Transistor biasing calculation problem.

[boylesg]

Columns are A -> G Rows are 21 -> 24 IB= column contains the formula =E21/C21 RB= column contains the formula =(B21-D21)/F21 They are based on the equations here: http://www.zen22142.zen.co.uk/Design/bjtsw.htm Transistor Vin hFE VBE IE IB = RB = BU931P 24.0000 300.0000 1.8000 8.0000 0.0267 832.5 2SC5446 24.0000 8.0000 1.5000 5.0000 0.6250 36 BU931P 12.0000 300.0000 1.8000 20.0000 0.0667 153 As you can see the only way that I can come up wihe 150R value for the resistor on the base of the darlington transistor is if I assume an emitter current of 20A. But this can't be right because they have a 10A fuse and the instructions state that they are limiting the current to 5A. So does anyone have any clue how they have come up with a 150R value for this resistor? Am I using the correct forumulas in my table? If I work backwards with a more sensible emitter current of 5-8A, this yields a base current of between 10-30mA, depending on whether I use 24V or 12V, and this in turn yields base resistor of some where around 2.2k for the BC327 as per the schematic. For the BC327 I am using the equations in here: http://www.kpsec.freeuk.com/trancirc.htm#ic **broken link removed** Transistor in switch mode - calculate IB and required RB
Transistor	Vin	hFE	VBE	IE	IB =	RB =
BU931P	24.0000	300.0000	1.8000	5.0000	0.0267	832.5
2SC5446	24.0000	8.0000	1.5000	5.0000	0.6250	36
BU931P	12.0000	300.0000	1.8000	20.0000	0.0667	153

 

[chuckey]

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

 

[boylesg]

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

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.

 

[chuckey]

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

 

[BradtheRad]

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.

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.

 

[boylesg]

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.

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.