

This will be intersting because the signals you want to measure are really
noisy and complex, unlike say the voltage on a battery as it slowly drains.

 > The voltage has to be scaled down and isolated, which
 > is the tricky part. Most likely I'll devise some kind
 > of voltage controlled oscillator which will send a
 > modulated tone over a beam of 38Khz IR light. Basic
 > stamp should be able to convert the tone (pulses) into
 > a digital value. The gap can then be adjusted such
 > that the maximum value is read from the capacitor just
 > before electrode presentation.

You may be able to turn this from a hardware into a software problem, and
change from measuring capacitor voltage which is probably going to be
complex and require mega-isolation, to measuring current flowing into the
capacitor from your power transformer with a current transformer on one HV
lead of your transformer. You can in software assume that:

- charging the capacitor to full voltage takes current. As long as current
flows from the transformer to the capacitor, it's charging, or discharging
providing reactive current back into the transformer, or you have some sort
of fault. We don't want this, we want to use this energy in the tesla coil,
not send it back into the mains.

- if current flow is zero, you have no power, the AC input is crossing zero
and he capacitor is fully discharged.

- if the direction of the current from transformer to capacitor is changing,
the peak AC voltage is dropping ( as the sine wave ramps to zero before
reversing) and it's a good time to fire that capacitor into your tank
circuit as it's fully charged and won't charge anymore.

I'm sure more logic is involved here, but software projects are never as
easy as they first look. Again, you're probably going to have to filter out
the RF noise and fun stuff like that as you'd need to with measuring the
voltage on the cap itself, but the possible advantage here is your are
measuring small currents with no complex isolation requirements, and direct
voltage readings may be useless. The lead from even a neon sign transformer
through some sort of magnetic core + a few turns of wire = instant current
transformer.

KEN