RQ Spark Gap
SRSG Xenon Timing Light
Radio Shack strobe
Synchronous Rotary Spark Gap

Spark Gap
 
The ubiquitous cylindrical spark gap.

Parts:
6" Dia. X 5" long, high density PVC pipe(blue)
12 - 1" X 3" Hard copper pipe
3" Dia. X 4" long PVC for air dam (white)
High volume muffin fan (not quiet type)
Misc. wood scraps

   This one has interchangeable parts. The parts are:
1. the fan + cover
2. the base + air dam
3. cylindrical spark gap
I can test gap changes easily and clean the gaps.
 I found it easiest to drill the 6" plastic pipe with a Dremmel tool. Using a router bit to cut slots for adjusting the gaps. Just make sure to use star type lock washers inside and out. For a permanent setting some sort of glue could be used.
 The fan is glued to the 1/8" masonite cover, and the cover is set on the pipe. The 4 mounting screws protrude about 1/8" and just fit inside the pipe. So the screws serve as tabs to hold the fan cover in place. 


Timing Light for Synchronous Rotary Spark Gap (SRSG)

 My timing light design is a single flash rate, variable phase angle stroboscope. It is intended for use with SRSG's, it will not be effective for Asynchronous rotaries.
Features:


Using the timing light:
  If you don't know the current phase adjustment of your 120 BPS SRSG them just point the strobe at the SRSG and adjust the PHASE pot until the rotor freezes. Read the phase angle off the scale.
  For 240 BPS SRSG's, the first setting that freezes the rotor will be on every other electrode. You can then readjust the PHASE pot to see the other set of electrodes and read the scale setting for the second set. This might be useful for unevenly spaced electrodes.
  If you use a variac to vary the phasing of your SRSG then the strobe is useful to determine exactly how much adjustment you are getting, and the point at which the motor looses sync. If the variac gives 20 degrees of lag, then you may want to put the initial setting of your rotor  at 10 degrees before the center, giving 10 degrees plus or minus.

Circuit Description and setup:
  The high voltage side is a simple (and common) line derived, cascaded voltage doubler. This charges a 350 uf photo flash cap to provide the DC power supply. The DC supply is used to charge the SCR trigger pulse cap, the Trigger transformer pulse cap, and the flash tube photo flash cap. CAUTION: This circuit is not isolated from the AC line. It is very dangerous. If you are not familiar with this type of circuit, don't build this project!
  The low voltage control is an LM555 timer chip, whose trigger is synched to the zero crossing point of the AC line. The PHASE pot controls the delay, from the zero crossing point, at which the strobe fires, and thus the phase angle.
 There is only one setting on the trim pot that allows a smooth transition from one end to the other. This is best set using an oscilloscope.
 The schematic does not show the toggle switch or the 5 Amp fuse required for safe operation.
 
Download using Windows 95 or 98
Click on thumbnail. When larger image loads then right click to bring up menu. Click on "save file as".
front panel drawing here  This scale works for the components shown above. It would be best to verify using an oscilloscope. You can use a paint program to print to proper size.

Here are the guts. Front panel.

Radio Shack strobe
 

Here is a modification to the Radio Shack strobe.
 Using a simple wall type light dimmer, this will give a variable phase angle adjustment.
 Disclaimer: I havn't actually tested this, as I don't have a Radio Shack strobe and don't care to buy one. This "should" work at 60 flashes per second.
 Download instructions:
 The picture is sideways so it will print out better. Just click on the thumbnail on left.   When the larger image appears, right click on it, a menu will appear. Click on "save image as". 
 Email me if the modification instructions aren't clear.


SRSG
 Choose the right motor. If you have a choice, don't use a motor with sleeve bearings (bushings). The shaft on a sleeve bearing motor can have a fair amount of end play. This causes the shaft to "pull in" when power is applied, causing misalignment of the electrodes. It is possible to minimize the end play by using proper sized fiber washers as spacers on the motor shaft. These will, eventually, wear down and the end play will return. Since one usually doesn't rack up many hours on the motor, fiber washers are a reasonable alternative, especially when, in my case the motor was free.
 The thing to do is use a motor with ball bearings. The ball bearings are press fit on the motor shaft, holding it steady. The only end play is the amount of play in the bearing itself. If your not sure how to identify ball bearing motors one simple method is the look for oil holes at the ends of the motor housing. If the motor has oil holes it is not ball bearing.
 Sizing the motor.
 Most coilers seem to prefer 1/3 to 1/2 horsepower motors for mediun size SRSG's. Alot depends on the diameter and thickness of your disk. The number of electrodes will greatly increase drag on the disk.
 What size disk? A small disk(4-6") will be suitable for lower power levels, approx. up to 2000watts max, I have used as small as 1/40 horsepower for these. Meduin disc(6- 10"), 1/3 to 1/2 horsepower to run smooth. Larger, thicker disks need 1 or larger horsepower.
 The balance of the disk will greatly affect vibration and general reliability.
 If in doubt it would be advisable to make a disk mock up, using lexan or similer material before cutting your expensive FR-4 or Linnen phenolic.
Electrodes
 Tungsten is the best. Tungsten TIG welder electrodes are available at all welding supply stores. They are only about $5 - $8 each. To cut them, just nick it with a file or cut off wheel. Then the electrode will snap off at the nick. The ends can be cleaned up with a grinder wheel.
 Brass works for lower powers.
Construction:
Photos are thumbnailed. Click to enlarge.
 This is the modified armature on my micro SRSG. It is a 1/40 horsepower Bodine ball bearing motor.
Specs:  diameter 1.65"   4 flats cut at .50" 
The flat width is about 1/3 the diameter if the armature. A good rule of thumb to follow.
 This photo shows the start windings(brown) and run winding(red). One flat can be seen at tha top, under the start winding. This is sometimes called the dead zone. Some advocate measuring the width of this zone and grinding flats to match. My experience is this may make the flats to wide. 
 The front ball bearing is firmly pressed on the shaft, eliminating any end play in the motor. The other noticable difference with ball bearings is the overall smoothness of the motor. I can't recommend ball bearing enough.
 This is the completed unit. It is mainly plexiglass with LE phenolic for mounting the electrodes. The 3.75" disk is also LE phenolic.
 The steel collar to connect the disk to the shaft was made by a local machinist. Cost $18.00. It is .75" diameter with double set screws at each end. 
 Since this photo was taken I have moved the copper ring to the other side of the disk to prevent arcing. I have also added heatsinks to the stationary electrodes.
Electrode gap = .008"(8 mils)

 Most of the SRSG construction was done with hand tools and a drill press.
 The motor was made synchronus by grinding flats on the armature with a side grinder. Some final smoothing was done with a coarse file.
Horray! The old motor died. So I had to get a better one from the scrap yard. Photos below are the new improved motor.
 
 The base is 1/4" steel plate. Everything is solidly bolted to the heavy base.
 The motor phasing is adjusted by loosening the clamps and rotating the motor in the cradle.
 That is the new motor.
 Rating are:  1/2 horsepower,  1625 RPM(before Mods), 120 volt
 I didn't want to invest in a chunk of copper to make a disk so I just used brass strips to connect the electrodes.
 The flying electrodes are 1/8" -  2% Thoriated tungsten TIG welder electrodes.
 The electrode holders are 1/4 X 20 brass bolts. I just drilled a hole through the center of the bolt with a drill press. Then I drilled a hole through the bolt head and tapped it. The electrode is held secure with a 4-40 set screw in the bolt head.
 Each electrode and bolt was weighted with a triple beam balance, excess material was removed until they all matched.
 The stationary electrodes are 3/32" - 2% thoriated tungsten TIG welder electrodes. It is secured with a 6-32 screw.
 Those tombstone shaped hunks of brass were part of an X-Ray machines I had.  The brass connectors are male Tweco welding cable connectors. I just drilled a hole in the brass hunk and tapped it with a 1/2" tap. The male Tweco was threaded with 1/2" threads. It is secured with a 6-32 set screw. I bolted a chunk of aluminum heat sink to the brass for cooling and to help prevent vibration.
I was lucky. The belt pulley on the motor was perfectly flat on the outside so I could use it for a mounting flange.