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I've got a touch sensitive table lamp that dims to three different stages (off, low intensity, mid intensity, full intensity). It has stopped working like that and now just being powered to the mains goes to full intensity, ignoring the touches to the base.

Touch plate connection is good, and I've gone checking connections throughout the circuit, and all seems good too.

I ordered a new dimmer IC and replaced the chip, but the circuit behaves the same. The circuit is very very similar to the one described in the IC sheet here http://www.micropik.com/PDF/tt6061a.pdf

I'm more interested in learning how to debug the circuit than the fix itself. Any help on steps to take would be appreciated.

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closed as primarily opinion-based by Trevor_G, RoyC, PeterJ, laptop2d, Sparky256 Mar 11 '18 at 23:58

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ Very very carefully..... LETHAL VOLTAGES PRESENT Not something you should play with if you are not familiar with working on such things. \$\endgroup\$ – Trevor_G Mar 9 '18 at 19:23
  • \$\begingroup\$ the TRIAC is probably shorted ... did you have a light bulb burn out? ... what happens with filament bulbs is that the filament burns and separates in the weakest spot ... if you unscrew the bulb, then the free end of the broken filament flops around ... if you unscrew the bulb with power applied, then the filament can touch the opposite post in the lighbulb and cause the bulb to "flash" briefly, and because of the shorter filament drawing a lot of power ... this condition will blow out triacs \$\endgroup\$ – jsotola Mar 9 '18 at 20:19
  • \$\begingroup\$ @jsotola With the mains disconnected I checked for continuity between all terminals of the triac in both polarities and I can only detect continuity between the gate and T1. This tells me that the Triac is not shorted, but please let me know if that test is not enough. I have not checked with power on (I'm not familiar with handling 220V AC and don't want to risk touching things randomly). I can always desolder the triac to test further, but I think it's ok. \$\endgroup\$ – palako Mar 11 '18 at 11:37
  • \$\begingroup\$ @jsotola my bad, double checked and the terminals that show connectivity (again, no current applied to the circuit) are T1 and T2, so it looks like it is actually shorted. I'll order a new one and see. Please add your comment as an answer and I'll mark it as accepted answer. Thanks! \$\endgroup\$ – palako Mar 11 '18 at 11:45
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The TRIAC is probably shorted.

Did you have a light bulb burn out?

What happens with filament bulbs is that the filament burns and separates in the weakest spot.

If you unscrew the bulb, then motion causes the free end of the broken filament to flop around.

If you unscrew the bulb with power applied, then the filament can touch the opposite post in the lighbulb and cause the bulb to "flash" briefly and since the filament is shorter than normal, its resistance is lower than normal and it draws a lot of power.

This condition can blow out a triac.

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This may help. If you verify the gnd symbol is Neutral and not Line reversed, you can measure all the pin Vdc and Vac levels during idle and touch conditions. Start with Vcc and record a table of results and share.

A clock frequency is applied to a touch plate which is partially ESD protected and a change in signal amplitude is detected and counted for a duration to advance the phase of the triac pulse to control the dim level.

enter image description here

      Idle        Touch
Pin   Vdc  Vac     Vdc  Vac
 1 
 2
 ...
 8
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