Transistor Operation

Thread Starter

jethro99

Joined Oct 31, 2020
30
In a conventional ignition coil, battery power flows through a primary coil. When the power is disconnected a high voltage is induced into an adjacent secondary coil. Causing a spark across the spark plug electrodes.

From what can be seen in the diagram below, is it possible to tell/guess anything about how the system functions?

Does a voltage being applied by the trigger cause the transistor to open or close?

TIA.

 

dl324

Joined Mar 30, 2015
17,022
From what can be seen in the diagram below, is it possible to tell/guess anything about how the system functions?

Does a voltage being applied by the trigger cause the transistor to open or close?
There's no need to guess.

It's an NPN transistor being used in saturation mode (as a "switch"), so you could think of it as "closing", but the proper terminology is that it turns on.
battery power flows through a primary coil
It's current that flows through the primary.

I'm not sure what the coil symbol is trying to convey. If there's a secondary, it's usually isolated from the primary. Don't know what the dangling diode is meant to do. In an ignition coil, it seems like you wouldn't want to snub back EMF.

From an electronics perspective, the transistor is drawn sideways/backwards. We like current flow to be primarily left to right and top to bottom.
 
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geekoftheweek

Joined Oct 6, 2013
1,249
I'm not sure what the coil symbol is trying to convey. If there's a secondary, it's usually isolated from the primary. Don't know what the dangling diode is meant to do. In an ignition coil, it seems like you wouldn't want to snub back EMF.
Ignition coils have the primary and secondary tied together at one end to the B+ as drawn. The diode is a strange one. I was always under the impression the coil produces a higher positive potential.
 

MaxHeadRoom

Joined Jul 18, 2013
28,782
Ignition coils have the primary and secondary tied together at one end to the B+ as drawn.
Not on the latest waste-spark dual-plug type systems now, they are totally isolated.
The coil has two spark plug connectors, one can be grounded if used for E-fence etc,
The older style (points operation) has a common connection point.


1702755616516.png
 
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MisterBill2

Joined Jan 23, 2018
19,095
I corrected the DWG, normally there is one coil for two plugs.
What is depicted in the drawing in the initial post is a reasonable depiction of what has served well for over 70 years.
It is not representative of the present scheme, but it does represent the most common scheme for the previous era. The new scheme avoids the rather expensive distributor, and so it is a real cost reduction, but it has a greater sell price..
 

LvW

Joined Jun 13, 2013
1,766
You need to learn how transistors operate. Here learn from the beginning. https://www.allaboutcircuits.com/textbook/semiconductors/chpt-4/bipolar-junction-transistors-bjt/
I cannot recommend the linked contribution because it contains wrong explanations.
Example (Quote): "The small current that controls the main current goes from base to emitter or from emitter to base, once again depending on the kind of transistor it is (NPN or PNP, respectively)."

This statement is wrong. A small current can never directly control a larger current. Surprisingly, we still can find such a false claim in some books or other contributions - hower, always without any justification.

In fact, the bipolar transistor is a voltage-controlled device Ic=f(Vbe). There are numerous proofs and explanations for this (formulas, transistor physics, circuit properties, etc.)
 

MisterBill2

Joined Jan 23, 2018
19,095
I cannot recommend the linked contribution because it contains wrong explanations.
Example (Quote): "The small current that controls the main current goes from base to emitter or from emitter to base, once again depending on the kind of transistor it is (NPN or PNP, respectively)."

This statement is wrong. A small current can never directly control a larger current. Surprisingly, we still can find such a false claim in some books or other contributions - hower, always without any justification.

In fact, the bipolar transistor is a voltage-controlled device Ic=f(Vbe). There are numerous proofs and explanations for this (formulas, transistor physics, circuit properties, etc.)
Then explain why the Vbe remains close to the 0.7 volts as the collector current changes. Certainly it is "some form of magic" related to the quantity of charge carriers between emitter and collector. It seems to be related to electrons, which are rather tiny and difficult to see.
Of course, current is usually related to voltage, although I find that most diodes do not seem to obey ohms law very well for other than very small changes in current.
 

LvW

Joined Jun 13, 2013
1,766
Then explain why the Vbe remains close to the 0.7 volts as the collector current changes. Certainly it is "some form of magic" related to the quantity of charge carriers between emitter and collector. It seems to be related to electrons, which are rather tiny and difficult to see.
Of course, current is usually related to voltage, although I find that most diodes do not seem to obey ohms law very well for other than very small changes in current.
From your question I derive that you really still believe that the BJT is current-controlled, correct?

My answer to your question:
It is the well-known Shockley equation which describes the relation Ic=f(Vbe). From this exponential equation you can derive that a change in Vbe will cause a corresponding Ic variation. The similarity to the function of a pn-diode is obvious.
And if you call such a Vbe-variation small (or "close to 0.7 volts") is simply a matter of interpretation. Do you think this would be a "counter argument" ?
To me, there is no "form of magic". And why do you refer to Ohms law?
Does this answer your question?
 
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