At present I am rebuilding an 1990 Elektor powersupply with a triac controlled preregulator able to handle inductive loads. My intention is to modernise the design but first I want it to be operational. The preregulator does not work as intented resulting in an hot transformer.

With the DUT connected to a safety transformer 1500W I hooked up a resistive load in the form of a 230V 250W halogen lamp. I was able to look at the some signals and found that the triggering in the quadrants 1 and 2 look ok. Triggering in the quadrants 3 and 4 behave differently.

Accros R33 the quadrant 1 and 2 pulse train is visible but appart from a few blibs no action in the quadrants 3 and 4.

Looking at the discharge slope of C1 I have have the feeling that Tri2 is not firing well in quadrant 3 and 4 . But I can not find the reason.

The information from the datasheets for the BTA41-800 the TIC206D and the DB3 did not gave me any clue.

The Elektor design appears to have originated from an SGS THOMSON design found in http://www.datelec.fr/secteur/ST%20AN308.pdf

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The image on the ocilloscope is measured accros C1 ( in the bridge)

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  • \$\begingroup\$ Spookey Circuit +1 \$\endgroup\$
    – Autistic
    Sep 13, 2017 at 11:01
  • \$\begingroup\$ Is there a reason you're using such a complicated circuit simply to be able to use a transistor output opto? A MOC3052 would get rid of 99% of the hullabaloo for a price that's less than a diac, two bridges, some zeners, a trimmer, and another triac, I reckon... \$\endgroup\$ Sep 13, 2017 at 11:48
  • \$\begingroup\$ @ThreePhaseEel. The reason is mentioned in the first line of the question. Can the MOC3052 handle inductive loads? If so it could be an option for improvement. \$\endgroup\$
    – Decapod
    Sep 13, 2017 at 13:34
  • \$\begingroup\$ @Decapod -- the MOC3052 drives the gate of Tri1 through a resistor :) (in general, triac output optocouplers drive other triacs, not loads) \$\endgroup\$ Sep 13, 2017 at 22:29
  • \$\begingroup\$ @ThreePhaseEel. Meaning that with your proposal all the "hullabaloo" for controling the starting angle and measures to preventing zero crossing current interuption must be solved elsewhere. \$\endgroup\$
    – Decapod
    Sep 14, 2017 at 6:24

1 Answer 1


The problem as mentioned in the question has been solved. As I have suspected Tri2 did not behave as wanted in quadrant 3 and 4. I replaced the original triac TIC206D with a sensitive gate triac Z0405NF. Now the triggering of the circuit operates as required.

For those who are interested, a short explanation of the circuit. The zener diodes limit the incomming mains up to a value of 66 V peak. This voltage starts to charge C1 through R30, P1 and BR2. The opto coupler connected to BR2 acts as a variable resistor thereby able to influence the time it takes to charge C1.

As soon as the breakdown voltage of BR1 is surpassed Tri1 triggers. At the same time Tri2 triggers ( voltage over R32) discharging C1 below the breakdown voltage of BR1. There after the story repeats until the voltage accros the zeners becomes to low to charge C1 high enough to trigger BR1.

For more info read the information in the Application note of SGS THOMSON.

Time to move in making the system function as intended.


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