I wanted to know whether connecting two or more simple diac-triac ac lamp dimmers in parallel with the same load cause any problems? I do not want to connect them in series, just parallel will be enough.

The dimmer circuit is the common diac-triac dimmers found in stores and wired in paralled like this . enter image description here

The load will be a dimmable led light bulb at 240v ac 12w.

Edit: Updated circuit diagram

  • 1
    \$\begingroup\$ This sounds indeed like a bad idea. Why would you want to put the triac dimmers in parallel, and how exactly do you plan to put them in parallel? Draw a schematic of at least 2 parallel dimmers so that we see which components you want to only have one time, and which components you want to have multiple times. \$\endgroup\$ Aug 27, 2017 at 15:59
  • \$\begingroup\$ I've updated the schematic to make it easier to illustrate. \$\endgroup\$
    – Kokachi
    Aug 27, 2017 at 16:07
  • 1
    \$\begingroup\$ ah, much better, thanks! Still confused what you need this for! \$\endgroup\$ Aug 27, 2017 at 16:10
  • \$\begingroup\$ I'll explain. I will set 3 different dimmers at different dim settings. Then I will connect each of the dimmers through their own individual relays. So that If I want a certain brightness I can switch that relay on and the saved dim setting will light the lamp. I wanted to know if by accident I turn on 2 or more relays at once will there be any problems. \$\endgroup\$
    – Kokachi
    Aug 27, 2017 at 16:13
  • \$\begingroup\$ ah, ok, but why don't you just use three different resistors / potentiometers and just switch between these? \$\endgroup\$ Aug 27, 2017 at 16:20

2 Answers 2


Parallel connection of 2-wire dimmer switches is fine.

Note that the first one to switch on will control the brightness. i.e., The one with the brightest setting will win. This is due to the way the triacs in the dimmers switch on at a certain point in the mains cycle.

enter image description here

Figure 1. Triac dimmer control. Note that the earlier in the mains cycle that the power is switched on the brighter the lamps.

I do not have a problem if the brightest one wins. I just want to know whether they will create a short or explode or destroy the load.

No, there will not be a short. The first one to switch on will drop the voltage across the other dimmers close to zero and the C1 on each of those will not charge any further and those triacs will not turn on.

The question changed somewhat after I wrote the answer in that OP can accept a "one triac" solution.


simulate this circuit – Schematic created using CircuitLab

Figure 2. A relay controlled dimmer with three preset power levels.

Figure 2 shows how to wire the dimmer for three-point relay control. The 4k7 resistor can be omitted provided the other resistors are greater than 4k7. I have laid the circuit out with the triac on the live wire as would normally be the case in a fixed installation. It will work either way.

  • \$\begingroup\$ by 2-wire dimmers you mean the dimmers which are connected to load only through the live wire? I've updated the circuit diagram to illustrate it better. \$\endgroup\$
    – Kokachi
    Aug 27, 2017 at 16:09
  • \$\begingroup\$ I do not have a problem if the brightest one wins. I just want to know whether they will create a short or explode or destroy the load. \$\endgroup\$
    – Kokachi
    Aug 27, 2017 at 16:10
  • \$\begingroup\$ While the parallel connection with work with three complete dimmers, you can significantly cost reduce the solution by having only one Triac. Then you'd have just the three R/C/Diac networks with the common for the Diacs connected to a single Triac. Whichever Diac fires first turns on the output Triac. \$\endgroup\$ Aug 27, 2017 at 17:13
  • \$\begingroup\$ @JackCreasey If possible can you show me a diagram. Cost is not a problem as these dimmers cost only half a dollar each including shipping. \$\endgroup\$
    – Kokachi
    Aug 27, 2017 at 17:28
  • \$\begingroup\$ @Kokachi: See the update. "... these dimmers cost only half a dollar each including shipping." Now I'm worried. \$\endgroup\$
    – Transistor
    Aug 27, 2017 at 18:10

There are multiple ways to tackle the problem of providing 3 pre-set light levels.

  1. Three R/C/Diac networks and one Triac


simulate this circuit – Schematic created using CircuitLab

Or even simpler still....

  1. Just three potentiometers, one DIAC and one Triac


simulate this circuit

You could fairly easily modify a single dimmer to satisfy either of the schemes above.

Note: The schematic you show is not the most reliable (the trigger point), and typically two R/C networks are used to provide much better reliability.
You should also include a snubber to prevent radiated noise.

enter image description here

  • \$\begingroup\$ I would go with solution no.2. I have a question,can the resistors used to replace the pot be of 1/4W? The single resistor before the pot is of 1/4W power rating anyway. \$\endgroup\$
    – Kokachi
    Aug 28, 2017 at 3:57
  • \$\begingroup\$ It depends on the potentiometer value for any given phase angle. You might see peak dissipation up to about 1/3 W, so to be overconfident, I'd suggest you use 1/2 W resistors. \$\endgroup\$ Aug 28, 2017 at 5:34
  • \$\begingroup\$ Ok, 1/2W resistors are available, I can go with that. You mentioned that my schematic was not that reliable, is it really a concern? \$\endgroup\$
    – Kokachi
    Aug 28, 2017 at 5:53
  • \$\begingroup\$ Reliable trigger point, rather than reliability (failure) of the dimmer device. I'd recommend the snubber be added, but you may not notice any effects from the trigger point movement. \$\endgroup\$ Aug 28, 2017 at 15:17
  • \$\begingroup\$ you mean the each time the circuit turns on the dimming level might be erratic or unpredictable? \$\endgroup\$
    – Kokachi
    Aug 28, 2017 at 17:14

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