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    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.

    Ignition Coil Driver Schematic

    555 Ignition Coil Driver Schematic

    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.

    The circuit

    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

    Q1: 2N4401
    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.

    ask a question

    23 Responses to "555 Ignition Coil Driver"

    1. Note updated schematic–D2 is added.
      Its function is to conduct coil current when power switch is opened–otherwise it flows through U1 when Vcc reverses due to the collapsing magnetic field.

      • Hey, an I use this…or something like this in place of my VS1400 Suzuki MC’s CDI unit?
        Please advise. Thank you.

      • Jim Keith says: on June 21, 2015 at 2:00 pm

        This produces a repetitive spark and is not suitable for timed engine applications. Soon, I want to publish a circuit that uses mechanical points to produce timed ignition pulses. Such needs an ignition coil that is designed for inductive discharge applications–while inductive discharge coils may function well in CDI applications, the CDI coil is not likely to work in inductive discharge applications.

    2. Is there really enough protection on the 555? From what I’ve been reading, there’s voltage transients and load dump and reverse-voltage to worry about under-the-hood.

      I’d love to hear some feedback, as I can’t decide if I need to go all out and implement something using a MAX16126 for a 555 PWM fuel pump motor controller I’m working on. Seems like there should be a reliable and cheap way for a circuit that is pretty flexible/resilient to voltage in the first place (555 can tolerate up to 18V).


    3. Looks like you have the 555 pin labels reversed on the schematic, for pins 7 and 3 ? Pin 3 is the output?

      • This is a valid way of using the 555 outputs–although not in the application notes, I have used it successfully many times.
        Will post a article on this useful detail soon–it will be of interest to all 555 buffs.

    4. I’m new to electronics so forgive me if this is a dumb question. Is Q2 two transistors or one? I’m seeing two front to back.

    5. My ignition coil has only a positive and negative terminal so how do I wire that up to your schematic? Also do I need a coil with the internal resistor?

      • The positive terminal connects to either the limiting resistor or +12V. The stencil on the side of my coil indicates: “USE WITH EXTERNAL RESISTOR.” If not so indicated, use without the resistor.

        Regarding the two transistor symbol, this is called a Darlington connection and, yes, it is two transistors in one package. Darlingtons have high current gain so that the required gate drive is low. Since high voltage transistors generally have very low current gain (hFE or Beta), a Darlington configuration is often used to increase the current gain to a more reasonable value.

    6. Thank you Jim. If I cant find that particular transistor, can you recommend a suitable replacement?

    7. jamesalx@bellsouth.net says: on October 19, 2013 at 4:37 am

      I want to turn an old Lincon 235 welder into a tig. I need a hf unit I am using a coil with a step down transformer wound on a ferate core to get the spark and wind a choke to keep the rf out of the cracker box I have some n channel mosfets for current control and an LT 4320 (serface mount) driver I am not an electronics guy, I can read a schematic I worked on large hv switches ac/dc motors/generators sets but mostly just changed cards in the regulators when they were sick then our engineer would repair the cards . However I do want to learn. Oh, lt states 300degs is the absolute max for the driver where can I get solder? thank you for the circuit.

    8. Excellent circuit! I just built it and it works. I have two comments:

      1.) The darlington transistor (RCA SK9109) dissipates some 10 W and requires a big heat sink (4 K/W or better). I think it is in part due to the clamping circuit. I would expect that a fast external freewheeling diode could help to reduce losses. The resistor on the other hand barely gets warm.

      2.) I find it difficult to reliably ignite (nearly stochiometric) air-propane mixtures at room temperature and atmospheric pressure. In an attempt to increase the spark intensity I increased the frequency, removed the series resistor, and increased the supply voltage to 16 V. This helped only marginally. Any other ideas? The ignition coil is brand new and should be pretty standard. I wonder if C3 could be chosen so that the circuit resonates at its natural frequency?

      • A free-wheeling clamp diode will prevent the coil from generating a high voltage. I am not familiar with the SK9109– even though it is a darlington, its hFE may not be very high– probably need more base drive to saturate the transistor– reduce the value of R5– this alone will improve the spark and reduce transistor temp rise.

        2. I have no clue as how to ignite your mixture. C5 is simply a bypass capacitor– this circuit does not resonate the coil primary inductance so the value of C5 has no effect.

        Good luck

      • Jim,

        Thanks for your response. You are 100% correct: There is not enough base current to saturate the darlington transistor that I found (I got it from the local electronics store; they said it’s the correct replacement for the BU323Z). The sparks got considerably more powerful with reduced R5 — they easily ignite the air-propane mixture. And the darlington does not get quite as hot. I think I will order a BU323Z online. Thank you for your advice. Happy New Year!

    9. I really appreciate your ingenuity with the inverted NE555 configuration.

      But regarding that inductive ignition was superior vs capacitor discharge. I have designed a multiple spark cdis a placed a manual transfer switch between the Inductive Ignition with Igniter driver. I find my mcdis have better engine performance than Inductive Ignition with Igniter driver. And its true that CDIS have more components and very complicated circuit to build.


      • Hi Roger, thank you for your comment.
        I am working on an enhanced inductive discharge system that is adapted for older engines with breaker point distributors. It will have superior high RPM performance due to two advanced inductor charging techniques. Unfortunately, I have no engine to test it on as my last breaker point vehicle was a Nova Bodied ’73 Buick with a 350″² engine that is long gone. It may be a match for CDIS …

      • Hi Jim,

        I’m searching for information about point ignition and reading the responses I see that you are working in a circuit for old engines with a points breaker ignition.
        You just have this finished? If is possible I want to see.

        Thanks in advance


    10. Dual Ignition Coil was great idea. I really believe that it will work and match cdis. There’s still a lot of old contact point car here in Manila, Philippines. We can try it here. LOL

    11. Hi, I think that Q1 must be a pnp type in order to avoid R5 to waste energy in the off cycle of Q2…. Don’t you think so?

      • Good observation–yes, the driver may be made more efficient using a PNP transistor. However, the 25mA wasted is hardly significant in a 4A circuit. One advantage of doing it with an NPN is that Q1 assists in the turn-off off of Q2 by grounding the base thus sucking the carriers out –reduces storage time.

    12. Hi again. Can you say me what is the max amps in the primary winding of the ignition coil? Thanks.

      • Jim Keith says: on December 19, 2014 at 5:39 pm

        The typical coil draws about 4 to 5A.

        High performance coils draw up to 10A or so –the higher current is not necessarily for increased spark energy, but for half the ampere-turns that reduces the inductance by a factor of 4, thus greatly reducing the charging time sorely needed for high RPM performance.

    13. Hello sir we are interested to manufacturing the ingition coil system with compete sheet metal components.pl.suggest and help me Thank you with regards.

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