Transistor biasing calculation problem.

They used this value to make sure the transistor is fully saturated at all values of Hfe/volt drops across D1/10 ohms o/p of NE555. . .
Frank

chuckey said:

They used this value to make sure the transistor is fully saturated at all values of Hfe/volt drops across D1/10 ohms o/p of NE555. . .
Frank

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If so it seems rather excessive. From what I have read you go roughly 2-3 levels below the resistance value that you calculate for this purpose.

If I do use a 150R resistor as in the specs then it appears that the darlington transistor is permanently conducting a current and I do not get any arcs from the ignition coil. Replacing the ignition coil with a car light bulb and replacing the timing capacitor with a 1000uF one reveals that the light bulb does not flash.

However I found, by trial and error, that a 2.2k base resistor on the darlington does cause the light bulb to flash however it does not drive the ignition coil efficiently. The arcs are minute and the darlington transistor seems to generate a great deal of heat on its large heat sink.

Can't seem to figure this particular circuit out.

The NE555 has a transistor connected between its PIN 3 and earth, so if this is leaky, then its output voltage will always be less then its VCC turning on TR1 and hence the darlington. I would go around this circuit with a meter and see whats wrong. Or you could just try connecting a 1K in parallel with the output at pin3 to Vcc.
Frank

boylesg said:

Replacing the ignition coil with a car light bulb and replacing the timing capacitor with a 1000uF one reveals that the light bulb does not flash.

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That large a cap value generates a cycle of 37 seconds (assuming your resistors are 18k as shown in the schematic).

An ignition coil for vehicles is made to spark at 10 to 100 times per second. Have you confirmed that your frequency of operation is inside that range?

An overly fast pulse speed could be responsible for the minute arcs. There is less time for current to build during switch-On. Extra cycles might need to go by, to develop sufficient current to make a spark jump at all.

BradtheRad said:

That large a cap value generates a cycle of 37 seconds (assuming your resistors are 18k as shown in the schematic).

An ignition coil for vehicles is made to spark at 10 to 100 times per second. Have you confirmed that your frequency of operation is inside that range?

An overly fast pulse speed could be responsible for the minute arcs. There is less time for current to build during switch-On. Extra cycles might need to go by, to develop sufficient current to make a spark jump at all.

Click to expand...

Put it this way, I can get good arcs from the ignition coil if I drive it with my mosfet based driver circuit at 90% duty cycle and about 7kHz (180nF timing cap)
But I get very little in the way of arcs if I drive it from the Darlington based driver circuit at the same frequency.

In both cases I have the 555 arranged as astable extended duty cycle with the only difference being what is between the 555 pin 3 and the ignition coil.

If I connect the BC327 collector to an LED then it causes that LED to flash visibly with an added 1000u timing capacitor. Without the additional 1000u timing capacitor the LED is permanently on due to the high frequency. So I am assuming that this part of the circuit is working as the designer intended, not withstanding from my alteration of the 555 circuit to extended duty cycle.

If I instead connect an automotive light bulb in place of the ignition coil and insert the additional 1000u timing cap, then the light bulb does not flash.

I did at one stage replace the darlington with a salvaged tv HR output transistor (saturation base current of about 600mA), re-calculated its base resistor (820R I think it was) and manged to get the automotive light bulb flashing. When I connected the ignition coil and removed the 1000u time cap, I got nice arcs from the ignition coil.

There is something strange about these darlington transistors the designer has specified, but I am damned if I can figure it out.

I suppose I could just run through my entire range of resistors until I find the correct value for darlington base, but what I pain that would be. But I would prefer to understand what I am doing wrong.