# Should serial triac dimmers work for resistive load?

I have a cheap 120V hot plate that I assume regulates its temperature using a triac dimmer (because the heating element vibrates/hums at lower settings). But at its lowest setting it still gets too hot for my application.

So my first reaction is to put another household dimmer between the device and the power. But I'm not sure how the second triac in the series will behave if it's seeing chopped AC from the first.

I think the capacitors in the second triac are going to hold their charge between "cuts," and so the cumulative effect of the dimmers will be multiplicative as expected: I.e., if both are set to "half" power the device will see "quarter" power. Is that correct? Or is it not that simple?

• You might want to look up how triac dimmers work first... Commented Feb 2, 2016 at 14:53
• @PlasmaHH - I did, and I'll admit I'm not very sharp on EE. So if something I'm describing is off I'd appreciate the clarification as part of any answer! Commented Feb 2, 2016 at 15:08
• just so far: look at the chopped ac, and think about what would happen if this was the input of another dimmer, and keep in mind what happens to the charge of the cap when the triggering diac activates Commented Feb 2, 2016 at 15:18
• @PlasmaHH - I'm not smart enough on EE to work this out, even with help. But I do enjoy reading good explanations. And since this is a real-world problem I would appreciate an answer even if I can't follow the explanation. (And additionally, in "Mythbusters" style, if the effect I want to achieve can't be produced by what I propose, I'd appreciate a suggestion of 120VAC 15-amp devices that could accomplish it.) Commented Feb 2, 2016 at 15:40

Stacked dimmers are probably not going to behave the way you want. Household dimmers expect a sine wave input: they look at the zero crossing and fire a triac at some fraction of the cycle. When the second dimmer receives the chopped up sine wave, it might try and fire at the same time, or it might get confused by the non-sine input. It may even have undervoltage trouble, depending on how the low-voltage DC for the control circuitry is controlled.

Instead of two dimmers, here's a couple different things you could try to accomplish your goal:

• Feed the hot plate from a variac. However, this could cause the hot plate to malfunction (again, feeding it a lower voltage might make the control circuitry unhappy), and large variacs with large power ratings are expensive, heavy, etc.

• Control power to the hotplate with an SSR. Because the hot plate has a slow thermal response time, you could basically "PWM" the power to the hot plate with a very low frequency (1Hz or lower).

• Break open the hotplate, and bypass the built-in temperature control (if possible, leave the internal temperature cutoff for safety), and control the hotplate with an SSR. This eliminates all uncertainty with the built-in control, and you have direct PWM control. Again, because it's a hot plate, you don't need high-speed PWM control, 0.2Hz or something will be fine. The nice thing about SSRs is that you can buy a cheap SSR with screw terminals, and you don't have to deal with any mains voltage other than wiring the thing properly.

• The above suggestion might also work with a household dimmer. However, make sure the dimmer is rated for your hotplate wattage, and keep in mind that some household dimmers don't go close enough to zero (you might have the same problem).

• Ah ha: Looks like SSRs are even sold in kits with PID controllers for this purpose! Commented Feb 2, 2016 at 17:39
• Yup. I've used one of those to control a space heater for temperature control of a small space. They work quite nicely. Get yourself a thermocouple, wire it up, decode the poorly written instruction manual, and you're in business! Commented Feb 2, 2016 at 17:50