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Why did the dimmer burned the linear transformer? It is a MOT transformer, secondary rewound with 7 turns and a resistive heating element of 800w. While the transformer worked fine at line 220AC, burned while connected to the dimmer(phase angle). enter image description here

enter image description here

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  • \$\begingroup\$ What’s the load on the other side of the transformer? \$\endgroup\$ – winny May 6 '18 at 20:00
  • \$\begingroup\$ a resistive element, aprox. 800w, spiral coil \$\endgroup\$ – johnger May 6 '18 at 20:04
  • \$\begingroup\$ What part of the transformer burned? \$\endgroup\$ – winny May 6 '18 at 20:23
  • \$\begingroup\$ the secondary, but cant say for sure. because the damage, can't confirm where ignited... \$\endgroup\$ – johnger May 6 '18 at 20:26
  • \$\begingroup\$ Ignited by prolonged heat or instantaneous flashover? \$\endgroup\$ – winny May 6 '18 at 20:41
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Do you remember when I told you about Quadrant IV on the triac?

Remember when I said that if you don't put enough current into the gate (>100mA) the triac will trigger only on the (positive) half-cycles?

That is equivalent to putting half-wave rectified DC through your transformer primary and "burning" is exactly what we would expect to happen.

You need to either stop trying to trigger in Q-IV or to put enough current through the gate to trigger in any of the quadrants.

And put a fuse in series with the transformer primary. It may not save the triac but it should save the transformer winding.

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  • \$\begingroup\$ i have modified the circuit, changed resistor values to allow enough current. with resistive load worked fine. i used a 10A fuse which blowed, so decided to test further without fuse, leading to burned transformer. \$\endgroup\$ – johnger May 6 '18 at 21:21
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    \$\begingroup\$ So the schematic above is not what you tested? \$\endgroup\$ – Spehro Pefhany May 6 '18 at 21:23
  • \$\begingroup\$ in that case, my only option is to use bridge rectifier instead of capacitive supply or can i use signal burst ? \$\endgroup\$ – johnger May 6 '18 at 21:23
  • \$\begingroup\$ the schematic is the exact circuit from the other post, following your advice about current just modified 2 resistor from driver to get more current. the board is assembled from that schematic. R4 and R5 were modified. \$\endgroup\$ – johnger May 6 '18 at 21:24
  • \$\begingroup\$ It still sounds like you were getting half wave across the transformer. The inductive load makes it harder to reach holding current if you are very close to the zero crossing. If your power supply can handle it you can try running 120mA continuous to the gate (get rid of R2 and make sure R5 is low enough value- about 30R and is rated for enough power (about 1W), and probably have to reduce R4 to maybe 300 ohms) \$\endgroup\$ – Spehro Pefhany May 6 '18 at 21:34

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