Does diac conduct in this schematics?

[eem2am]

Does diac conduct?

Hello,

On page 3 of the following is a CFL circuit which is started by a diac

https://www.st.com/stonline/books/pdf/docs/3706.pdf

i was wondering, do you agree that the diac always conducts whenever the top transistor is on?

-and when the diode is conducting at such times, it is conducting current into the base-drive transformer of the bottom transistor?

--------------------------------------------------------------------------------
some back-ground

(the diac initially feeds base current to the bottom transistor to start things off)

the transistors are actually turned off by the load current which saturates the base drive transformer to which they are connected.
the lamp current , just after transistor turn off, is through the opposite anti-parallel diode.
as the lamp current goes through zero the base drive transformer "de-saturates"....then because it is falling it turns on the other transistor- hence oscillation.

but do you know if the diac conducts whenever the top transistor is on?

 

[FvM]

Re: Does diac conduct?

do you agree that the diac always conducts whenever the top transistor is on

Clearly no, see the diode polarity. It keeps C2 discharged and prevents further triggering of the DIAC after startup.

 

[eem2am]

Does diac conduct?

thankyou FvM,

-I do see that the diode nearest the diac will discharge the cap C2 when the bottom switch is on.

and i presume you are saying that even though the diode near the diac is not discharging the cap C2 when the top BJT is on, ,......the time of the top BJT being on is not long enough for the cap C2 to charge up to the ~2V that would turn the diac on?

 

[FvM]

Re: Does diac conduct?

charge up to the ~2V that would turn the diac on?

No DIAC turns on at 2V. 30V is a typical voltage.