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Protecting Driver Transistor from Transformer/Coil Back EMF

Discussion in 'General Electronics Discussion' started by nube_Tom, Sep 20, 2013.

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  1. nube_Tom


    Sep 20, 2013
    I found a circuit on the web that uses a 555 timer feeding to 2 transistors; the final one a NTE261. The NTE261 controls a step up transformer/coil used in a snowmobile for the spark plug. I want to use the circuit to run an old hit-and-miss farm engine. The circuit did not work, so I made some changes and now it works.

    My question :confused: is; from what I have read, and believe to know, the coil field collapsing produces a return voltage spike (actually a few spikes) back into the NTE261 transistor. One is supposed to put a diode across a "coil" to prevent this, but when I do the circuit fails. The original “web found” circuit also did not have a diode, but it did have a 330 ohm resistor and LED from the collector to +6v. Would that path through the LED be enough to protect the NTE261? – no it appears spikes are still present after testing.

    Any thoughts would be helpful.
    Thanks Tom
  2. davenn

    davenn Moderator

    Sep 5, 2009
    Hi Tom
    welcome to the forums :)

    show us your circuit ( or the link to it)
    you may have put the diode in the wrong way around

  3. duke37


    Jan 9, 2011
    Perhaps you could give us details of the NTE261, it is not in my transistor book.

    The transistor rapidly turns off the primary current and the inductance forces this current to continue for a short time. As a result, a high voltage is generated which can force current through the turned off transistor, damaging it, or the current can go into the secondary giving the spark.

    The high voltage is necessary for operation.

    There are two ways of limiting the voltage to safe levels.
    1. Put a Zener diode across the primary, try one or more to make up 300V
    2. Put a capacitor across the coil as used in automobile ignitions.

    A fet is prefered for the switching transistor and there are NTE fets.
  4. nube_Tom


    Sep 20, 2013
    Circuit Update

    First I spent a number of hours on the web looking up different driver options and intend to try both a MOSFET such as SIHP18N50C-E3 and a IGBT such as IRGB14C40LPBF.

    As for the circuit all I have at this time is a sketch. But in words a 555 timer is pulsing a npn transistor which is controlling the base of the NTE261: The emitter grounded and collector to the spark plug coil primary, the other end to +6vdc. It is sparking very healthy. If I put anything, LED, diode, .... etc., across the NTE261 emitter to collector it all stops. The diode is in the direction to supposedly conduct any reverse pulse from the coil.

    The NTE261 Darlington data sheet can be found at; I check the voltage on the collector with a scope and it was about 75v on the collector: The NTE261 is rated at 100v, so if I read and understood right it should be ok.

    If you see anything wrong with what I believe please let me know.

    Thanks, Tom
  5. duke37


    Jan 9, 2011
    I don't have broadband, only slimband so it takes me ages to look up details of devices which you could doubtless provide with little difficulty.

    You have not said what the transistor is driving, I would expect a car ignition coil to generate more than 100V on the primary. A high voltage is necessary and anything across the coil will inhibit the spark.

    What is the engine? We had one when I was a child which we called the 'put put bang'. My grandfather used to boil an egg in the cooling water when he felt a bit peckish. He then got into trouble for not eating his dinner. The engine had a low voltage ignition with a push rod to open the points inside the cylinder to draw a spark.

    Looking at my stock of fets, I see I have one IRF830, 500V, 4.5A, this may be more suitable and there will be many more newer types. A fet will need 10 or 12V to drive it.

    A diode across the coil as is used when driving relays will kill the spark.
  6. nube_Tom


    Sep 20, 2013
    Some information from last reply.

    Table 2. Absolute maximum rating(1)
    1. For PNP types voltage and current values are negative.
    Symbol Parameter Value Unit
    NPN TIP122
    PNP TIP125 TIP126 TIP127
    VCBO Collector-base voltage (IE = 0) 100 V
    VCEO Collector-emitter voltage (IB = 0) 100 V
    VEBO Emitter-base voltage (IC = 0) 5 V
    IC Collector current 5 A
    ICM Collector peak current 8 A
    IB Base current 0.12 A
    Total dissipation at Tc ≤ 25 °C
    Tamb ≤ 25 °C
    Tstg Storage temperature -65 to 150
    TJ Max. operating junction temperature 150

    Attached Files:

  7. nube_Tom


    Sep 20, 2013
    The Engine - When Finished (I hope)

    The engine when finished will look something like this. The spark would be using a Model T Ford spark vibrating coil, driven from a rotating contact providing ground.

    My intention is to replace the Model T Ford coil with an electronic circuit. The values shown are not perhaps the final value. But it does spark using the darlington final transistor TIP122 (in the diagram is is only shown as 1 transistor).


    Attached Files:

    • 18.jpg
      File size:
      37.8 KB
    Last edited: Sep 27, 2013
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Hi Tom.

    A few comments on your schematic in post #6.

    1. You NEED to put a resistor in series with the pushbutton. The way you have it drawn, when the pushbutton closes you will be applying 6V across the base-emitter junction of the "51A" transistor. This will cause excessive current to flow in the transistor and the essential magic smoke will escape from it. The value is not critical; 10k will do nicely.

    At the same time you could change the 360k base-emitter resistor to 10k as well, and move the transistor so it sits just above the "66F" transistor in the diagram and connects directly to the 6V rail with its emitter at the top. This is a more common way to draw a circuit section like that one.

    2. I would not connect an LED the way you show, mainly because it will draw some current and will cramp the flyback voltage at the collector somewhat.

    3. I recommend connecting some kind of overvoltage protection network in place of the LED. The peak flyback voltage at the collector will be limited by the secondary voltage and the spark gap, but if that becomes disconnected, the collector voltage might shoot up so high that the transistor could be damaged.

    A typical suppression circuit would be a high-speed high-voltage diode with its anode to the collector and its cathode connected through a resistor (330 ohms or so) to a zener diode (cathode end) with a capacitor (e.g. 10 nF) in parallel with it, returned to the +6V rail.

    The flyback pulses pass through the diode and charge up the capacitor, but the voltage across the capacitor is clamped by the zener diode, so the flyback voltage can never exceed the zener voltage. You choose the zener voltage so it's a bit less than the rated maximum Vce of the transistor. You may want or need to use several zeners in series to get the required voltage; also, the zeners should have low leakage current, as any leakage current represents wasted energy.

    Edit: Another option is, as Duke suggested, to put a low-value capacitor directly across the primary of the coil. This forms a parallel L-C tuned circuit which produces a flyback pulse with a controlled shape. This capacitor corresponds to the "condenser" used in engines from (I guess) the mid-1900s. I don't know whether older engines used a "condenser" or not.

    4. I'm pretty sure you have the primary connections on the coil connected the wrong way round. The tap (marked WH and BK) should connect to the transistor's collector, and the bottom end (which you show connected to the core) should connect to the +6V rail.

    5. You should investigate the duty cycle of your drive waveform. Generally flyback transformers are operated with a wide current pulse and a relatively narrow flyback time, because the flyback voltage is so much higher than the voltage applied while the coil is "charging" (charging up its magnetic field).

    Your design currently has a duty cycle of around 58% at the 555 output and therefore 42% at the coil driver transistor's base. When the transistor turns OFF, the flyback pulse will decay relatively quickly, and the remainder of the 58% of the cycle time will be wasted. I think you should have at least 90% duty cycle at the base of the coil driver transistor. There are many ways to achieve this. Let me know if you want suggestions.

    6. What's the operating frequency of your 555? A 0.1 µF capacitor with those resistor values is going to give a relatively low frequency...
    Last edited: Sep 28, 2013
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I have seen some transistors made for this application which have a high voltage zener diode between the collector and base. This, along with a few other protections, allows the voltage at the collector to rise to approximately the zener voltage and no higher (the transistor turns on to carry most of the current).

    I would recommend a reverse biased diode from emitter to collector to protect the emitter junction from reverse voltages.

    edit: (removing silly error and adding image)

    Attached Files:

    Last edited: Sep 28, 2013
  10. nube_Tom


    Sep 20, 2013
    Great Suggestions!

    First- Yes I would like some suggestions on the duty cycle. I basically am using the design from the 1st web page I found.

    Second- I will implement the changes suggested, and apply the above suggestion (First-) when received.

    The data on the IGBT that I think I will be using has a Zener inside it and is rated at 100v. Would I need external protection as well if the spark plug remains in the circuit? Or perhaps if it gets disconnected the extreme voltages would still damage it?

    I did get a little confused about the protection circuit; In words; from the collector to the anode + of the fast diode, then to a 330 ohm resistor, then to the cathode – of the Zener, then to +6v. The capacitor shunts the Zener.

    I have placed a diode from the base to the collector, as suggested and the circuit failed. This was with the Darlington transistor, not the possible IGBT. I like the idea about the Zener from the collector to the base. Would I do that if I use an IGBT?

    Note: Old Ford vibrating coils for spark did have an internal capacitor. You might get a kick out of this web site;

    Thanks for your time helping I appreciate it.
  11. nube_Tom


    Sep 20, 2013
    Thank you for your time. I have replied with another post.
  12. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    OK, I'll write something up.
    Limiting the collector voltage to 100V is probably not going to work - that voltage is too low. When the IGBT turns OFF, the flyback voltage at the collector needs to go high enough to produce the required voltage at the top end of the ignition coil.

    Assuming the transformation ratio of the coil (the turns ratio between the bottom part below the tap and the total coil length) is 1:20 and you need 8 kV (these numbers are both guesses), you'll need 400V at the collector. Fill in the correct numbers, if you know them, and you can calculate the peak collector voltage. I think you'll find 100V is too low.

    What is the part number of the IGBT you are considering using?
    Yes, but perhaps Steve's suggestion is better - a zener from the collector to the base (cathode of the zener goes to the collector). This uses the transistor itself as the voltage limiting component. When the collector voltage tries to exceed the zener voltage, the zener starts to conduct and forward-biases the transistor, causing the transistor to pull its collector voltage down, and limiting the current.

    My concern about that idea is the capacitance in the zener. It also has implications for the base drive circuitry. I've seen this zener arrangement used with ignition circuits, but they have been the tuned circuit type (with a "condenser"), where the collector voltage does not rise steeply and there is only one flyback per cylinder firing. I think you intend to have multiple ignition pulses per cylinder firing, right?

    I think so. I've never worked with IGBTs. Steve, can you advise?

    In what way did the circuit "fail"?
  13. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Re improving the oscillator, I think your best option is the circuit described in under the heading "Improved 555 oscillator duty cycle".

    This design adds a diode (e.g. 1N914) from pin 7 to pins 6/2 (anode to pin 7) and provides nearly independent control of the high and low times of the output waveform.

    In your design, there's an inversion stage after the 555, so the coil driving transistor will be ON when the 555 output is low; this is the period that should be relatively long. The flyback and ignition pulse occurs when the 555 output is high; this is the period that should be reduced.

    Since that circuit allows you to set the high and low times independently, adjusting either of them will affect the frequency. There's a circuit that lets you vary the duty cycle while keeping the frequency (relatively) constant, but that circuit doesn't allow you to vary the frequency. So I think the circuit I've pointed to you is probably your best option.
  14. nube_Tom


    Sep 20, 2013
    IGBT Type

    It is a: International Rectifier (942-IRGB14C40LPBF) 430V

    I will check out the site you suggested as well. Also, if my old scope works I did see about 75V peak on the collector of the Darlington I am currently using. The transformer is from a snowmobile Artic Cat.

    Thanks, Tom
  15. Rob_K


    Sep 20, 2013
    Just to help anyone here with the mechanicals of an ignition system. The spark at the plug is usually between 20,000 to 40,000 volts and condensers are used in points ignition systems to prevent arcing at the points and help boost the spark at the plug. One of the diagnostics you can do is to look at the points and if they are black, chances are the condenser is the problem and if the points are blue, it is the coil that has failed.

    Modern electronic ignitions work using magic and little pixies as far as I know, but they work well and never seem to need to be dismantled or fed.
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