Camera Flash Circuit Explanation

[Firefox123]

These two websites:

http://www.increa.com/reverse/dc/

and

http://thesignalpath.com/blogs/2011/06/12/camera-flash-circuit-and-nixie-tube-tutorial/

have very similar circuits for a standard camera flash circuit...and overall do a decent job explaining them.

There is one question I have though...one thing they do not explain in detail and I'm not quite sure what the answer is.

The question is WHY the transistor ever shuts off? WHY does it go into the cutoff mode for a short period of time? What is causing this?

So...what causes the transistor to go into cutoff? Does anyone have a good (detailed) explanation?

Thanks!

 

[Firefox123]

Anyone have any thoughts on this?

 

[jim hardy]

No takers yet ?

i think it has to do with inductance of the little transformer.
Recall current flowing in an inductance causes magnetic flux
but iron is non-linear it can only hold so much flux,
so at some amount of current an iron core inductor quits behaving like an inductor , its flux quits increasing with current...



from your first link (and credit to those folks at http://www.increa.com/reverse/dc/ )

when transistor conduction starts,
current begins to increase in the transformer primary increasing flux in its core
and voltage in every turn of every winding is in proportion to rate of flux increase

as he said, voltage in secondary during increasing flux interval turns transistor on more(note dots) and that's positive feedback

now here's the key to it -
when transformer core has all the flux it can handle
it "saturates" and flux stops increasing
so there's no more volts induced in any of the turns of either winding
so secondary winding no longer promotes primary current
and when primary current begins to drop,
flux starts to decrease,
reversing polarity of induced voltage in all windings
so secondary winding now turns the transistor off and there it stays until flux settles back near zero.

note his positive feedback operates in BOTH half cycles ! That's why it gives that snap-action full-on full-off cycling.

we are taught to usually think of transformers as processing sinewaves
but they are at heart just inductors.

old jim

 

[Firefox123]

No takers yet ?

i think it has to do with inductance of the little transformer.
Recall current flowing in an inductance causes magnetic flux
but iron is non-linear it can only hold so much flux,
so at some amount of current an iron core inductor quits behaving like an inductor , its flux quits increasing with current...



from your first link (and credit to those folks at http://www.increa.com/reverse/dc/ )

when transistor conduction starts,
current begins to increase in the transformer primary increasing flux in its core
and voltage in every turn of every winding is in proportion to rate of flux increase

as he said, voltage in secondary during increasing flux interval turns transistor on more(note dots) and that's positive feedback

now here's the key to it -
when transformer core has all the flux it can handle
it "saturates" and flux stops increasing
so there's no more volts induced in any of the turns of either winding
so secondary winding no longer promotes primary current
and when primary current begins to drop,
flux starts to decrease,
reversing polarity of induced voltage in all windings
so secondary winding now turns the transistor off and there it stays until flux settles back near zero.

note his positive feedback operates in BOTH half cycles ! That's why it gives that snap-action full-on full-off cycling.

we are taught to usually think of transformers as processing sinewaves
but they are at heart just inductors.

old jim

Jim...thanks for the reply. I need to think about this more, but on a first look I think this might be what is happening.

Thanks again!

 

[jim hardy]

I need to think about this more, ...

indeed that's the thing to do.
You need to get to where you can work a machine in your head
and check the formulas that naturally fall out from your understanding against textbook formulas
when they converge you are probably right.

Observe that so long as flux is increasing, the induced voltage helps push current through the 220 ohm resistor.
When flux stops increasing due to saturation,
induced voltage drops off
reducing base drive
decreasing base current hence collector and primary current
which begins the 'decreasing flux' half cycle.

That works for me.

Now consider this drawing from another site

i think he has his 15 turn winding connected backward , 220 ohm should go to the tap not the end (or else he needs another dot on bottom of winding and reverse direction of those turns)

that had me stumped last night.
Somebody please correct me if I'm wrong ?

old jim

 

[Firefox123]

indeed that's the thing to do.
You need to get to where you can work a machine in your head
and check the formulas that naturally fall out from your understanding against textbook formulas
when they converge you are probably right.

In my short career...I have found that to usually be the case.

Observe that so long as flux is increasing, the induced voltage helps push current through the 220 ohm resistor.
When flux stops increasing due to saturation,
induced voltage drops off
reducing base drive
decreasing base current hence collector and primary current
which begins the 'decreasing flux' half cycle.

That works for me.

I think you are right...I also think this is why the OFF time of the transistor is so short compared to the ON time...the reverse EMF quickly dissipates and the process starts all over again...

Now consider this drawing from another site

i think he has his 15 turn winding connected backward , 220 ohm should go to the tap not the end (or else he needs another dot on bottom of winding and reverse direction of those turns)

that had me stumped last night.
Somebody please correct me if I'm wrong ?

old jim

That looks wrong to me also...

 

[Firefox123]

Okay I have been thinking about this a bit more...

And now I am thinking it is not due to the transformer saturating...

I think it is the transistor.

Once the transistor saturates we have maximum current flowing through it...so the current stops increasing and is constant...this causes the flux/induced voltage in the secondary to quickly drop to zero...and this is what causes the transistor to go into cut-off.

Of course the transistor goes into cut-off before "zero volts" is reached...

So I think it has more to do with the transistor saturating than the actual transformer itself...

Let me know what you think.