I'm having trouble trying to build this circuit: (Step 5) - (diagram below).

I built it, and everything works fine, but the coil doesn't spark.
I then used a scope-meter to check, and there is a almost perfect square wave outputted from pin 3, but after the resistor, the wave gets messed up, reduced to almost just background.

enter image description here

  • 1
    \$\begingroup\$ DO NOT use a scope or meter on the HT coil - it will almost certainly be destroyed. | Are you using a 2N3055 or? | Have you made any changes to the cct? What voltage supply? \$\endgroup\$
    – Russell McMahon
    Apr 4, 2015 at 15:17
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    \$\begingroup\$ I'm no expert in ignition coils, but the - and + look suspiciously oriented. Also, I reckon some kind of freewheel diode would be in order. Have you maybe toasted your transistor? IF BE is shorted or toasted, you'd just get junk measuring it. \$\endgroup\$
    – Dan
    Apr 4, 2015 at 15:22

2 Answers 2


After the resistor (at the basis of the 2N3055) the voltage can never get more than 0.6V, which is the BE voltage of the transistor.

IMO this circuit is highly suspicious. As Dan remarks, the +/- of the coil are the wrong way round, and there is no protection for the transistor. A freewheel diode would be a bad idea as it would suppress the spark too. AFAIK a zener is a good solution.

The 555 lacks the highly recommended 100nF decoupling capacitor at pin 5.

But my main objection is the base drive of the transistor. A 2N3055 has a lousy Beta: a datahseet shows a minimum of 5 at 10A. This circuit seems to call for 6A, so at the very minimum I would calculate with a Beta of 10, which would require 600mA base current. With the 100 Ohm resistor the base current will be ~ 10/100 = 100 mA. (Don't lower the resistor, 200 mA is the maximum for a 555).

The circuit should use a differnt transistor, or a darnington, or a FET. The next circuit on the page that you link to shows a FET. (But its high voltage protection does not look good to me.)


Several issues here.

1) You don't have anywhere near enough base drive into your power transistor. At the current levels that you are going to be running, you can figure on a current gain of 10 - 30. That means that you need hundreds of mA into the base of the transistor.

2) A 2N3055 does not have anywhere near a high-enough voltage rating for this application. Your coil uses flyback action to create the high voltage - that means that the collector voltage will reach hundreds of volts.

3) A square wave signal is probably not appropriate. I would modify your oscillator circuit to give you variable duty-cycle so that you can start with a very narrow pulse, then increase it as you tune the circuit.


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