# Problem understanding a section of a circuit

#### francesco_rossi

Joined May 31, 2013
4
In the schematic shown in this page: http://www.pavouk.org/hw/en_atxps.html I wound like to understand the section of the circuit who powers the IC1 TL494.
From what I understand IC1 is powered through D30 by the "first" secondary of T6. What happens in the primary section of this transformers who induces current in the secondary section? Maybe Q12 act as a switch and generate the current in the primary who induce the current in the secondary needed to power IC1.

Someone can explain me how the TL494 is turned on in this circuit?

#### wayneh

Joined Sep 9, 2010
17,153
Confusing to be sure. I see that power for IC1 is also available from L1 in the secondary of T3.

#### KeithWalker

Joined Jul 10, 2017
1,851
In the schematic shown in this page: http://www.pavouk.org/hw/en_atxps.html I wound like to understand the section of the circuit who powers the IC1 TL494.
From what I understand IC1 is powered through D30 by the "first" secondary of T6. What happens in the primary section of this transformers who induces current in the secondary section? Maybe Q12 act as a switch and generate the current in the primary who induce the current in the secondary needed to power IC1.

Someone can explain me how the TL494 is turned on in this circuit?
What is the function of the circuit? I would hesitate to analyse it until I have some idea.
The power is generated in the transformers to be distributed to the rest of the circuitry by the free running multivibrator formed by Q1, Q2 and T2.

#### francesco_rossi

Joined May 31, 2013
4
What is the function of the circuit? I would hesitate to analyse it until I have some idea.
The power is generated in the transformers to be distributed to the rest of the circuitry by the free running multivibrator formed by Q1, Q2 and T2.
Sure but the main component IC1 have to be powered in some manner and I don't understand how. The only way is through T6, but how?

#### ian field

Joined Oct 27, 2012
6,539
In the schematic shown in this page: http://www.pavouk.org/hw/en_atxps.html I wound like to understand the section of the circuit who powers the IC1 TL494.
From what I understand IC1 is powered through D30 by the "first" secondary of T6. What happens in the primary section of this transformers who induces current in the secondary section? Maybe Q12 act as a switch and generate the current in the primary who induce the current in the secondary needed to power IC1.

Someone can explain me how the TL494 is turned on in this circuit?
In most I saw when I serviced them for a living; they deliberately unbalanced the primary push-pull so it would "kick" when power was first applied - this kick was just enough to get the controller chip going. often the 2 reservoir cap bleeds would be unequal, but they appear to be the same here. A small capacitor charging at power up can get the job done.

The simple flyback switchers in some stuff have a start up resistor to get the chip going till the Aux winding starts producing its Vcc.

Another example is the basic CFL circuit - its more or less 2 blocking oscillators stacked one on top the other. at switch on, neither is eager to do much - a diac relaxation oscillator pulses the base of the bottom transistor so its conduction wakes the other one up. The diac capacitor is diode clamped to the output so its shorted out every time the bottom transistor saturates, so it never charges enough to fire the diac once the circuit is running.

Inhibit circuits sometimes turned up in bigger appliance start up circuits - they were quite capable of being a right PITA!

#### wayneh

Joined Sep 9, 2010
17,153
The only way is through T6, but how?
Not true. Did you even look at what I said?

#### Jony130

Joined Feb 17, 2009
5,241
From what I understand IC1 is powered through D30 by the "first" secondary of T6. What happens in the primary section of this transformers who induces current in the secondary section? Maybe Q12 act as a switch and generate the current in the primary who induce the current in the secondary needed to power IC1.
Q12 and T6 forms ringing choke converter (standby supply). Here you can read more:
And the circuit in the link works exactly the same as the circuit form this ATX supply.
https://web.archive.org/web/20170614183401/http://mmcircuit.com/understand-rcc-smps/

Do you need more explenation?