2.3 Low-Energy Capacitive Spark Ignition Systems
2.3.1 Short, Fixed-Length Spark Ignition System
The discharge circuit used in the short spark ignition system was based on the ideas of Ono et al. (Ono et al., 2005, 2007, Ono and Oda, 2008). The basis of the design was a simple capacitive discharge circuit, but many features were implemented to improve the system performance in terms of reliability, consistency, and repeatability so that the spark energy could be reasonably predicted and measured.
The capacitive discharge circuit consisted of a Glassman model MJ15P1000 high- voltage power supply (0–15 kVDC range) connected to two 50 GΩ 7.5 kV charg- ing/isolation resistors in series with a Jennings CADD-30-0115 variable vacuum ca- pacitor with a range of 3 to 30 pF. The capacitor was then connected in parallel with the spark gap, so that when the capacitor was charged to the gap breakdown voltage it would discharge through the gap producing a low-energy spark. The high-voltage power supply output was controlled by supplying a 0–10 V input voltage provided by a function generator. The function generator output a ramp signal that rose from 0 to 7.32 V in 50 seconds, which caused the high-voltage power supply to output a ramp voltage increasing from 0 to 11 kV in 50 seconds. The ramp time was chosen to be more than 10 times longer than the maximum capacitor charging time. This choice of ramp time allowed sufficient time for the capacitor to charge so that the voltage could be measured at the output of the high-voltage power supply instead of measuring the voltage directly on the capacitor. It was important to be able to measure the voltage in this manner because of the extremely large isolation resistance (100 GΩ); if a probe with much lower impedance was connected directly in parallel with the capacitor, a voltage divider was formed where the probe draws the majority of the current. By using a sufficiently long voltage ramp to charge the capacitor, it was possible to meausre the capacitor voltage on the other side of the resistors. A Tektronix P6015A high-voltage probe was connected to the output of the power sup- ply to measure the capacitor voltage at breakdown, and the output of the probe was
digitized by a Tektronix TDS460A oscilloscope at a sampling rate of 1 MS/s. The spark current was measured using a Bergoz CT-D1.0 fast current transformer, and the current waveform was digitized by a second oscilloscope (Tektronix TDS 640A) with a sampling rate of 2 GS/s. The faster oscilloscope was triggered by the spark current directly and then triggered the second oscilloscope to record the breakdown voltage.
It was necessary to implement a high-voltage relay in the circuit to disconnect the capacitor from the high-voltage power supply after a spark occurred to prevent multiple sparks. A Gigavac GR5MTA 15 kV load switching relay was connected between the positive output of the high-voltage power supply and the first 50 GΩ charging resistor. The relay required 12 VDC to close, which was provided by a lab power supply and a Grayhill 70-ODC5 solid-state relay mounted on a Grayhill 70RCK4 rack. A timing diagram illustrating the triggering of the devices and the opening of the high-voltage relay is shown in Figure 2.3. A 4 V power supply and a delay generator were used to provide the logic inputs to the relay; the 4 V signal leaves the relay closed during charging, so that the high-voltage relay receives the 12 V signal and remains closed. When the spark begins, the current signal triggers the oscilloscope which in turn triggers the delay generator to open the solid state relay. This causes the high-voltage relay to open, disconnecting the charging circuit from the high-voltage ramp and preventing multiple capacitor discharges. A schematic of the circuit is shown in Figure2.4and the important circuit features are indicated in the photograph in Figure 2.5. All the circuit components were mounted on a 0.5 inch thick acrylic plate, and the resistors, capacitor, and high-voltage relay were mounted on Teflon standoffs to limit any leakage current. A round acrylic face plate was attached to the end of the circuit board to hold all the connections to the external power supplies, delay and function generators, and high-voltage probe. All electrical connections with corners or sharp edges were coated with high-voltage putty to prevent corona losses.
The spark gap, shown in the photographs in Figure 2.6, was constructed using brass and stainless steel rods that were threaded at the ends so that different electrode tips could be used. One of the brass screws was mounted in a piece of fiberglass in
VBreakdown
4 V to solid state relay
→HV relay open
1 s
falling TTL to delay generator and 2nd oscilloscope (measuring voltage) spark triggers oscilloscope
35 s
50 s HV Power
Supply
Spark Current
Oscilloscope
Delay Generator
falling TTL triggers LabVIEW VI and camera
Figure 2.3: Timing diagram illustrating the triggering of the oscilloscope and the opening of the high-voltage relay after the spark discharge
front of the other electrode tip on a stainless steel extender rod. The spark gap could then be adjusted by threading the brass screw further in or out through the fiberglass.
The brass screw and extender arm were mounted on brass rods fed through Teflon bushings in a circular fiberglass plate, and on the other side of the plate high-voltage leads were attached to the rods for connecting the spark gap to the discharge circuit.
The fiberglass plate mounted on an aluminum fixture that held the spark gap on one side and the circuit board on the other side. The fiberglass plate, teflon bushings, and feed-through rods were all mounted using O-rings ensuring that the assembly was vacuum tight. As with the circuit board, all sharp edges on the connections were insulated with high-voltage putty. An acrylic tube enclosed the circuit board and air from a desiccant dryer was pumped through a connection in the face plate and into the enclosed circuit. The dry air was necessary to control the humidity so that the extremely sensitive high-voltage components, particularly the capacitor surface, remained dry while testing to minimize leakage current. Every time the tube was removed and adjustments to the circuit made, the surfaces of the resistors, capacitor, and Teflon parts were cleaned using isopropyl alcohol. The spark gap side of the aluminum fixture fit through a flange on the combustion vessel and clamps were used to hold the fixture against the flange with an O-ring seal. The spark ignition system
Current Transfomer (Bergoz CT-D1.0) 3.7 VDC
15 kV Load Switching Relay (Gigavac GR5MTA)
50 G7.5 kV
50 G7.5 kV 3 – 30 pF
Variable Vacuum Capacitor (Jennings CADD-30-0115)
Spark Gap High Voltage
Power Supply
- 0 - 15 +
kV DC
12 VDC
Opto- Isolated
Relay (Grayhill 70-ODC) VCC
GND
CONTROL + 4 VDC
from Delay Generator
(Glassman MJ15P1000)
Voltage Ramp from Function
Generator
To oscilloscope (spark current) To oscilloscope (to measure voltage at
breakdown) HV Probe (Tektronix
P6015A)
Figure 2.4: Schematic of the short, fixed-length, low-energy spark ignition system
mounted on the combustion vessel is shown in Figure 2.7.