What is the functionality of this circuitry

[Efe_114]

What do these components i circled do in this circuit? 

[inse]

My interpretation: depending on the output voltage the additional winding of the transformer is switched with the TRIAC in order to reduce the power dissipation in the linear regulator.

[Efe_114]

How does it increase the voltage though? Doesn't it just increase the current?

[Terry Bites]

Put a lot of dirt on your mains supply among other things.

[inse]

@terry:
I don't think the circuit will cause interference as the OPAMP is configured as comparator and turns on the TRIAC depending on the relation between a reference and the output voltage, no phase cutting.

@efe:
The transformer windings are connected in series, lets assume each delivers 15V.
So the TRIAC acts as a switch between 15 and 30V input for the linear regulator.
With a interesting arrangement of the bridge rectifier, once the bridge itself for low voltage and a divided bridge consisting of 1/2 of the bridge and the two diodes at the TRIAC.
The OPAMP on the right inverts the reference voltage for the negative branch.

[moffy]

I must agree with inse. The main Triac(Q3) is only triggered on the -ve side, when:
1. Vref is > than the divided feedback voltage from the output of Q1(ignoring the small amount of hysteresis around U23B)
and
2. The voltage on T2 exceeds the output of CR7 +ve or T2 is less than CR7 -ve.(ignoring voltage drops of CR5,6 etc)

It is set up for a constant voltage through Q1, Q3 will provide extra voltage/current on the input side to the regulator I would assume in the case of a high load, where the ripple on the caps has become excessive. In this arrangement I would expect that the voltage J5-1 to J5-2 would be less than J5-2 to J5-3, because J5-1 to J5-2 is only supplemental, but that doesn't have to be the case.

[pqass]

inse is correct on all points.

ADC_REF is generated from a LT1021-5 +5V precision zener (see "(B) Bias Supplies for +25V&-25V Power Circuit..." in the E3631A Service manual).  Therefore, as per the inverting op amp at lower right, vref is -5V.

When the user commands (through the user interface) that the voltage difference between COM and +25VOUT (before the current shunt; ie. before current is factored in) is greater than +13.9V (+5V÷(5.6k÷(5.6k+10k))), the LM393 (U28B) comparator turns on the triac (Q10) via Q9 and opto (U29).  It stays on until the voltage difference goes below +13.9V; this is not a phase-dimmer-type pre-regulator.  The triac turns on the top-most winding of T1 (J5-4) into CR31&CR30, doubling the unregulated DC input to the bulk caps etc.  Since double the voltage is present, the top half of CR29 is reverse biased and the middle tap (J5-5) is effectively disconnected.  Similarly for the negative supply except the threshold is -13.9V.

[moffy]

Thanks for the info, a bit more complex than I thought.

[Berni]

It just looks like switching taps on the transformer to select the appropriate input voltage.

Most lab power supplies use relays to do the same thing, you can hear them clunking away as you turn the voltage knob up and down. Here the job is done in solid state using a triac. So no clicking and no relay to wear out.

[moffy]

It just looks like switching taps on the transformer to select the appropriate input voltage.

Most lab power supplies use relays to do the same thing, you can hear them clunking away as you turn the voltage knob up and down. Here the job is done in solid state using a triac. So no clicking and no relay to wear out.

Used to have one of those, really nice power supply.