I recently bought a DIY 1000W AC motor dimmer meant to be powered off 220-240VAC. I was hoping to use this to control a transformer which in turn powers a heating element.

The heating element needs 60W and i think the resistance is about 5mOhm. My derivation of this is at the bottom. Rather than try get the windings on the transformer perfect i thought using this dimmer control would give me enough flexibility.

This is the circuit that has been supplied:


simulate this circuit – Schematic created using CircuitLab

I am new to triac and diacs but my first question is more about the 1000W rating on this circuit. With my heating element only needing 60W is it a good idea to change the values for some of the resistors and/or caps? or is this 1000W just an absolute max with no relevance to the magnitude of control i would get with a lower powered load?

Secondly, the terminals were not labelled on the diagram and are not labelled on the PCB either. If somebody could figure out the orientation of the IN and OUT terminals i would greatly appreciate it.

Deriving Resistance of the heating element:

  • Connected the element in series with a 100 (99.3)Ohm resistor powered by a 4 (3.965)V supply.
  • Measured the voltage drop over each of them
  • Over element \$V_e = 0.2mV\$
  • Over resistor \$V_1 = 3.94V\$
  • \$ R_e=V_e * R_1/V_1 =5.05 mOhm \$
    the 0.2mV drop over the element is very close to the minimal 0.1mV that my DMM is able to measure so i do not know how accurate it is. If anyone knows a better way to test the resistance please let me know.
  • \$\begingroup\$ P = U^2/R -> P = 230^2/0.005 = cca 10MW!! \$\endgroup\$ Nov 10, 2015 at 8:28
  • \$\begingroup\$ sorry perhaps i should have made it clearer, the mains power is not going directly to the heating element. it is being stepped down via a transformer so a much lower voltage of about 0.5V. P=0.5^2/0.00505 = 49.5W \$\endgroup\$ Nov 10, 2015 at 8:41
  • 1
    \$\begingroup\$ The 1000W is the maximum rating of the dimmer, it is related to the max current the TRIAC can handle. These dimmers usually can work with loads between 20W - 1000 W. No need to change any components ! The transformer might be a problem for the dimmer though, some dimmer don't like inductive loads. But you can just try that, if it works it works and you can use it. The dimmer won't break due to the inductive load. \$\endgroup\$ Nov 10, 2015 at 9:11
  • \$\begingroup\$ You can get a better handle on the resistance by measuring the current through the string and the voltage across the string. That'll give you the resistance of the entire string, and if you subtract the value of the resistor you'll be left with the resistance of the heater. \$\endgroup\$
    – EM Fields
    Nov 10, 2015 at 9:38

2 Answers 2


These are nominal values for power, if you need less then there will be no major issues. But...the triac has a holding current, more powerful means greater holding current, meaning: when the current will fall below the holding current the triac will turn into non conductive mode, this is at th end of each cycle, while it starts conducting with a single pulse trigger. Therefore a larger triac will cut-off the power a little bit sooner than smaller one (but I wouldn't bother in your case).
If the load is inductive (your case) then the triac might re-trigger on accidentaly and to suppress this phenomena a snubber circuit is used.
In your schematics something is weird: C2,R1 is a snubber which should be placed parallel to the triac, not in that position where it is now (this is only my opinion). Look this similar schematics and see the arangement: http://www.electronicecircuits.com/electronic-circuits/filament-light-dimmer-circuit

  • \$\begingroup\$ I see what you mean about the snubber section, strange that it is connected differently. The build came with its own PCB so i assumed it had been tested to work with inductive loads (especially being labelled as motor dimmer). based off the link you provided this also indicates that in my circuit the top line is positive right? \$\endgroup\$ Nov 10, 2015 at 10:23
  • \$\begingroup\$ As said it was my opinion only, wait until some other people post their answers. \$\endgroup\$ Nov 10, 2015 at 10:34

Your light dimmer is very basic and not unorthodox .The triac ratings are fine.BUT I would like you to think what happens if the triac firing is a bit Assymetrical which is generaly the case because the triac has slightly different characteristics in each of its operating quadrents.On a normal incandescant lamp nothing bad happens ,whats a bit of DC between friends .The transformer will have a reasonably low primary DCR so a small DC voltage can cause a significant DC current to flow through the primary in addition to the load current .This can and will saturate a normal mains transformer .A toroid would be even more susceptable .The saturating transformer will take huge primary currents buzzing loudly ,possibly blowing the triac .Oddly this phenominine is worse when the dimmer is set at somewhat less than full power.Without close protection the transformer will go on fire .Example 50 watt LV halogen on 230V 5A lighting circuit and house burning up .SO use dimmable transformer or use transformer with bigger gap or place lamp ballast reactor in series with primary and get your fusing right.As for measuring your resistance use more current to get more mV to get more meter accuracy.


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