This ignition coil driver is a HOT one! From my recollection, it delivers a nastier spark than the legendary Ford Model T ignition coil. The circuit uses an inverted 555 oscillator that is coupled to an ON Semiconductor BU323Z Darlington transistor (350V, 10A) that drives a conventional inductive discharge ignition coil. In this topology, the inductive discharge voltage developed across the transformer primary is multiplied by the turns ration (factor of 100) to easily deliver a 25kV voltage to the spark gap.
Background of the inductive discharge ignition system
Developed by the brilliant automotive engineer Charles Kettering over 100years ago, the inductive discharge ignition system continues to be the method of choice and is used on most automobile engines today. The past 50 years has brought a number of improvements. 1. In the mid 1960’s, the automotive aftermarket industry offered transistor ignition products that replaced the breaker points with germanium power transistors. 2. Industry wide, breaker point contacts were eliminated in the mid 1970’s when high voltage silicon transistors came of age. This eliminated the requirement of the capacitor across the primary switch thus increasing the output energy significantly. 3. In the mid 1980’s when microcontrollers came of age, the ability to optimize the dwell (charge) time substantially improved high speed performance. 4. More recent improvements include multiple inductive discharge ignition coils (one for each cylinder) thus eliminating the requirement for the distributor and further improvement of high speed performance.
Inductive discharge vs. capacitor discharge
I have never seen an intelligent discussion of the pros and cons of inductive vs. capacitor discharge techniques. While some say that capacitor discharge is superior, the issue is actually complex.
The major drawback of early inductive discharge systems was that the inductor charging time was the limiting factor at high RPMs where the reduced dwell time reduces spark energy. However, there is a little known advantage to inductive discharge—this is the high rate of change of the discharge current (di/dt) that far exceeds that of the capacitive discharge system. This occurs because the flux is already present in the magnetic circuit so all that must occur is that the current be transferred from primary to secondary. This high rate of change of current causes the leading edge of the ignition pulse to be superior. Also, the inherent simplicity and reliability of the inductive discharge system is unparalleled. In the muscle car era (1960’s & 70’s), the capacitor discharge system brought significant high RPM improvement, but today’s advanced inductive discharge systems may actually be superior.
I used a variation of the inverted 555 oscillator to provide proper output signal polarity. http://www.electroschematics.com/7114/inverted-555-timer-circuit/
While the circuit could have been designed without Q1 using the specified high hFE transistor for Q2 (BU323Z), the transistor I actually had on hand was the low hFE BUW13 that required the additional stage. As is, the circuit is able to accommodate either device.
How it works
Q2 is normally turned via the base drive flowing through R5 so the coil primary current integrates to maximum. The 1Ω series power resistor is required because the coil is rated for 9V—this reduces the power dissipation inside the coil resistance. 1mS pulses from the 555 timer repeatedly turn on Q1. Q1 subsequently removes the base drive to Q2. The 555 operates over the frequency or pulse repetition range of 10 to 200HZ. When Q2 turns off, the collector voltage spikes to about 250V as the inductor attempts to keep the current flowing. The secondary voltage is equal to the primary voltage times the turns ratio (100) thus resulting in a secondary voltage of 25kV.
Differences in the circuit that was actually tested
Q2: BUW13 (non-darlington power transistor, 15A, 850V)
Clamp diodes: Visible in the photo, but not on the schematic are a pair of 200V silicon transient suppressor diodes (1.5KE200) in series. These were added to protect the power transistor—I did not want any device failures…
R5: 40Ω, 12W
R4: 510Ω, 0.5W
Recommendation for Q2
The On Semiconductor BU323Z high voltage power Darlington transistor is currently available from DigiKey for $2.78 each. Integrated into the device is a 360V clamp zener device that turns the transistor on in the event the voltage becomes excessive. Data sheet link: http://www.onsemi.com/pub_link/Collateral/BU323Z-D.PDF
DigiKey offers a number of other suitable devices—you can do the research on their powerful parameter search engine.
Other potential devices include the obsolete BUW13 or BUV48 transistors that are available on eBay at a reasonable price—if used, note the circuit differences in the previous section.
How much voltage can the circuit generate?
That is your problem—as the spark gap dimension is increased, the required breakdown voltage also increases. Eventually, the current will flow in an undesired path—arc across the coil HV terminal to the primary terminal—coil destruction (internal arc)—power transistor failure. While the BU323Z boasts internal protection, make sure you have a spare—Murphy’s Law comes into play especially when there are no spares…
Per the oscillographs, the VCE actually reaches 400V peak—I do not know if the clamp devices absorbed any current. In my car ignition system, the VCE runs at a more reasonable 250V peak. Also, I was unable to understand the secondary current waveform so I conveniently omitted it—the data may be bogus. Strange things can happen when attempting to instrument high power plasma discharge. I also suspect that my coil may be defective is some way—the output pulse was of negative polarity, indicating that it was not connected internally as a true autotransformer. However, my experience on this detail is so limited that I do not know what to expect.