# Control a resistance in AC circuit without generating current harmonics

I am currently working on a project aiming to control the amount of current flowing through a resistance from 0 to nominal in a continuous manner. The catch is that the power is 230 VAC 50 Hz. My first iteration at it was through phase angle control. However, the current look is horrible (many harmonics).

Thus, I am trying to lessen the amount of harmonics in order to comply with IEC 61000-3-2 (EU norm on current harmonics).

My first thought was to use a full bridge rectifier with a high frequency PWM controlled MOSFET. As the load is only resistive, it creates an in phase, very sinusoidal looking current. Here is the circuit (with 1/10 of input voltage to not overdrive the only MOSFET I knew in the library of the simulation software):

Here is the look of the current:

Thus I thought about filtering those high frequency harmonics by putting a filter in place. However even with a good cutting frequency, I don't find good values for LC filter cause with a smaller cutting frequency comes a bigger current / voltage phase and thus reducing power factor.

Next thing I thought about is to place an active PFC between the full bridge rectifier and the resistive load. I think I would work but is to my mind quite overkill (and quite expensive for high power: it’s around 2 kW).

• Welcome! How about a CLC filter on the input? May 9 at 19:22
• That's a tough one- An input filter might work if your switching frequency is high enough, but any buck type solution will have discontinuous input current. Active PFC would work, but  and size. Does the load have to be driven by DC? Could you turn it on for one full AC cycle and off for N cycles to get the average power you want? That way at least the power factor would be unity for one cycle and you wouldn't draw current for the off cycles. Not sure if that's acceptable resolution or control for your application, but if it's a heater, maybe. May 9 at 19:23
• what range of current do you want to control? May 9 at 19:27
• @JohnD the final solution you talked about could work very well. The only thing is that I am trying to not mess up to much the grid. The resistive load switched is a 2kw heater (for building thermal experimental purposes) and for one house we could use up to 5 of them... So 10kw switched on and off may be not the best idea. May 9 at 19:27
• Ovens, kilns, big stuff, is routinely controlled by 'burst fire' controllers that let through a few 'whole half cycles' from zero cross to zero cross. No harmonics, and the collossal size of the grid, which already has to handle 100000 kettles being turned on simultaneously at half time, will handle the tiny multi-kW fluctuations that your single controller would cause. If you're bothered, sync the 5 heaters to turn on and off at different half cycles. May 9 at 19:33