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I have a table lamp which has a busted dimmer control.

When I bought it the dimmer used to kind of work but then after a few days it worked only when the rotary knob was turned all the way high. After a while it broke completely and won't even light up.

I removed the lamp and tested it - the lamp works fine. So I decided to do a tear down and see what's inside.

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I managed to trace the circuit. Here it is. I hope it's correct. If it's wrong do let me know.

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I did some testing with my cheap multimeter and found out the values of 2 resistors, one being 1k and the other 5k. The potentiometer showed no resistance at all which means the potentiometer is probably the part that went bust, but the potentiometer switch works fine in continuity test.

Pleas explain how this circuit works to me and what went wrong here.

Update: ZY 406 might be an SCR, PCR406 in TO-92 package like user "Spehro Pefhany" suggested. I have updated the diagram with this change in symbol.

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    \$\begingroup\$ ZY406 is probably an SCR. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Aug 29, 2017 at 14:03
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    \$\begingroup\$ @SpehroPefhany yeah, it could be a PCR406 in a TO-92 package. \$\endgroup\$
    – Kokachi
    Commented Aug 29, 2017 at 14:14
  • \$\begingroup\$ The pot is 500k, I found it selling in China market as DIY lamp dimmer kit. The shop link is below. [Link to item](m.intl.taobao.com/detail/… \$\endgroup\$
    – Bravo
    Commented May 6, 2022 at 8:03
  • \$\begingroup\$ Double check the relative polarity of the SCR and the bridge. \$\endgroup\$
    – Autistic
    Commented May 6, 2022 at 9:58

1 Answer 1

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The pot resistance and the cap form an R-C charging ramp voltage. When this voltage, divided by 83% (1K and 5K voltage divider) exceeds the SCR trigger voltage, the SCR conducts. Thus, for each half-cycle of the power line, the light cones on some time delay after the zero crossing. As the pot resistance gets smaller, the ramp time shortens, and the light comes on earlier in the half-cycle.

From your symptom history it sounds like the pot wiper was not making good contact with the resistance element, eventually not making contact at all, but still was making contact with the element end point where it is attached to the pin. Now, not even that is making contact with the wiper.

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  • \$\begingroup\$ I see, thanks for the explanation. So I guess I was right about the pot being the broken component here. \$\endgroup\$
    – Kokachi
    Commented Aug 29, 2017 at 16:38
  • \$\begingroup\$ That would be my first guess. Of course, everything else on the board is easier to remove and test. If you know the open-circuit value of the pot, you can tack a resistor of about 1/2 that value across the terminals and see if the lamp comes on. \$\endgroup\$
    – AnalogKid
    Commented Aug 29, 2017 at 17:33

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