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I'm designing a LP filter that should be used for a load of up to 2000W. I want to filter the AC voltage that supplies a controlled heater which uses a TRIAC for control. I've tried one approach but found it useless. First, such an inductor, on series with 13A must be very big and possible costly. Second, high losses of power. Third, since I'm working with DC I should use a non-eletrolytic capacitor so, for 220V and such capacitance, it will be very big. Clearly, although I got good filtering on o'scope, it's not the best approach for high power loads.

LP filter

So, finally, is there a better way to design a LP filter for high power that is more electrical efficient and cheaper?

Also, my cutoff frequency was about 60Hz (planned higher, but had to adjust the inductor to guarantee the right current on load). Is there any rule of thumb of which fc should I use when planning to filter AC voltages?

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  • \$\begingroup\$ I am sensing an AB problem, which makes me ask why you want to filter it at all? \$\endgroup\$
    – PlasmaHH
    Commented Mar 19, 2015 at 14:57
  • \$\begingroup\$ English is not my native language, does AB can be understood as a conception problem? \$\endgroup\$
    – Pedro Quadros
    Commented Mar 19, 2015 at 14:59
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    \$\begingroup\$ Seems to be referred to XY problem in english: meta.stackexchange.com/questions/66377/what-is-the-xy-problem \$\endgroup\$
    – PlasmaHH
    Commented Mar 19, 2015 at 15:00
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    \$\begingroup\$ Simple question: Why are you wanting a filter? What purpose? \$\endgroup\$
    – Andy aka
    Commented Mar 19, 2015 at 18:14
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    \$\begingroup\$ Some components blowing is the symptom, not the problem. What PlasmaHH and Andy getting at, is that a filter is probably the wrong tool for the job. Use a zero-crossing switch to turn the heater's triac on or off for complete AC cycles ... nice clean waveforms, no spikes. What you haven't told us yet is the thermal inertia of the heater - can you switch it on once per minute, once per second, or at every half-cycle to get the heat control you need? \$\endgroup\$
    – user16324
    Commented Mar 19, 2015 at 18:22

2 Answers 2

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You have a 68mH inductor and a 100uF capacitor forming a low pass filter. Well, that is your intention. The problem is that these two components also form a series tuned circuit and this will completely "short" out the AC at 61 Hz. If your AC supply happens to coincide with 61 Hz it's a dead short.

What if your AC supply never rises above 60Hz? The inductive reactance is 25.63 ohms and your capacitive reactance is 26.53 ohms - they are of course subtractive and the net impedance is about 0.9 ohms. On a 220VAC supply this means a current of nearly 250 amps. Not good.

If your AC frequency is 50 Hz the inductor has an impedance of 21.36 ohms and the capacitor has an impedance of 31.84 ohms - net impedance is 10.5 ohms - it's still going to take nearly 22 amps of current and blow your fuse.

You have to compromise or there might be a fire. Choose a resonant frequency that is significantly higher than your AC frequency and if this doesn't provide enough filtering, design a two stage filter or a 3 stage filter.

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  • \$\begingroup\$ "Is there any rule of thumb of which fc should I use when planning to filter AC voltages?" How much higher, in ratio? \$\endgroup\$
    – Pedro Quadros
    Commented Mar 19, 2015 at 18:33
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    \$\begingroup\$ @PedroQuadros do the math like I did and decide how much leakage current you can tolerate. Presumably you know that \$X_L = 2\pi f L\$ etc? \$\endgroup\$
    – Andy aka
    Commented Mar 19, 2015 at 18:35
  • \$\begingroup\$ Sure I do. Just having a very hard time as a fresh Engineer. But I'm studying a lot and very different scopes at same time, trying to be at my best fast as it's possible. Thx for the help! \$\endgroup\$
    – Pedro Quadros
    Commented Mar 19, 2015 at 18:48
  • \$\begingroup\$ On my undergraduate I've decided to work with controls, specially on the software design. But I found I have to know controls, programming but also electronics, machines, line transmission... Finally, all the Electrical Engineering course and a lot of more things. \$\endgroup\$
    – Pedro Quadros
    Commented Mar 19, 2015 at 18:53
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    \$\begingroup\$ I think if you use your sim tool you should be able to design something to take out the major harmonic problems but put a small resistor in series with the 100uF to measure current thru this limb. Maybe an acceptable leakage might be 2 amps. \$\endgroup\$
    – Andy aka
    Commented Mar 19, 2015 at 19:17
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Directly filtering AC mains is a design decision typically used only on the grid itself, where reactors can be the size of cars and need no special considerations for cooling and will last for 50 years. Even then, that is typically a technique for reactive grounding.

You already said that you are probably going to modify the heater, so let's look at the overall problem. At this point in the design process, we have to solicit all alternatives and make choices based on effectiveness and economics. Depending on scale and severity of the problem, you have to consider the cost of a big mains filter versus the cost of other techniques. Bear in mind that that big filter is going to need cooling, either directly or by a potentially more expensive air conditioning bill. Its not very likely, but the expenses might balance out in favor of the added benefit of a double converting UPS. Large installations do this too, but when they get really large a motor generator with an attached diesel engine (commonly called a diesel UPS) may be a better option.

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